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Quincy

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  1. Quincy
    As the pattern over North America shifts, an extended period of potential severe thunderstorms targets the central U.S. Although this update covers May 6-10, thunderstorms are ongoing today (May 5th) and were prevalent in prior days as well. The difference here is that we are gradually starting to see more and more potentially potent setups on the horizon.
    Wednesday, May 6th: This day has been on the radar for a while and things are coming into somewhat better focus. As shortwave energy pivots from the High Plains to central/northern Plains on Wednesday, an area of surface low pressure will develop. With more forcing in place than prior days, that alone signals an enhanced severe weather threat.
    —
    By Wednesday afternoon, a portion of the surface low should be crossing over from far eastern Colorado into western Kansas. The strongest forcing should reside from northwestern Oklahoma into central Kansas. However, the greatest instability will likely be displaced further south from central Oklahoma down into north Texas.
    Partial to considerable cloud-cover will tend to limit the amount of destabilization that takes place. That combined with modest flow at 500mb, progged to be on the order of about 35 to 45 knots, will tend to limit the extent of the severe weather potential. The surface low itself is also a bit elongated. If it were more concentrated and also deeper, that would signal a greater severe thunderstorm potential. Nonetheless, with a tongue of 1,000 to 2,000+ J/kg CAPE, favorable speed and directional shear, and some forcing aloft should favor at least scattered severe thunderstorms by mid to late afternoon.
    —
    In terms of severe threats and the areas to watch…
    It comes down to northwestern Oklahoma into central Kansas for what should be the greatest severe threat. More isolated severe storms are still possible along a dryline down into western and central Texas. Large hail, perhaps very large in a couple of storms, appears to be the most significant risk. Damaging winds will be possible, especially into the overnight as storms may tend to merge into lines. There is a tornado threat, which should be maximized during the early evening hours, as the low level jet ramps up. As has been the case many times this year, while there is an evident tornado threat, the intensity and duration of any tornadic storms will be dampened somewhat by the limiting factors mentioned above. There is a fairly good likliehood that multiple tornadoes (but not a major outbreak) will be reported late Wednesday afternoon into Wednesday evening.
    Thursday into Friday, May 7th and 8th, also provide some severe weather threat, but in a more isolated and sporadic nature. Upper level heights should remain fairly neutral through the period with no significant forcing noted in the models. Each day is likely to feature moderate instability and at least marginally supportive wind shear for supercell thunderstorms. The corridor for this period should be narrowed in on the Texas panhandle, portions of interior Texas and into Oklahoma. While some severe threat may nudge into portions of Kansas, the greatest (still somewhat limited) threat should hang back further south. Each day, storms may produce large hail with some damaging winds and a few tornadoes.
    —
    Each day, watch for a few things: First, where is the greatest instability? If cloud-cover and convective debris are limited during the morning, expect a somewhat enhanced severe weather threat during the afternoon. Also, look for any outflow or other mesoscale boundaries that can be the focal point(s) for storm development. Finally, notice any upper level perturbations, even if seemingly minor, as they could provide just enough forcing to locally increase the severe threat.
    Saturday, May 9th has the potential, but the keyword is potential, to be a significant severe weather outbreak with numerous tornadoes. The overall pattern is favorable, as a trough swinging from the Four Corners region to eastern Colorado/New Mexico by late-day will provide ample forcing from the High Plains into portions of the central to southern Plains. Caution must be applied when looking at the analogs, but there is a fairly strong signal from the analogs that favors widespread severe thunderstorms and at least isolated significant severe events, including strong and/or long-track tornadoes.
    —
    Now, even though some major events show up in the analogs, there are a few limiting factors that will not favor a high-end outbreak. While the flow and forcing increase aloft, 70-80 knots at 300mb and 50-60 knots at 500mb will fall well short of an event such as 4/14/12, which showed up as a GFS-based analog. As far as instability, even though moderate CAPE values are predicted, we’re not looking at strong to extreme instability. There seems to be a common theme, with this week especially, that the atmosphere is not quite taking full advantage, resulting in a less extreme “than possible” setup.
    —
    With all of this said, even if the timing is not exactly right, and the timing looks pretty favorable right now, severe thunderstorms are strongly favored on Saturday. What will be the difference between a low-end event and a more significant outbreak will come down to mesoscale details. What also favors severe weather on Saturday is how the models, overall, have been in very good agreement with the general setup on Saturday for quite some time. The focus on Saturday ranges from north Texas through much of Oklahoma and Kansas, to perhaps as far north as portions of Nebraska. All severe weather modes are anticipated, with an enhanced risk of tornadoes.
    Sunday, May 10th is a bit further out there, but yet another severe day is quite possible. Details are a bit more muddled, as the evolution of Saturday will have a sizeable impact on what transpires on Sunday. The threat zone inches somewhat east, ranging from portions of Texas up through central and eastern Oklahoma, eastern Kansas and perhaps into portions of Missouri and Arkansas.
    In summary, expect severe weather threats for the rest of this week and likely through most of this weekend across the central U.S. At least a few severe thunderstorms are expected each day, with scattered severe storms on Wednesday and perhaps a mode widespread and significant event on Saturday. Tornadoes are possible each day, but the day with the likelihood for the most tornadoes is also Saturday. In terms of storm chasing, there should be many quality opportunities to get out in prime real estate, from the High Plains into the Plains west of I-35. With the threats covering a long period over some similar areas, it may be worth while to hunker down and start thinking of possible targets or even just places to stay (if you’re planning a multi-day trip). Without giving too much away, and since details may change, I tend to favor a corridor from the Texas panhandle into southwest and central Kansas. Climatology also favors this zone for the greatest severe weather threat and trends this year have kept the dryline a bit further west, mainly across these areas, during most events.
  2. Quincy
    As the calendar passes deeper into June, there is typically a steady drop-off in the frequency of severe weather threats across the U.S. Last year featured a grand finale to the severe weather season with a fairly significant sequence or tornado events across portions of the north-central U.S. from June 16th to 18th. While this year will likely not see quite an intense show as June 2014, there could very well still be some respectable severe activity with tornadoes.
    Mid-June of this year has been on the radar, so to speak, for quite some time. While there are still some model differences, it would appear that at least a couple of days between Thursday, June 18th and Sunday, June 21st should feature at least localized severe weather events. Note that there is some isolated potential before June 18th, this article will focus more so on the 18th onward.
    With it only being the 15th and plenty of time for forecasts to change, it is too soon to get into specifics. There are at least two main ingredients that would appear to favor severe weather, more-so, perhaps, than the U.S. has seen over the past week or so. Strong to possibly extreme instability is projected to be in place. Both the GFS and Euro show sizable areas of of 3000+ J/kg MLCAPE. The upper level flow is shifting toward a more zonal pattern, at least in the larger scale. An increasingly tight height gradient is likely from the central/northern Plains into portions of the Midwest by the end of the week and into the weekend. This is expected to result in a stronger component to the winds in the upper levels. The result is favorable forcing aloft and enhanced wind shear with any shortwave energy that ejects through.
    There are always caveats and red flags. While there may be more severe weather ingredients to work with, the overlap of those parameters may be relatively small. For example, while 500mb flow may be in excess of 50 knots for a time during this period, the best instability will almost certainly remain displaced well to the south and east. This in itself should tend to favor a more localized severe weather risk. Nonetheless, the Euro and especially the GFS progs over the past several runs highlight(ed) the period in and around June 18th to 21st. The potential exists for severe weather, possibly significant, including tornadoes somewhere in the region between the northern half of the Plains to the western Great Lakes. At the very least, some high CAPE/low shear setups are likely. The questions will become, how large can the threat zones extend and will some of the more impressive shear overlap with stronger instability?
    Take a look at some forecast soundings for this weekend if you want some severe weather eye candy. It is silly to take these progs too seriously at this point, so we will have to wait and see how the forecasts evolve. A broad geographical area is highlighted here and it will not be until the short-term that exact threat zones can be confidently identified. Nonetheless, at least one or two solid chasing days are predicted between Thursday and Sunday. If you are willing to travel or on a chasecation, there is reason to be cautiously optimistic about the storm chase potential coming up.
  3. Quincy
    April is seen as a transition month during the severe weather season, as the frequency of tornadoes typically picks up rather quickly through the month. Despite what the calendar has to say, tornado activity has been fairly lackluster so far this month. In fact, after moving well above the average to-date tornado count in late February, the U.S. has steadily been losing ground. As it stands now, through April 18, the U.S. is near average this year for tornadoes, but with a relatively quiet rest of the week forecast, it looks like the country will fall below average for the first time since February. (The 1986-2015 climate period of 30 years is used for this average)
     
