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Quincy

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  1. 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.
  2. 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!
  3. 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.

  4. 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.
  5. 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.

  6. Quincy
    A severe thunderstorm threat is expected to develop late this afternoon into the evening hours across portions of the northern Plains. The setup includes an area of surface low pressure ejecting from Wyoming toward the Nebraska/South Dakota border late today. At the same time or shortly thereafter, an embedded upper level shortwave swing through the Dakotas. The combination of kinematic support and strong instability will likely result in several severe thunderstorms with all severe weather hazards possible. The severe weather threat may continue well into the night as storms shift east-southeastward.
    The computer models are in good agreement with the synoptic setup and even most of the mesoscale details seem fairly clear. The HRRR/NAM/RAP show an area of low pressure moving into east-central Wyoming by late afternoon with isolated thunderstorm development across much of eastern Montana. The initial development is closer to the upper level forcing at the time and in association with a frontal boundary interacting with moderate instability and modest wind shear. Further east, although stronger surface heating/instability are progged, a fairly strong cap should limit convection for most of the afternoon.
    Between about 22z and 00z, surface heating looks sufficient to begin breaking the cap. At this time, robust thunderstorm development appears likely across southeastern Montana and far northeastern Wyoming. In the immediate downstream environment, a secondary warm front/differential heating boundary is showing up with the high resolution guidance. As it looks now, that should be draped roughly near the I-90 corridor from far northeastern Wyoming into the western half of South Dakota. It is here that strong instability and increasing wind fields should be very favorable for severe thunderstorms. Any discrete thunderstorms between 23z and 02z near this secondary boundary are expected to have the highest probability to produce tornadoes. The higher end instability scenarios appear overdone, but even the less dramatic GFS shows in excess of 2500 J/kg MLCAPE overlapped with 60 knots of bulk shear through much of western South Dakota.
    Given such instability, the initial threats appear to be large to very large hail, followed by isolated tornadoes. A more substantial damaging wind threat likely develops during the evening hours further east. As the low level jet really ramps up and upper level winds increase, the kinematic environment should support a significant MCS, possibly a derecho. The exact placement of this convective system develops on how and where thunderstorms grow upscale. It appears that beginning in northwestern and moving into central South Dakota is where the model consensus stood as of this morning. This is also supported by an intense inflow environment with moderate elevated instability and a vigorous low level jet (in excess of 50-60 knots) ejecting from northern Nebraska into portions of South Dakota. While the convection-allowing models differ slightly with solutions, they all show an elevated threat of fairly widespread damaging winds into early Saturday morning across much of South Dakota.
    This is a fairly interesting case, as this portion of the (western) northern High Plains does not have many significant tornado cases dating back to the early 1960s. One of the red flags for tornado development includes storm coverage as a result of capping. Nonetheless, regardless of geographic region, it is possible for a strong tornado to develop given the expected environment. If a strong tornado were to form, this would most likely be in western South Dakota or perhaps far southeastern Montana/far northeastern Wyoming. Compared with climatology, the projected severe weather parameters tonight fall into the high end of prior tornado cases for the region. Although wind fields intensify into the evening hours, a gradual loss of heating and a trend toward storm mergers may also mitigate the threat. The threat of most widespread impact will likely end up being damaging winds, even if a derecho does not form. For what it’s worth, the 09z SREF derecho probabilities are in excess of 50% for a large portion of South Dakota by 06z Saturday. A few severe thunderstorms are also possible across southern North Dakota and a rogue supercell cannot be ruled out in the Nebraska panhandle either.