    The month of April started with the “dreaded” trough of low pressure across the Northeast. Locally, it was cold and snowy after a winter that was relatively mild and snowless. Likewise, after an active winter season for tornadoes, severe activity has been sporadic over the past several weeks. With all of this said, a pattern change is finally on the not-to-distant horizon, with troughiness returning to the West and the welcomed return to ridging across the Southeast U.S. This pattern will broadly support a more active period of weather across the central states, including an uptick in tornado potential.
     
    If we’re keeping track, a pattern change was slated for mid-April a long time ago, back as early as mid-March. That pattern change has been delayed somewhat as pesky troughing across southeastern Canada hangs on and the models display a glaring ridge (positive height) bias. This past week saw a shuffling of the pattern, but an omega block led to a messy setup and only a few isolated tornado reports in parts of the High Plains. The models often have issues handling larger scale pattern changes, but now that such a change is effectively inside of a week, confidence is increasing and the models are getting into good agreement.

    By this weekend, the first in a series of troughs is likely to swing across the western half of the country with some modest ridging to the east. Watch for lee-side cyclogenesis in the central High Plains vicinity between Sunday and Monday. While this is encouraging, the setup is not without flaws. This first trough ejection and evolution of the system is kind of junky (for the lack of a better word) in terms of tornado potential. There’s some instability as early as Saturday progged in the north-central states, but shear is forecast to be lackluster. The similar is the case for Sunday and Monday from the Plains into the Upper Midwest – generally marginal overlays between modest shear and favorable instability as a frontal system slides east. This is subject to change, but the trends are not particularly impressive. It’s the middle and latter portion of next week that really bears watching, figuring that this first system may at least get the gears turning for building a warm, moist air-mass over the Plains.

     
    With the Euro and GFS in good agreement (including the ensembles), it’s fair to use the GEFS analogs as a baseline for what type of setup we could be looking at, somewhere in the Tuesday to Saturday (April 26-30th) time-frame. Even before looking at specific dates, the string of digging troughs in the West with increasing moisture return in the central states sets the stage for a classic period of near-peak/peak tornado season activity. This does not necessarily mean we will have a massive number of tornadoes or that there will be a high-end setup, but at the very least, it is probable that tornado activity will markedly pick up as the week wears on. Now speaking of those analogs, there are some big dates and periods that show up, in order from ranking:
    5/10/1953: A couple of days removed from the day 6-10 analog, but this date featured a localized outbreak of strong tornadoes in the Upper Midwest.
    4/25 – 4/29/1994: 124 tornadoes in five days, including an outbreak from the 25th into the 26th.
    5/12 – 5/13/1980: Not an outbreak, but strong tornadoes in Missouri followed by a pair of EF-3 tornadoes in Michigan the next day.
    4/17/1995: AR/OK/TX outbreak with 21 tornadoes.
    4/26/1991: Plains outbreak with 53 tornadoes, including the Andover, KS EF-5 tornado.
    5/2/1994: The only analog in this bunch that didn’t have notable tornado activity.
    5/7 – 5/11/2008: 122 tornadoes in five days. (2008 has been an analog on the radar for a while, also one of the busiest spring seasons of this century)
    5/8/1979: 21 tornadoes, though mostly in Florida, two were reported in Iowa, including an EF-3.
    4/30/1978: 15 tornadoes in the southern Plains, including 6 strong tornadoes and a mile-wide EF-4 on the northwest side of Oklahoma City.
    5/4 – 5/6/1960: 71 tornadoes in three days from the Plains into the Midwest, including a long-track EF-5 tornado in eastern Oklahoma.
    If the pattern unfolds closely to what is modeled, it would appear that April certainly has the potential to go out with a bang.

     
    Through some of the research I’ve done with a May tornado outlook for USTornadoes.com, I noticed that the analogs have things in common. Without giving away too many of the details, one of the common themes in the analog was for the potential for violent tornado events in May. It could be the case that we’re just getting warmed up next week…
  4. Quincy
    Here is a snowfall map using reports from various sources. Many of the reports came from this forum and the National Weather Service. Only social media reports that passed through quality control were considered. All reports gathered were carefully considered and compared before being included.
     
    Snow reached southern Connecticut during the predawn hours on January 23rd. The snow gradually moved inland, dropping the most persistent bands of moderate to heavy snow on an axis from Fairfield County, northeastward into New Haven County and parts of eastern Connecticut. As low pressure slowly moved east to the south of Long Island, precipitation never reached the far northwest corner of the state.
     
    During the peak of the storm, winds consistently gusted to between 30 and 40 mph along the shoreline. No Connecticut stations officially reached blizzard criteria, but near-blizzard conditions affected southwestern Connecticut at times.
     
    Breaks in the precipitation shield across southeastern Connecticut resulted in locally lower amounts of snow, particularly across New London County. Snow quickly came to an end in all areas early on January 24th, ending from west to east across the state.
     
    The Blizzard of 2016 affected a large portion of the United States from the Arklatex region, eastward to the East Coast. The most intense snowfall fell from the Mid-Atlantic states into the New York City metropolitan area. Snowfall totals of 2 to 3 feet were common here, with a few locally higher amounts. All-time single event snowfall records were set at Allentown, Baltimore, Harrisburg and New York City’s JFK Airport. A record daily snow depth was also set at Washington Dulles International Airport. Both Harrisburg and JFK Airport reported 14 straight hours of snowfall rates of 1 to 3 inches per hour during the height of the storm. JFK Airport observed 30.2 inches of snow in one calendar day on January 23rd.

  5. Quincy
    A strong low pressure system is forecast to move up into the Great Lakes by Thursday and eventually into southeastern Canada late Friday into Saturday. A potent cold front associated with this storm will move towards the Appalachians by the second half of Friday. (For Wednesday and Thursday, severe thunderstorm activity is probable for the Mississippi Valley and portions of the western Ohio River Valley.
    I see a marginal setup for severe storms from Central N.Y. into the middle Appalachians. Even though a strong shortwave and upper level jet is expected to move into New York state and Pennsylvania, there are several factors which will likely limit the severe threat...
    With low-level moisture and clouds, daytime heating will not be maximized along the East Coast. Right now, the areas shown in green can expect gradually steepening lapse rates. However, dew-points are only expected to be in the mid-50's to around 60 north of Washington D.C., along with 2-meter temperatures in the mid-60's to lower 70's. eastern Pennsylvania and New Jersey could get well into the 70's, but with the front lagging further to the west, there really isn't much of a trigger to kick off any storms in that area. The SREF/NAM/GFS/Euro are actually fairly similar in timing and overall evolution of this storm system.
    Without much heating, relatively low CAPE values are forecast. The SPC SREF does show a ribbon of slightly negative (0 to -2) Lifted Index values from central Pennsylvania southward. This indicates a marginally unstable atmosphere. As one goes further south into Virginia, the severe weather indices increase, including dew-points into the 60's. This is why that area is outlined in a "low" threat potential.
    Wind shear values are a bit more supportive for severe weather, but even here, the storm system loses some of its punch as it reaches the Appalachian mountains. Effective bulk shear of 40-45kts is forecast across the region. Some strong winds aloft are expected with the front, but this area of stronger winds lags far enough back to really minimize the severe weather threat.
    Overall threats...
    -Damaging wind gusts could be supported anywhere in the green or yellow shadings.
    -In the yellow shading, there's a very slight risk of hail and isolated tornadoes.
    -In New England, gusty winds are possible with a band of rain and thunderstorms late Friday night into early Saturday, but damaging winds are not terribly likely.
    As the spring season goes forward, I will be making more severe weather outlooks. Any feedback is appreciated and I am still working on the outlook/threat scale. I would like to compare forecasts to verification in the future, so some thought will be needed to address just how best to do so. Learning is an ongoing process as I learned a lot about severe weather forecasting last spring/summer and there's still a lot more to cover. Experience also helps!
  6. Quincy
    I see this event as a widespread 2-4 incher across most of interior SNE. The tough call comes along I-95 for BOS-PVD-GON, as often is the case. I do think most areas even along the South Shore get a thump of snow/sleet at the beginning.
    I'm not really all that confident on widespread 4"+ amounts, but I've outlined an area with a black dotted line that could see locally 5-6 inches.
    I also expect precipitation to quickly shut off in the morning (by 7 or 8 a.m. across much of Connecticut) and taper to drizzle near and SE of I-84 with pockets of "snizzle" or flurries/freezing drizzle across the higher terrain.
    Connecticut Forecast:
    On the shoreline...(< 2 inches)
    Although there may be a period of snow and sleet, temperatures near the ground will likely remain above freezing through most of the event. Combine this with with a changeover to plain rain and areas near the coast should see less than 2 inches of snow/sleet.
    Inland...(generally 2-4 inches)
    Most areas should see a period of moderate snow before a changeover to sleet. Precipitation may actually end as a period of drizzle late Wednesday morning, with pockets of freezing drizzle in the hills. Before the changeover, generally 2 to 4 inches of snow/sleet is expected.
    Highest elevations...(locally 4"+)
    Due to colder air in place, Litchfield County and northern Tolland County may stay near or below freezing throughout the entire event. The end result is NEAR 4 inches of snow with localized totals that could reach perhaps 5 or 6 inches.
  7. Quincy
    An early season storm is expected to bring a heavy, wet snow to the interior hills of the upper mid-Atlantic region and New England on Wednesday into very early Thanksgiving morning. While the big cities from Philadelphia to New York and Boston can expect some snow, the greatest amounts will be to their north and west.
    Low pressure will develop along the Carolina coast this morning and ride up the East Coast. With the low passing close to, if not over Cape Cod, no strong high pressure center in place to the north and the fact that we're still in November, a warm surge of air should penetrate up to and even a bit northwest of I-95. While temperatures from 850mb down to the surface may hover near freezing, a warm layer up around 800-700mb will likely cause melting aloft, meaning there will be sleet and rain. The result is anticipated to be heavy wet snow well inland, but quite a bit of sleet across areas such as New Haven, Willimantic and the western Boston suburbs, with a change to rain in areas including Providence and Taunton. New York City and Philadelphia look to be in a bit of a different scenario, where rain will try to change to wet snow during the day on Wednesday. The time of day will not be favorable, along with the somewhat inland 700mb low track.
    The heaviest snow should fall from late morning through the afternoon across northern New Jersey and the mid-Hudson Valley. The hills of interior Massachusetts and central New England should see widespread snowfall amounts over 10" with the heaviest falling from the afternoon into the evening hours. 10 to 12" will be common with a few locally higher amounts possible. Due to the localized nature of 12"+ totals, they have been omitted from this map. Snow will taper off to snow showers early Thanksgiving morning and a few flurries could linger into the day in some locations.
    Snowfall forecast totals for the big cities:
    Boston: 2 to 4 inches
    Worcester: 8 to 12 inches
    Providence: 2 to 3 inches
    Hartford: 6 to 9 inches
    New York City: 1 to 3 inches
    Philadelphia: 1 to 3 inches

  8. Quincy
    Here is a final snowfall forecast for the upper mid-Atlantic region and southern New England for the Blizzard of 2015. No major changes were made to the forecast, but the western fringe was trimmed somewhat.
    It's becoming a nowcast situation, as the computer models are relatively all over the place. The 12z Euro is one extreme (heavy west), with the 00z RGEM further east and much less impressive. Based on radar trends, the previous forecast map and a consideration to the model consensus, here is the latest thinking:
    The shield of heavy snow is backing west through Long Island and southern New England as of 11:35 p.m. Monday. The back edge should make it to roughly the New York border before it essentially stalls. Interior southern New England should remain within the steadiest snow for the longest period of time. There are still unclear details about a mesoscale band of enhanced snowfall totals, but a general 20 to 26 inch snowfall is expected from the hills of eastern Connecticut into much of interior eastern Massachusetts. Banding may result in some 30"+ snowfall amounts, somewhere between northeastern Connecticut and the Worcester-Boston area.

  9. Quincy
    Outside of the hilly terrain across the interior, this snow event does not look like a big deal. I expect 3-6"+ across the east slopes of the Berkshires and much of Worcester County. Connecticut only sees a brief period of rain, with generally 1-3" across the northern hills.
    The boundary layer temperatures are simply too marginal for much to change, especially with dew-points in the upper 20's as of early Tuesday evening. Downstream observations don't indicate that cold air damming across New England will verify stronger than modeled.
  10. Quincy
    Map above is based on reports from various sources, including this board, the National Weather Service and data viewers from across the state sent in.
    Event Summary:
    Scattered snow showers reached portions of western Connecticut by late morning on the 29th. This snow was associated with weakening low pressure over Pennsylvania.
    The main storm began to develop east of Virginia during the afternoon.
    Steady precipitation overspread the state from southwest to northeast between about 1 and 3 p.m. Most areas saw snow, but some ice pellets and graupel were reported near and southeast of I-95. Coastal New London County even switched to rain for a time.
    Colder air moved in and a heavy band of snow set up over central and eastern Connecticut.
    Snowfall rates between 1 and 3 inches per hour hammered portions of New Haven, Middlesex and New London Counties. This area of heavy snow eventually moved northeast and also impacted Tolland and Windham Counties. This band was a bit more intense than some predictions, resulting in higher snowfall amounts than forecast.
    Extreme western Connecticut was too far west to be affected by this heavy snow.
    The heaviest snow fell between about 4 p.m. and 9 p.m. before the shield of precipitation began to break apart. After midnight, the only leftover precipitation was occasional snow showers. By then, the bulk of the accumulation was over.
    I still find the Feb. 7, 2003 analog to be quite good for this event, with respect to Connecticut. It's clear that with a warmer solution, southeastern Mass. could not have had such high snowfall amounts.
    Shift the axis about 75 miles SW and you get a VERY good match-up. This analog showed up as a strong match about 2-3 days before the event: The point was that interior eastern Conn. and NW R.I. was favored for the highest amounts.

    Forecasts for the storm were decent within 24 hours of the event, but before that, most were playing catch-up. What was expected to be a minor event turned into a moderate one with several snowfall totals in the 10-12 inch range, especially across eastern Connecticut.
    My own forecast (from 5 p.m. the night before the storm) was too high for areas SW of Conn. and was generous as well for eastern Mass. With that said, I could have also pushed the 6-10" zone back further west:

    The image below shows approximate snowfall totals from across the region:

    Heavy snow focused in on the eastern half of Connecticut and the radar image below shows moderate to heavy precipitation pounding southern portions of the state. At the same time, some observed 2 to 3 inches of snow per hour for a few hours.