  7. 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.
  8. Quincy
    I expect light snow to reach the NY/CT and NY/MA border around 9 a.m. or so and then overspread the area from west to east. The heaviest precipitation amounts impact northern Connecticut into central and western Massachusetts.
    Quick warming at 850mb should push most areas over to sleet and freezing rain. The cold layer aloft up to about 925mb looks deep enough where the inland hills may get a thump of sleet, while the interior coastal plain could see several hours of freezing rain.
    By late afternoon precipitation tapers off, but freezing drizzle could linger for many inland areas through much of the night.
    Basically expecting 1-3" of snow and sleet, followed by a glaze of ice for inland areas. Best shot at 3" is across the higher terrain of the Berkshires and Worcester County. Keep in mind that this total accounts for snow AND ice. The only area that is a bit of a question mark is the I-95 corridor (approximately). Will have to keep a close eye on surface temperatures and see how cold air damming is verifying. Right now I think that the immediate coast gets a brief period of snow/sleet/ice and then changes to plain rain. Also, somewhat lighter precipitation amounts here may also result in less of an ability for near-surface temperatures to cool. In a marginal setup, heavier precipitation can help result in more freezing/frozen precipitation.
  9. Quincy
    A major snow storm is projected to bring blizzard conditions to much of New England Monday night through Tuesday. When all is said and done, many areas in and around the Boston area can expect two feet of snow with locally higher amounts.
    Despite the European model staying the course with greater snowfall amounts for areas around New York and New Jersey, it appears that most other data favors sharply decreasing snowfall amounts west of southern New England. As low pressure rapidly intensifies and moves toward Cape Cod and the islands, snow will become heavy with strong winds and blizzard conditions along the I-95 corridor from New York City to Boston and downeast Maine.
    As low pressure nearly stalls and does a loop, similar to historic blizzards in the past, a band of intense snowfall should drop excessive to possibly historic snowfall amounts. It appears that the likely positioning of this band would be on a SSW to NNE axis between central/eastern Connecticut and interior eastern Massachusetts. This may include the Boston metropolitan area. Although mesoscale details of such a band are not 100% clear, the potential exists for some 30"+ totals where this band sets up. Snow in this area will persist through the second half of Tuesday, before finally winding down early Wednesday morning.
    Here is the thinking for this storm.