  11. Quincy
    The final few days of April finished on the quiet side in terms of severe and that continued into May 1st. With that said, a larger-scale pattern change is likely to be accompanied by multiple threats of severe weather in the coming days.
    Saturday 5/2: Not seeing anything major here. Although we should finally see some modest instability up into the central Plains, dew-points are only expected to rise into the upper 40s to lower 50s across the area with some weak forcing during the evening. One plus is that wind shear will be supportive of supercell structures, assuming other variables line up.
    Verdict: A few, rogue severe thunderstorms are possible from late afternoon into the evening, but even those may only be marginally severe. The focus is on the central Plains

    Sunday 5/3: The setup looks slightly better than Saturday, but that does not say much. A larger area can expect modest to moderate instability, but the drawback here is near-neutral height tendencies. Upper level progs show little focused area of forcing. The only thing that looks somewhat interesting is a surface low forming near the Colorado/Kansas border. This could narrow the focus on some isolated severe thunderstorms to the Texas and Oklahoma panhandles, but even here the prospects are not terribly exciting. If there is enough of a trigger, a few isolated strong to severe storms could also initiate up into the central Plains and as far east as the upper Midwest. Wind shear is questionable throughout the regions outlined.
    Verdict: Isolated severe thunderstorms are possible anywhere from the southern High Plains into the central Plains and upper Midwest. If there is one area to focus in on, it might be just ahead of a surface low, in the vicinity of the Texas/Oklahoma panhandles. While the ceiling may be somewhat higher than Saturday, storm coverage and intensity is not expected to be particularly impressive.

    Monday 5/4: This looks like the beginning of what may be an extended period of active weather. A trough should begin digging into the West Coast, with some modest height falls reaching the High Plains. Moisture will likely be a positive and negative thing here. Although dew-points should reach the upper 50s to lower 60s at a minimum, cloud-cover, convective debris and increasing precipitable water values throw a wrench in the severe storm potential. Likewise, wind shear is not expected to be terribly impressive either. What may end up ultimately happening is that a few pockets of greater instability could lead to a few supercells reaching severe limits.
    Verdict: Mixed signals somewhat hamper the severe potential, but it would appear probable that at least a few isolated to widely scattered severe thunderstorms could develop from the Texas/Oklahoma panhandles into Kansas.

    Tuesday 5/5 through Thursday 5/7: The Euro and GFS are actually in very good agreement overall through the period with the general pattern. The West Coast trough continues to dig and a piece of energy swings through from the High Plains on Tuesday into the central Plains on Wednesday. Tuesday looks muddled at best with meager instability and limited kinematic support. Wednesday may offer the best potential, if things line up. Shortwave energy should be pivoting into the central Plains, along with a surface low, but that low looks awfully elongated. The instability setup may suffer too, from limited instability and on-going clouds/sub-severe storms. With that said, a low level-jet should be cranking with both the Euro and GFS suggesting at least 40-50+ knot winds at 850mb. Thursday becomes a bit more unclear, despite continued model agreement. Storms may be ongoing Thursday morning, based on current trends, and the models seem, perhaps, a bit overdone with instability in the warm sector. Even so, wind shear looks to slacken off, which could be another factor to limit the severe threat. Timing is yet another concern, as a faster evolution could really hinder the Thursday risk, whereas a slower scenario might be better.
    Verdict: Some isolated to scattered severe thunderstorm potential each day, but details are still unclear. Wednesday may offer the greatest potential with Thursday being a wild card.

    The worst case is that this upcoming week, there are just a few severe thunderstorms, but enough of a threat to track for a few days in a row. The best case is that things come together for one to possibly two decent chase days.
  12. Quincy
    A cold front is forecast to move from west to east across the East Coast on Wednesday. The result could mean scattered thunderstorms along the front, along with a few severe thunderstorms.
    (Marginal threat)
    Middle Appalachians into DE, PA, inland NJ and interior NY.
    A similar setup to this past Friday is expected on Wednesday, with a cold front moving across the area during the afternoon and evening hours. The only difference here is that the cold front is not as strong as that scenario (last Friday) and less convective activity is expected.
    Ahead of the cold front, some marginal destabilization is expected as near-surface temperatures rise into the 70's, slightly steepening lapse rates. As a result, Lifted Index values between 0 and -2 are forecast. MUCAPE is meager, with values generally less than 500 J/Kg. Although dew-points are forecast to be in the mid to upper 50's, wind shear and helicity is forecast to be somewhat less than the last event. Bulk shear of 30 to 50kt is forecast across eastern Pennsylvania, coupled with storm relative helicity values of 100-200, so some isolated damaging wind gusts are possible.
    (Low threat)
    Interior Virginia, Central Maryland and portions of eastern Pennsylvania.
    In this region, the timing of the cold front corresponds closely with the peak daytime heating. There's also a sliver of 500 to 1000 J/Kg CAPE values expected in south-central Pennsylvania, down into Maryland and Virginia. It's also in this general area that there is a marginal tornado threat, however the tornadic setup on Wednesday is also less impressive than it was last Friday.
    Graphics to differentiate between the damaging wind and tornado threats:

    (No appreciable severe weather threat)
    Southern New England, Long Island and coastal plain of New Jersey.
    The cold front does not make it into this area until midnight or a few hours afterward. At that time, moisture is very limited, wind shear values relax and there should be little to no instability to fuel thunderstorm activity.
    Threat definitions...(Severe thunderstorm = wind gusts to 58 MPH and/or >1" hail)
    Marginal: Conditions are marginally favorable for severe thunderstorms.
    Low: Conditions are somewhat favorable for severe thunderstorms.
    Moderate: Conditions are moderately favorable for severe thunderstorms.
    High: Conditions are very favorable for severe thunderstorms.
    Extreme: Conditions are extremely favorable and dangerous thunderstorms are likely.
    As the spring season goes forward, I will be making more severe weather outlooks. Any feedback is appreciated and I am still working on the outlook/threat scale. I would like to compare forecasts to verification in the future, so some thought will be needed to address just how best to do so.
    *Disclaimer* The intent here is somewhat different than SPC severe weather outlooks. The green, yellow, etc. shadings do not directly correlate between SPC and my own outlooks. I'm still getting a feel for these outlooks, but I imagine that in more impressive severe setups, I will use red and orange colors a bit more generously than SPC might. Also, green colors on my maps indicate a marginal severe weather threat, while on SPC it corresponds to a general thunderstorm (not necessarily severe) threat.
  13. Quincy
    A pattern change is soon underway that will leave much of the U.S., especially the eastern two thirds, feeling spring fever. Unlike patterns in recent years, a prevalent ridge of high pressure off or near the East Coast (“Southeast ridge”) will maintain generally at or above average temperatures for the region. The only exception may be parts of the Northeast, where some backdoor cold fronts could keep temperatures cooler. To the west, troughing across the West Coast should help keep a much-needed stormy pattern in place for California and surrounding areas. In the middle is where things can get interesting in terms of severe thunderstorm potential with the possibility for above average tornado activity.
     
    After taking a close look at past mid to late March patterns over the last 20 years, two periods can be identified as close analogs to what is forecast to unfold, March in 2006 and 2009. The short and medium range guidance is in agreement that the first springlike setup over the Plains gets going on Sunday and lasts into early next week. Beyond that, the longer range ensembles agree that the basic pattern, one with ridging in the Southeast and troughing over the western third of the country, continues. The Euro weeklies show this right through the end of March, while the 16-day GEFS has this setup solidly through the third week of March.
     

    Without spending too much time on details, both 2006 and 2009 saw a fair amount of tornado activity to the north and west of a “typical” March, where activity is favored over Dixie (Deep South/Gulf Coast). There was a significant early season tornado outbreak from March 9-13, 2006 across the Middle Mississippi Valley and Ozarks, particularly over Missouri. In 2009, activity was not nearly as concentrated, but there were notable tornado events across the Plains and Midwest. Essentially, both years had a relatively active March, but the biggest story was how far north and northwest tornado events took place.
     
    Both 2006 and 2009 featured ridging across the Southeast and warmer than average temperatures over much of the central and eastern U.S. This is important for the rest of this month. Aside from the warmth, moisture (think instability) is able to travel farther north, which is often a key limiting factor for severe events in the Plains and Midwest in late winter and often early spring. Assuming the pattern that shows up early next week is repeated through the month, we may have a very active March on our hands.
     
    Summary
    The rest of March may feature persistent Southeast riding and western troughing
    Above average precipitation is expected in California
    Warmth is likely from the southern Plains to the Tennessee Valley/Southeast/Mid-Atlantic
    Above average severe activity is anticipated, overall, nationally
    The threat exists for multiple tornado events over the southern/central Plains and parts of the Midwest

    It is not clear at this point if we will have a significant March outbreak like 2006. Also, there are still some limiting factors for Sunday-Tuesday coming up that may preclude a bigger event. Nonetheless, severe weather, including at least a few tornadoes are anticipated into early this upcoming week. Through mid and late March, expect at least a couple of more severe events, continuing the pace for near to above average tornado activity in the U.S.

  14. Quincy
    A tornado outbreak appears likely on Tuesday into Tuesday night across portions of Alabama, Mississippi and Tennessee. Over a broad area from the lower Mississippi Valley to the Tennessee and lower Ohio valleys, scattered severe thunderstorms are expected to develop Tuesday afternoon, continuing into Tuesday evening. The threat will shift east overnight, gradually lessening, especially after midnight. The setup looks favorable for several tornadoes, including at least one or two strong tornadoes.
     
    Overview: Discrete to semi-discrete thunderstorms are anticipated to develop over central to northern Mississippi and western Alabama as early as midday Tuesday. The severe threat quickly ramps up over Mississippi by early afternoon, although it not be until mid-afternoon that the earliest for a severe threat over western Alabama. (Early convection may be elevated) Other storms are likely to fire into western Tennessee, while a new line of thunderstorms initiates near the Mississippi River during the afternoon. Basically, you have storms right ahead of a cold front (the western line of storms) and warm sector supercells. The supercells in the warm sector have the greatest risk at producing strong, long-tracking tornadoes. I do not see this as a high-end or historic outbreak, but my best estimate at this point is for 8 to 12 tornadoes with at least one or two strong (EF-2+) tornadoes. The other point to make is that the tornado threat will continue after dark across northeastern Mississippi, northwestern Alabama and middle Tennessee. As storms merge and are eventually overtaken by a squall line, the tornado threat should lessen after midnight.
     
    The greatest risk for tornadoes is expected to extend from central to northern Mississippi into western and northwestern Alabama, western to middle Tennessee and western Kentucky. Strong tornadoes are most likely to form across northeastern Mississippi and northwestern Alabama into west-central Tennessee. A couple of brief tornadoes cannot be ruled out to the east or even north of this purple area, but the tornado threat is much lower in those locations.

     
    The models showing greater instability (NAM, SREF, RGEM) are verifying quite well. Given dew-points already into the mid-60s in southwestern Mississippi by 11 p.m. CST Monday and dew-points around 70 in parts of Louisiana, moisture return is going to plan or slightly greater than prior forecasts. The biggest variable affecting the extent of any outbreak tomorrow is instability. It looks like much of northern Mississippi and perhaps western Tennessee will see SBCAPE values of 1000-1500 J/kg by early afternoon. This instability, when coupled with favorable wind shear, would absolutely favor tornadic supercells. The HRRR is trending even greater with instability, but the progs of 1500-2000 J/kg by midday may be a bit overdone. Still, it should be noted that 1000 J/kg is about the average amount of CAPE for cold season tornadoes in Dixie, especially with the magnitude of shear that is expected to be observed later Tuesday.
     
    The point is to be prepared for severe weather, including tornadoes, across the areas mentioned above, anytime from Tuesday afternoon onward. There may be some weak shower activity in the morning, but that will most likely not be severe. Trends with any early showers will need to be monitored, as convective/cloud debris could have an affect on destabilization. later in the day.
  15. Quincy
    What a challenging forecast and I still think the models have more to resolve, mainly on Sunday with respect to enhancement of snow along a trough.
    Essentially, a light mixture of rain and snow overspreads the region through later today. Across the lower Hudson Valley, Connecticut, Rhode Island and southeastern Massachusetts, I expect virtually no daytime snow accumulation. Low pressure develops south of the area tonight, but the heaviest precipitation and best lift also stays south.
    Some snow falls early Sunday morning and a period of light to moderate snow may develop for a good chunk of the second half of the day, especially across northern Massachusetts and central New England. Here, snowfall totals are forecast to finish in the 4 to 8 inch range by 12:00 a.m. Monday. (Could snow possibly linger later? Yes.)
    We'll see how things continue to change!
  16. Quincy
    A notable threat of severe thunderstorm activity in early February has been showing up in the models for several days now. As the event gets closer, confidence is increasing that a setup favorable for severe weather is likely to occur. However, there still are a lot of details left to be nailed down. The broad pattern involves a vigorous trough in the jet stream digging across the Four Corners region on Monday, February 1st and swinging east to northeast across the United States into Groundhog Day. The greatest severe weather threat will be focused on where energy around this trough ejects with the orientation (tilt) of the trough being a factor of important consideration.
     
    Lee-side cyclogenesis is likely on Monday across the southern High Plains with low pressure rotating from the Texas/Oklahoma vicinity into the Middle Mississippi Valley and eventually the Great Lakes. Given the low-level jet projected with this system, ample moisture return should bring 60+ degree dew-points northward into the Gulf Coast states. Instability forecasts have varied from weak to modest and the shear signal has been significant, with a sizable area of 50+ knots of 0-6km shear likely.
     
    Similar setups in the past (analogs) have mixed signals, with some showing significant severe outbreaks and others with little to no severe activity. My main focus at this point is the orientation of the trough. A more negatively tilted trough will yield an increasingly unidirectional wind field. If the trough is closer to a neutral tilt or slightly positive, that would maximize the amount of directional shear. The trends appeared to favor the trough becoming negatively tilted on Tuesday, February 2nd, meaning that the storm mode could get messy and trend toward a squall line. The new 00z ECMWF shows a trough closer to a neutral tilt as the upper level flow is southwesterly, not south-southwesterly. If this were to be a hiccup and the trough is more negatively tilted, then the severe threat will not be maximized. It is important to note that severe thunderstorms, including tornadoes, will still be possible in this scenario, but if the ECMWF is correct, we may have a more significant threat to monitor.
     
    Based on the latest data, I would expect an isolated, conditional severe threat to develop across the southern Plains late Monday into the overnight, mainly from parts of North Texas into the eastern half of Oklahoma. Given cold advection and steep mid-level lapse rates, the major threat would likely be large hail, although damaging winds and perhaps a couple of tornadoes would also be possible. Tuesday is the day of focus, as any discrete to semi-discrete thunderstorms that develop in the warm sector could become tornadic. It appears that this area will encompass northeastern Louisiana and eastern Arkansas into Mississippi, far southeastern Missouri, far southwestern Kentucky and western to middle Tennessee. Although the threat could punch north into southern Illinois, southern Indiana and more of Kentucky, meager instability amidst an increasingly pinched warm sector, there is lower confidence in this scenario. Also, the severe threat could reach Alabama later in the day, but the last several runs of the ECMWF have suggested a somewhat slower evolution, keeping the threat a bit farther west, perhaps only reaching northwestern Alabama.
     