    The greatest bust potential lies across far western New England, New York State, New Jersey and points west. Although this forecast leans closer to the more eastern GFS/NAM/RGEM solutions, it will still be interesting to see how the 00z Euro trends. It is very rare for the Euro to cave to the other models inside of 72 hours, but it is also hard to believe that it will stay as far west with the heavy snow as previous runs have.
    Another consideration is that liquid to snow ratios, especially across the interior high terrain, will be greater than 10:1. Ratios in the range of 14-18:1 across western and central Massachusetts should help offset some lower precipitation amounts with a considerable fluff factor.
    8:30 a.m. Monday edit:
    The models have compromised a bit overnight, but the RGEM is still rather light with snowfall amounts across western New England and points west.
  10. 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.
  11. Quincy
    I just want to quickly share my latest thoughts for Connecticut and surrounding areas.
    There's little doubt that a strong storm with plenty of precipitation will impact the tri-state area and southern New England Friday into early Saturday, but there are still a few details to discuss.
    Concern #1:
    Mixing along the shoreline. An initially "warm" boundary layer may cause mixed precipitation to fall near the Connecticut shoreline and coastal Rhode Island for several hours. With that said, as low pressure really intensifies southeast of Cape Cod, winds shift to the north and cold air pours on down. This will suddenly change everyone back to snow, even across eastern Long Island.
    Concern? Well, right now I have a wide range of 6 to 12 inches in the forecast. I'm leaning on the higher end of those numbers for coastal sections of Conn. and R.I., but there's still some room for change. These numbers could go higher, although I really doubt they'd trend lower...
    Concern #2:
    Deformation band of intense snowfall. These bands can be difficult to predict with accuracy and a slight shift could be the difference between 8 inches and 18 inches. With that said, I expect this band to impact the eastern half of Conn., especially areas like Tolland, Union and Woodstock that stay all snow and hang onto precipitation the longest. In that band, I am predicting more than 20 inches of snowfall.
    Bottom line...heavy snow, strong winds and coastal flooding are likely. I can see widespread blizzard conditions verifying for IJD, ORH, BOS and surrounding areas. (many areas not included in this map, such as northeastern Mass. and southern N.H.
  12. Quincy
    This isn't really a big event, but due to the amount of discussion it's had, I felt a map was warranted.
    Low pressure moving through the Great Lakes brings an usual push of generally light snow towards the Appalachians. A new area of low pressure is forecast to form east of the mid-Atlantic region, enhancing snowfall amounts ever so slightly around the coastal plain. This map does not show it, but light snow should extend southward into Virginia.
    I believe that most areas will see an inch or less of snowfall, with interior Mass. seeing little or no snow at all. I'm not even all that confident for 3" amounts, but I added them as a potential outcome for northern Pennsylvania, the outer Cape (Cod) and the Islands of southeastern Massachusetts.
    The column is fairly cold, from the surface right up through 850mb and 700mb, promoting snow growth and higher than "typical" liquid to snowfall ratios. I could see 15:1 ratios on the shoreline and 20:1 further inland. The end result is a light, fluffy snow that pretty much sticks on contact to most surfaces.
    Basically, most areas may see up to an inch, with the highest probability of 1"+ amounts occurring along and south of a line from Binghamton to Bridgeport to Groton to Providence to Taunton.
  13. Quincy
    Just a very brief discussion:
    Low pressure slowly moves east of the mid-Atlantic coast Wednesday night into Thursday. A relatively broad storm with possible multiple low centers spin around and pinwheel periods of snow (some rain mixing in at the coastal plain) through the region from Wednesday into Friday.
    This looks to be an elevation-dependent event, but heavy amounts or precipitation across eastern Mass., eastern Conn. and much of R.I. will help offset some of the "snow losses." In those areas, 1 to perhaps 2" liquid equivalent precipitation could still result in a solid 5-10, perhaps 12" heavy, wet snowfall. The highest risk for 12" would probably be in southern Worcester County.
    Power outages are a concern, especially in southern and eastern areas. There, the water content will be higher and stronger winds are expected.
    Forecast confidence is moderate...the shoreline and southeastern Mass. could see an even sharper gradient. The banding nature of snow and timing with respect to day vs. night will be crucial in determining snowfall amounts. The snowfall totals above are cumulative, so a total of 10" may actually result in a snow-depth of 7" (for example) by midday Friday.
    The computer models are in decent agreement, although the GFS is a lot more conservative than all other guidance. I feel the GFS has had issues with this storm, despite originally bringing it back when the other models were keeping it south. Recall that the GFS backed off on the blizzard the day it started and totals were close to double what it had predicted.
  14. 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

  15. Quincy
    A series of supercell thunderstorms moved through central and eastern Massachusetts this afternoon. Some of the storms prompted tornado warnings and there were multiple significant severe weather reports as a result. Some of the thunderstorms originated as far west as eastern New York and later reached peak intensity as they moved into portions of central and southern New England. Even the coastal community of Boston was hit with some regionally impressive severe weather.
    The prelude to these afternoon storms actually came in the form of early-day severe storms this morning. A line of severe wind-producing thunderstorms affected portions of southeastern Connecticut, Rhode Island and Cape Cod. An area that does not often see severe weather, especially during the morning hours. Once those storms moved out, there was some clearing, which led to plenty of daytime heating to fuel another round of thunderstorms.
    By early afternoon, there was an area of moderate to strong instability developing across the eastern half of Massachusetts, coinciding with seasonably strong wind shear. The mesoanalysis showed a corridor of moderate buoyancy coinciding with more than adequate wind shear for supercell thunderstorms. Bulk wind shear was in excess of 50 knots across the region. With clusters of thunderstorms ongoing across portions of the area, the storm mode was going to be critical in the evolution of the event. If storms merged, lines would form, suggesting more of a damaging wind threat. If the storm mode was messy, the inflow region and overall environment could be disrupted, resulting in mainly sub-severe storms. However, if any storm could remain discrete and from on the southeastern fringe of the activity, in order to take better advantage of stronger instability, then things would become a bit more interesting.

    Two thunderstorms began to split in northern Worcester County shortly before 2 p.m. The northern most storm tracked east-northeast into far southern New Hampshire. This storm merged with surrounding storms and did not strengthen much. The southern storm took a bit of a right turn and continued in a generally eastward direction. This cell moved through northern Worcester County, organizing quickly into a robust supercell. Velocity and hail signatures increased, indicating the likelihood of 1-2″ diameter hail. This storm further organized with rotation becoming more focused. A hook was noted on radar and a Tornado Warning was issued. The storm continued into northern Middlesex County, dropping more large to significant hail, but the velocity signature was marginal at best for tornadic development. The main story was hail.
    Speaking of hail, a second supercell developed on in west-central Worcester County a short time later. This storm was semi-discrete, but remained that way for quite some time. It also dropped large hail and continued for two more hours, moving right through downtown Boston. Large hail was reported throughout portions of the city and surrounding areas. Overall, there were numerous reports of golf ball or larger hail across Massachusetts, with at least three confirmed reports of hail at least two inches in diameter. A few additional discrete storms developed later in the afternoon, but earlier storms had overturned the atmosphere a bit. These storms were generally strong to only marginally severe in nature.
    No tornadoes have been confirmed as of 10:00 p.m. Although there was some localized backing of low-level wind fields, the lack of favorable low level helicity was a limiting factor in the tornado potential. Nonetheless, a fairly uncommon significant hail event affected the eastern half Massachusetts, an area that based on a 1980-2006 average, only reports significant hail once every 6.75 years. This is based on four reports across all of eastern Massachusetts, during the time. While there were four significant hail reports in that 27 year span, there were three such reports this afternoon alone. Keep in mind that the time of record is relatively short, as reports from 2007 to this year were also not included.
    A look at the 2 p.m. mesoanalysis showing 1000-1500+ J/kg MLCAPE and locally backed near-surface winds in the vicinity of the most robust supercells:

  16. 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.
  17. 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.
  18. Quincy
    A tornado threat is still being closely monitored for Tuesday, as computer model forecasts and trends have been fairly consistent with the potential for severe weather on February 2nd for quite some time. There really are no major changes from the previous forecast, but there are still a few question marks that need to be resolved. Some new high resolution model guidance that came out Sunday night suggested that the threat may be leaning towards a higher impact event than a lower impact one.
     
    Overview of setup and tornado potential:
    Low pressure tracks into the Midwest on Tuesday
    Warm, moist air to the south and southeast
    Strong wind fields aloft with this system Thunderstorms expected 2nd half of Tuesday
    A few tornadoes are probable
    There is a strong, long-track tornado threat
    Area of focus is still the Lower MS Valley to TN/OH Valley

    The purple area shaded below is where tornadoes are most likely on Tuesday. A conditional “wildcard” risk exists from parts of central to southern Mississippi into western Alabama, but confidence is not very high yet in discrete supercells being able to develop in that area.

     
    PROS – Favoring a tornado event: Speed shear is very favorable for tornadoes, with more than 50 knots of 0-3km shear and 30+ knots of 0-1km shear likely. The directional shear is decent, with a southwest flow in the mid and upper levels with winds locally south to south-southeast in the warm sector. A delayed start to convection will likely result in a discrete thunderstorm mode early on. If the warm sector has less convective contamination early in the day, that could also allow instability to bump up a bit from recent forecasts. Moisture return is very solid for February, as 60-65+ degree dew-points are likely across much of Mississippi, Alabama and western Tennessee. A few analogs have consistently showed the potential for a tornado outbreak given the forecast setup. The 21z SREF and 00z NAM/RGEM also looked a bit more impressive than previous model runs.
     