    With all of this said, severe weather seems likely on Tuesday with at least a few tornadoes. The wind fields would support a strong tornado. If one or a combination of the following conditions are met, then this setup could turn into a higher-impact event with numerous tornadoes, some strong and long-tracking: More instability than currently forecast would support a more robust setup, particularly in terms of localized vs. larger scale. A near-neutral or even slightly positive tilted trough would increase the amount of directional shear. Discrete thunderstorm development in the warm sector, especially early in the day on Tuesday, would suggest an elevated tornado threat.
     
    First thoughts:
     
    -Conditional threat of a tornado or two late Monday night/early Tuesday.
    -At least a few tornadoes on Tuesday; better than 50/50 odds of a strong tornado.
    -Tornado threat is maximized on Tuesday from far northeastern Louisiana and eastern Arkansas into northern Mississippi, western to middle Tennessee, far southeastern Missouri, far southwestern Kentucky and northwestern Alabama.

  17. Quincy
    As a warm front lifts into the Northeastern United States on Wednesday, a warmer, more humid air-mass floods into the Northeast. Some thunderstorm development is forecast and some of those storms may be capable of reaching severe limits, particularly across portions of New York and west-central New England.
    Low threat (Conditions are somewhat favorable for severe thunderstorms)
    Interior New England, New York, northwestern New Jersey and norther Pennsylvania:
    As a warm front clears, an increasingly unstable air-mass overapreads the area, along with increasing dew-points. Once lingering showers, patchy fog and drizzle clear, some daytime heating will begin to fuel the atmosphere. The severe thunderstorm threat relies on a few factors. Although daytime heating is important, it should be noted that bulk wind shear values are forecast to be low to moderate, with the strongest helicity displaced northeast into central and northern New England.
    A line or broken lines of thunderstorms are expected to develop along a trough during the afternoon. The focus is on central and eastern New York, as well as western Massachusetts and southern Vermont. This area has a fair amount of overlap with respect to wind shear, instability, helicity and proximity to a shortwave passing near the Canadian-United States border. MLCAPE and lifted index values are somewhat supportive of severe thunderstorms, especially assuming there is a fair amount of sunshine to promote destabilization.
    It is also in this region that both the ARW and NMM model forecast reflectivities indicate a relatively strong squall line dropping southeast between about 5 p.m. and 9 p.m. local time.
    The threat appears to be mainly strong winds, but isolated hail is another possibility. Although the tornado threat is low, should wind shear coincide enough with higher helicity values, there could be a spin up or two.
    This area is being monitored and may be upgraded to a moderate threat of severe thunderstorms, should model guidance trends continue.
    Marginal threat (Conditions are marginally favorable for severe thunderstorms)
    Elsewhere in green:
    A few scattered showers and thunderstorms are possible across the region, as far south as the middle Appalachians. Across the coastal plain of New England, decaying thunderstorms could hold together as they approach Interstate 95.
    Threat definitions...(Severe thunderstorm = wind gusts to 58 MPH and/or >1" hail)
    Marginal: Conditions are marginally favorable for severe thunderstorms.
    Low: Conditions are somewhat favorable for severe thunderstorms.
    Moderate: Conditions are moderately favorable for severe thunderstorms.
    High: Conditions are very favorable for severe thunderstorms.
    Extreme: Conditions are extremely favorable and dangerous thunderstorms are likely.
    *Disclaimer* The intent here is somewhat different than SPC severe weather outlooks. The green, yellow, etc. shadings do not directly correlate between SPC and my own outlooks. I'm still getting a feel for these outlooks, but I imagine that in more impressive severe setups, I will use red and orange colors a bit more generously than SPC might. Also, green colors on my maps indicate a marginal severe weather threat, while on SPC it corresponds to a general thunderstorm (not necessarily severe) threat.

  18. Quincy
    As a warm front lifts into the Northeastern United States on Wednesday, some thunderstorm development is forecast and some of those storms may be capable of reaching severe limits.
    Low threat (Conditions are somewhat favorable for severe thunderstorms)
    Central Pennsylvania and portions of Maryland:
    A warm front is expected to lift into the Northeast on Wednesday. The focus is across the warm sector where some strong wind shear combines with surface/low-level instability to enhance the severe thunderstorm threat. The strongest wind shear and helicity values are projected to move across New York and New England, but there may be enough overlap across portions of the mid-Atlantic states to initiate some severe thunderstorms. The threats at this time look to be for damaging winds and isolated tornadoes. This is heavily dependent on timing and the ability of enough daytime heating to steepen lapse rates. The 15z SREF was showing an area of 1000-2000 J/kg MLCAPE across southern Pennsylvania during Wednesday afternoon.
    Marginal threat (Conditions are marginally favorable for severe thunderstorms)
    Elsewhere in green:
    Although a large area is outlined in green, no organized area of severe thunderstorms is expected here. However, there is marginal atmospheric support for a few damaging wind gusts.
    Threat definitions...(Severe thunderstorm = wind gusts to 58 MPH and/or >1" hail)
    Marginal: Conditions are marginally favorable for severe thunderstorms.
    Low: Conditions are somewhat favorable for severe thunderstorms.
    Moderate: Conditions are moderately favorable for severe thunderstorms.
    High: Conditions are very favorable for severe thunderstorms.
    Extreme: Conditions are extremely favorable and dangerous thunderstorms are likely.
    *Disclaimer* The intent here is somewhat different than SPC severe weather outlooks. The green, yellow, etc. shadings do not directly correlate between SPC and my own outlooks. I'm still getting a feel for these outlooks, but I imagine that in more impressive severe setups, I will use red and orange colors a bit more generously than SPC might. Also, green colors on my maps indicate a marginal severe weather threat, while on SPC it corresponds to a general thunderstorm (not necessarily severe) threat.
  19. Quincy
    As a cold front moves towards the East Coast on Saturday, some thunderstorm development is possible during the afternoon and early evening hours.
    Low threat (Conditions are somewhat favorable for severe thunderstorms)
    Mid-Atlantic region (east of Appalachians), lower Hudson Valley and SW New England.
    Conditions will destabilize in a warm, somewhat moist air-mass before an approaching cold front swings from west to east through the area. The biggest question mark is how much daytime heating will take place. Across Virginia, there will likely be more heating and with higher dew-points here as well, in the mid-60's, some thunderstorm development is likely. Some of these storms could reach severe limits.
    Further north, across eastern Pennsylvania, interior New Jersey, southeastern New York, northwestern Connecticut and the Berkshires, the threat of severe thunderstorm activity relies heavily upon daytime heating. The NAM continues to show some scattered showers and mid-level moisture limiting the destabilization process. With that said, the SREF/NAM/GFS still show a narrow finger of somewhat unstable air in this region. CAPE values between 250-500 J/kg, LI values around -2 and bulk shear around 30kt could result in a few thunderstorms reaching severe limits. Dew-points are progged to reach the lower 60's by the NAM. The 18z NAM simulated radar shows a squall line developing from Baltimore to Philadelphia to Poughkeepsie. However, this line is relatively short-lived as it collapses upon reaching the coastal plain and western Connecticut.
    It should be noted that helicity values increase towards the northeast, especially in New England. This may work to offset the slightly less unstable atmosphere.
    Bottom line, some thunderstorms may produce damaging winds from the mid-Atlantic region into southwestern New England. Some hail is possible here, but there's a stronger potential for severe hail further south.
    The tornado risk is relatively low, although Significant Tornado Parameter (STP) values indicate that a brief tornado or two could form anywhere in the yellow shading. Local topography across the Appalachians could aid in some rotation with any cells. When running the numbers, the tornado parameters are a bit below the mean tornado environments for historic tornadoes in the region, but are higher than what verified on Thursday with the brief EF-0 tornado in eastern Massachusetts.
    A few renegade thunderstorm cells could fire ahead of a cold front Saturday afternoon, especially across Virginia, where there may be a few supercells. A squall line along the cold front is more likely, advancing through eastern Pennsylvania early in the afternoon and towards the NYC metropolitan area and the coastal plain by mid to late afternoon. Instability will be key in the formation of and ability of this line to maintain itself.
    Marginal threat (Conditions are marginally favorable for severe thunderstorms)
    Mid and upper Appalachians and portions of southern New England.
    Shower and thunderstorm activity is likely across the entire area, but less daytime heating will limit the severe weather threat. As one moves eastward, especially towards the coastal plain of New England, the threat decreases significantly.
    Threat definitions...(Severe thunderstorm = wind gusts to 58 MPH and/or >1" hail)
    Marginal: Conditions are marginally favorable for severe thunderstorms.
    Low: Conditions are somewhat favorable for severe thunderstorms.
    Moderate: Conditions are moderately favorable for severe thunderstorms.
    High: Conditions are very favorable for severe thunderstorms.
    Extreme: Conditions are extremely favorable and dangerous thunderstorms are likely.
    *Disclaimer* The intent here is somewhat different than SPC severe weather outlooks. The green, yellow, etc. shadings do not directly correlate between SPC and my own outlooks. I'm still getting a feel for these outlooks, but I imagine that in more impressive severe setups, I will use red and orange colors a bit more generously than SPC might. Also, green colors on my maps indicate a marginal severe weather threat, while on SPC it corresponds to a general thunderstorm (not necessarily severe) threat.