    CONS – Limiting factors and areas of question: Capping could ultimately delay the start of convective initiation, resulting in sparse thunderstorm activity Tuesday afternoon. It is unclear how many discrete cells will form in the favorable environment and how large of an area that will cover. The upper level and surface pattern is not ideal, as the trough is more negatively tilted than what is optimal for an outbreak and the surface low will already have become occluded by early Tuesday. Near-neutral height falls across central Mississippi to central Alabama would limit the ability of storms to initiate early in the event. Although mid-level lapse rates are favorable near the Mississippi River, they become less impressive with eastward extent, based on most model projections. Any mesoscale boundaries that setup on Tuesday could also be factors. Instability is another issue. Although significant levels of instability are not necessary for a dynamic winter event in Dixie, forecast CAPE for Tuesday still appears to be marginal to modest.
     
    Sunday night model data: The 00z 4km NAM was very concerning, showing an extensive line of supercells developing by early Tuesday evening from Mississippi to Kentucky. Although the NAM did not really show any convective initiation to the south across central Mississippi to western Alabama, this the environment in that area will be supportive of tornadoes. It’s just that a lack of forcing and little to no height falls make this a conditional threat. With that said, the 00z RGEM does imply discrete to semi-discrete storm development on both sides of the Mississippi/Alabama border area. The NAM hinted at this, especially with some locally backed winds and elevated severe weather parameters. The GFS did not show any major trends one way or the other, but with its coarse resolution, it is not being used as a primary model of consideration. The SREF has also been gradually trending more impressive with the environment over the region for Tuesday.
     
    The analog data has not changed much either, with January 19th, 1988 still being the top analog of choice. “It only takes one,” meaning that even if this event is largely a dud, the environment could easily support a significant, long-tracking tornado. The odds of at least one strong tornado are increased to 70% with this update.
     
    Periscope video briefing on this potential eventhttps://www.periscope.tv/w/aX7aEDFsWktwcm1ud29Fbm18MWRqR1hhZFJrcE9KWt2OHgSuGqDHLDJbVuDezoEH2_k4l2FthEZuqXLrLa7J
  19. Quincy
    The United States has a greater frequency of tornadoes than most other counties. For many factors, the vast majority of tornadoes occur east of the Rocky Mountains across the continental U.S. Although the central and southern Plains region is widely considered to be "Tornado Alley," there are other areas that see just as many tornadoes, if not even more.
    The two graphics below are adaptations of NOAA/NWS SPC graphics found in their Tornado Environmental Browser. A broad area from the east slopes of the Rockies east to the west slopes of the Appalachians typically see the most days with tornadoes per year. Two local maximums can be identified in northeastern Colorado and central Florida. While these areas may see numerous tornadoes in any given year, they are generally low on the EF-scale. The central Appalachians feature a local minimum of tornadoes, but as one travels east, the frequency of tornadoes increases.

    While terrain by itself will generally not have much of an effect on a tornado, especially a significant (EF/F-2 or stronger) one, there are reasons why terrain affects tornado frequency. In the Plains, lee cyclogenesis combined with a surge of warm, moist air from the Gulf of Mexico and drier air from the Rockies tends to create a favorable setup for severe thunderstorms and tornado development. Across the East Coast, there is less real estate to work with when considering the proximity to the Atlantic Ocean. Also, wind out of the south to southeast in the lower levels will often mitigate the risk of severe thunderstorms, especially in the spring and early summer months from the mid-Atlantic region into the Northeast. The placement of a "Bermuda" high and/or a "southeast ridge" of high pressure will tend to limit the formation of strong non-tropical low pressure systems along the East Coast during the warmer months, as the jet stream is often displaced further northwest.
    The frequency of days with significant tornadoes is in some ways similar to, but also has differences in comparison with the frequency of days with all tornadoes. While portions of Colorado and Florida may see the most tornadoes overall, there are two distinct areas that feature a much greater frequency of significant tornadoes. The southern Plains into Dixie Alley (lower to mid-Mississippi Valley) will on average experience the most significant tornado outbreaks. Here, there are three factors that are probably most responsible for this. First, their proximity to warm, moist air from the Gulf of Mexico fill favor higher dew-points and greater instability. Second, the tornado season is a bit elongated here. While April into May have featured the most significant tornadoes in Dixie Alley, the late fall into winter months have also included several major tornado outbreaks. Finally, there may be at least some influence from tropical systems. While tropical storms and hurricanes often do produce tornadoes, it should be noted that most of these tornadoes are generally not significant.