  20. Quincy
    There have been no major changes to the previous forecast. Some of the snowfall totals have been trimmed back ever so slightly. A fairly widespread area of 5 to 10 inches of snow is expected, with a jackpot across much of interior Massachusetts, where most or all of the precipitation should fall as snow.
    Snow becomes heavy at times early Monday morning and begins to mix with sleet and freezing rain by the morning rush around New York City. The mixing then spreads into southern New England by mid morning. After a few hours of sleet and freezing rain across the coastal plain of Connecticut, precipitation should quickly begin to lighten up by late morning to midday.
    Across northeastern Massachusetts, snow will continue through early afternoon before tapering off by early evening and as a result, around a foot of snow is forecast here. The greatest threat for icing extends from northern New Jersey into the lower Hudson Valley and portions of southern and central Connecticut.

  21. Quincy
    Another vigorous through is forecast to sweep through the south-central states over the next few days, bringing a threat of thunderstorms to the region. Although this threat looks substantially less impressive than the event in mid-November, there still exists the potential for at least a few severe thunderstorms, especially Saturday.
     
    The setup for Saturday has had a lot of question marks and red flags from the start, but now that the event is approaching, things are coming into better focus. The upper level pattern is split with a fairly complex setup forecast to develop. It looks like two pieces of energy will factor into the southern Plains severe threat. The first will be a strong daytime low-level jet across eastern Texas with vorticity maxima ejecting north-northeast toward the Ozarks Saturday afternoon. The second area of energy hangs back with a cold front and the more substantial height falls across the southern High Plains.
     
    The greatest severe threat, although still somewhat limited, should develop from eastern Texas into perhaps southeastern Oklahoma/far southwestern Arkansas during the afternoon hours on Saturday. Aided by a 40-50 knot low-level jet and modest instability, at least a few supercell thunderstorms could fire. Given the wind profiles, all severe hazards appear possible with perhaps a few tornadoes. The threat here shifts to damaging winds as storms merge and should tend to wind down after dark as daytime heating fades and the strongest forcing moves into the Ozarks and Middle Mississippi Valley.
     
    The secondary severe threat looks to be from central to North Texas, into central and western Oklahoma. This could extend into Kansas, but with northward extent, instability will become increasingly limited. There are three issues with this area, with the first being a continued messy look to the wind profile. Model soundings have shown a veer-back profile in the mid-levels, producing some jagged, criss-crossing hodographs. Also, some warmer temperatures in the mid-levels should keep the atmosphere capped in most areas until early in the evening. Finally, the trough has slowed down a bit on the models, which also keeps the best forcing farther west until after dark. The timing is analogous to 11/16/15, but instability is significantly less impressive and shear is not as robust. Although a tornado cannot be ruled out, the threat is fairly low.
     
    For this area, thunderstorms will try to develop around or shortly after sunset, but may have trouble organizing given the above-mentioned factors. Still, given cold advection aloft and moderate shear, a few briefly discrete/semi-discrete severe thunderstorms could develop. The most likely scenario is that thunderstorms form into a squall line and feature damaging winds as the main threat overnight.
     
    Speaking of mid-November, the Sunday threat has a lot of similarities to 11/17/15. Not only with timing and placement, but with the synoptic setup. Keep in mind that 11/17 had more instability and was an “elevated” 10% tornado threat, but the event struggled, mainly producing sporadic damaging wind gusts, although a few overnight tornadoes were reported in Mississippi. This Sunday, the initial squall line looks to be only marginally organized (unlike the robust 11/17 AM line) and the potential for warm sector supercells out in front, across the Lower Mississippi Valley, is fairly low. Still, a few damaging wind gusts and perhaps a tornado or two appear possible. With the warm sector becoming increasingly narrow (pinched) through the day, the aerial extent of severe threat should fade during the second half of the day, despite impressive wind fields aloft.
  22. Quincy
    A strong cold front approaches the Appalachians Friday afternoon and evening. Along that cold front, a squall line with thunderstorms is currently expected is develop.
    (Marginal threat)
    Middle Appalachians into Pennsylvania, northwestern New Jersey and interior New York:
    Daytime heating is expected to push temperatures into the low to mid-70's in the valley locations with mid to upper 60's in the higher elevations. Low and mid-level clouds will likely limit just how unstable the atmosphere can get. As a result, relatively low SBCAPE below 500 J/KG combined with near-zero or marginally negative Lifted Index values are expected. Despite strong winds aloft and effective bulk shear values increasing to 35 to 50kts along a cold front, the lack of instability will likely minimize the threat of severe weather. In addition, dew-points are forecast to stay in the 50's, perhaps coming close to 60° in Maryland and eastern Pennsylvania.
    However, a few isolated strong thunderstorms could develop Friday afternoon ahead of a cold front. There is a marginal threat of a few damaging wind gusts associated with a squall line along the cold front Friday afternoon into Friday night.
    (Low threat)
    Interior Virginia, east of the Appalachians:
    The atmosphere is likely to become slightly more unstable further south. SBCAPE values of 500 to 750 J/KG are forecast based off of SREF guidance. Lifted Index values as low as -3 combined with dew-points in the 60's and higher helicity values would support more strong to severe thunderstorm activity.
    Based off of this, there is a low threat of severe thunderstorms with damaging winds and large hail in this area. Some isolated thunderstorm cells are possible Friday afternoon, but locally severe storms could develop along a squall line Friday night as the cold front approaches.
    There is a marginal threat for an isolated tornado or two, but confidence in this potential is not particularly high at this point.
    (No appreciable severe weather threat)
    Connecticut (southern new England), NYC, Long Island and coastal plain of NJ/DE:
    With more of a marine influence allowing for less daytime heating, there is no appreciable threat of severe weather in these areas. Also, with the cold front not expected to move through until after midnight Friday (into early Saturday), this further implies that the severe weather threat is virtually non-existent.
    With that said, some locally strong wind gusts are possible as a decaying squall line and associated cold front moves through late Friday night into early Saturday morning.
    As the spring season goes forward, I will be making more severe weather outlooks. Any feedback is appreciated and I am still working on the outlook/threat scale. I would like to compare forecasts to verification in the future, so some thought will be needed to address just how best to do so. Learning is an ongoing process as I learned a lot about severe weather forecasting last spring/summer and there's still a lot more to cover. Experience also helps!
  23. Quincy
    I've been creating daily 6-day forecasts for the better part of this year, with a focus on inland Connecticut. Before making any forecast, I take a close look at the computer model forecasts through Day 6, including a few forecast techniques to see how verification pans out. In April, I had 29 days worth of data, out of a possible 30, to measure forecast accuracy.
    As expected, forecast error generally increases with time. It is interesting to note a spike at Day 5 and a decrease at Day 6. That goes back to two particular days that had poor Day 5 forecasts vs. actual temperatures. The spread is relatively uniform as well. The error with the NAM model does seem to increase faster with time than the others, which is not a surprise. DGEX data was used for Days 5 and 6.
    With respect to my own forecasts, I measure verification as a mean of inland temperatures across the state. When I look at the computer models, I choose Meriden (KMMK) as a central point. This is due to its location near the center of the state. With that said, since my own verification is slightly different than the control (KMMK), this may skew results slightly. For that reason, I will be creating 6-day forecasts specifically for Meriden as a go-forward.
    The Euro and MAV MOS rank fairly close, but it is very interesting to note that the negative (cold) bias the Euro has is almost a mirror reflection of the MAV MOS positive (warm) bias:

    The MAV MOS appears to correct some of its bias towards Days 5 and 6. That can perhaps be partially explained by the fact that MOS is skewed towards climatological temperatures. The NAM also seems to have somewhat of a cool bias. I re-project highs from the NAM for Day 1, but that re-projection seems to over compensate the bias, at least in the case of April.
    Explaining the models/forecasts...
    Q: My forecast high temperatures for inland Connecticut. (mean of inland stations)
    MAV MOS: Forecast high temperatures for KMMK. (06z model run)
    ECMWF: Forecast grid-point high temperatures for KMMK. (00z model run)
    NAMDGEX: Approximate high temperatures for KMMK. These values are interpolated off of a graphical forecast, so the numbers are estimated. I use the NAM for Days 1-4 and the DGEX for Days 5 and 6. (06z model runs)
    850mb: An 850mb forecast technique that I have been working on for quite some time. Because this technique is based off of Danbury (KDXR), that station is used for verification.
    LAMP MOS: Forecast high temperatures for KMMK. (most recent run in morning)
    NAM Re-projection: This takes into account the actual 9 a.m. temperature vs. the 06z forecast for 9 a.m. for KMMK. That error is then re-projected into the high temperature forecast. Example: If the 9 a.m. temperature was 2°F warmer than forecast, then 2°F is added to the high temperature forecast.
    Consensus: A mean of each forecast above, including my previous forecast (continuity)
    How accurate was a Euro/MAV MOS blend? Well, not only do the opposing biases balance out close to zero, but the overall forecast error was less than any other forecast technique for Days 2-6:

    It's pretty interesting to see the results. It goes beyond comparing computer model verification. In order to become a better forecaster, I want to see what forecasts have worked out, which ones haven't and if I have any biases. This is only one month's worth of data, so more will need to be compiled over the long-run to see how models perform. I also expect that different models/techniques will perform differently depending on the season, weather pattern, etc.
  24. Quincy
    The threat for severe thunderstorms, including tornadoes, continues for Tuesday. The area of focus is the lower Mississippi Valley into portions of the Tennessee Valley and lower Ohio Valley. Nothing significant has changed from the computer models, suggesting that a severe weather event is still probable Tuesday into Tuesday night with at least a few tornadoes possible. There are still details to nail down, but at least a few key ingredients are in place for tornadoes across the region.
     
    Thinking since Friday morning’s update has not really changed. The computer models have gotten into better agreement with timing, as the Euro stopped its westward/slower trend and both the GFS and NAM are similar with the overall timing. There still are big questions about instability, as greater instability than forecast would support a higher-end event. As usual, the GFS and to some extent the Euro show marginal to modest instability, while the NAM (as often is the case) is a bit more robust. Wind shear, both directional and speed, is very favorable for tornadoes and the overall upper level pattern is supportive as well. Backing of low-level winds to the south/south-southeast should be coupled with southwesterly winds in the mid and upper levels. The one notable trend on the models has been for more of an initially discrete mode for convection Tuesday afternoon and early evening. If initiation is delayed, that could allow for further daytime heating, juicing up the setup even more. Also, if the storm mode is more discrete versus a squall line, the tornado threat would be elevated. It appears that the storm mode will eventually trend to a squall line regardless, especially as the shear pattern becomes more unidirectional overnight.
     
    Current thoughts: Discrete and semi-discrete thunderstorms fire in far eastern Arkansas and northeastern Louisiana Tuesday afternoon. The storms moved northeast and have the greatest threat of producing tornadoes from near the Mississippi River, northeastward to southwestern Kentucky, western Tennessee, northern Mississippi and northwestern Alabama. Given the expected wind fields, strong, long-tracking tornadoes remain a possibility. Odds for at least one strong tornado are set at 60% with this update.

    A tornado threat exists outside the red shading above, mainly in two areas. One being near the triple point in eastern Missouri to southern Illinois, but meager instability suggests this threat is highly conditional. Also, a few discrete cells could fire in the warm sector farther south in Mississippi (possibly western Alabama), but weak height falls suggest that the best forcing will be farther northwest.
     
    Once more high resolution data can be reviewed Sunday night, hopefully more details on the magnitude of this threat can be given with higher confidence. When reviewing analogs of similar setups in the past, most have produced at least some severe weather, but when instability was moderate to strong, such a setup often produced a significant tornado outbreak. At this point, this appears to be a low to moderate tier winter tornado event, not too uncommon for early February. One recent trend that may support a higher-impact event is that surface temperatures on Saturday were much warmer than forecast across much of the Mississippi Valley and Ohio Valley. If this trend were to continue in the coming days, it could allow for more instability than currently forecast on Tuesday, enhancing the tornado threat.
     
    To give a rough idea of what this event might look like, below is a reasonable analog from the January 19th, 1988 event. That setup also had lower end instability, especially over western Tennessee. It's not uncommon for winter Dixie events with high shear/low CAPE to produce tornadoes, some strong and long-tracking.
  25. Quincy
    For the fourth year in a row, overall severe thunderstorm activity, including tornadoes, finished below average across the Lower 48. This graphic shows the severe thunderstorm watch departure from the 13-year average. The vast majority of the U.S. saw at or below average numbers of severe thunderstorm watches. The greatest departures from average were across the central Plains into parts of the Midwest. The only state with more severe thunderstorm watches than average was Texas, with a higher instance of watches over parts of central to southern Texas. What was the cause for this, aside from the seemingly long-term lack of severe thunderstorm activity?
     
    The 2015 severe weather season actually got off to a relatively fast start from late March into early April, remaining active into much of May. Attention shifts toward June, which is usually the the peak of the season. June 2015 featured substantial ridging across the western U.S. While not the sole factor, that ridging was a big player in the relative lack of severe thunderstorm activity into June. The severe season climatologically focuses in to parts of the central and northern Plains in June, but the 500mb height pattern in June 2015 was not the most favorable for severe thunderstorms. Some troughing evident over the Great Lakes favored multiple severe weather events across the Midwest and Ohio Valley. However, even here, the season, overall (not in all cases), was on the quieter side.

    Above image shows 500mb heights and anomalies for June 2015.
    The overall tornado activity in 2015 was below average, but some parts of the country did see an increase in activity compared to recent years. Portions of Iowa and northern Illinois saw a very active tornado season. This included an early season outbreak on April 9th and a fall event on November 11th. Texas also saw an active season, particularly in North Texas. The High Plains saw a broad area of increased activity, particularly from the Texas panhandle into eastern Colorado and western Kansas. The other part of this graphic that stands out is the lack of tornado watches across the Deep South. Given recent trends and forecasts, it is conceivable that the severe season over the Gulf Coast states, including Florida, may remain on the active side over the next few weeks. Deeper into the year, it is a bit more unclear if March and April will see an uptick in activity across Dixie Alley. Only time will tell.

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