    Eastern Nebraska into the Midwest also averages an elevated number of days per year with significant tornadoes. Here, while the tornado season typically peaks in June, at least some fall tornado events have spawned significant tornadoes this far north. In 2013, October 3rd-4th recorded six significant tornadoes from eastern Nebraska into Iowa, with the major tornado outbreak of November 17th featuring 32 significant tornadoes in the Midwest.
    Other noteworthy observations:
    Missouri is an interesting case. The western portion of the state is often considered to be in tornado alley and some maps will place southeastern Missouri in Dixie Alley. The state has seen plenty of significant and even violent tornadoes, with the Joplin tornado of 2011 being one of the more recent examples. However, a small section of central Missouri features a local minimum in terms of both tornadoes and significant tornadoes. One possible explanation can be their location being in a "dryslot" of sorts for tornadoes. For storm systems developing just east of the Rockies, the eastward extent of associated severe weather will often fall short of central Missouri. Likewise, systems developing in the lower Mississippi Valley often form a bit too far east to target central Missouri with the most tornadoes.
    West Virginia has less tornadoes than most states east of the Rockies. One factor that immediately comes to mind would be the population density and terrain, which may limit some tornado reports. However, their location along the Appalachians certainly plays a role in the lower frequency of tornadoes. The higher elevations will typically have less instability. Likewise, moisture pooling will typically favor higher dew-points west and east of the Appalachians, leaving West Virginia in another tornado dryslot. According to the NWS, West Virginia has no recorded EF/F-4 or 5 tornadoes. With that said, a long-track F-5 tornado in southeastern Ohio narrowly missed passing into West Virginia before lifting. Also, an F-4 tornado that dropped southeast of Pennslvania into Maryland also lifted shortly before it would have passed into West Virginia.
  20. 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.
  21. Quincy
    I expect mostly snow for just about all of southern New England (pictured on this map). Although there may be a brief mixture along the I-95 corridor in Conn./RI, I expect crashing temperatures and heights to bring a wind-driven snow right down to the coast and into eastern Long Island.
    As far as snowfall totals go, I think that higher ratios will help push NW Conn. and W Mass. to 20"+. Further east, heavy precipitation will offset somewhat lower ratios for 20"+ from IJD to BOS. There is still some uncertainty with areas further south on this map, such as NYC, BDR, GON...lower ratios and possible mixing don't give me enough confidence to go with higher snowfall amounts. With that said, I could see a scenario where even the Conn. coast gets 20". The Euro is basically a 15-25"+ event region-wide and this model has been outstanding with overall consistency in this forecast.
    Bands of heavy snow are expected to form and some orographic enhancement may bring this heavy snow right back into the higher terrain of western Conn. and the Berkshires. A snow-shadow effect may limit totals somewhat in the Conn. River Valley, but even there I think totals will come close to 20". Potential exists for 2+ feet, especially across central and eastern Massachusetts.
    The other story is the strong winds and power outages are a real concern along and SE of I-95. I think drier snow and somewhat lighter winds inland will limit power issues there.
    What really captures my interest here is that expansive moisture field aloft. Past storms, like Feb. 2006, for example, had a relatively narrow area of moisture. This was able to produce intense banding and 20-30" totals from NYC into W Conn. However, with this storm, the moisture field backs into most of NY and covers just about all of New England. The end result is several bands of heavy snow, perhaps even a "mega" band. Snowfall rates of 2-3" per hour will be likely for several hours and convection/isolated thunder is possible.
    Winds may gust to 50 MPH along and NW of I-95, with higher gusts to the east. I think we'll see many stations verify official blizzard conditions and Cape Cod/Islands could see hurricane force wind gusts.
  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
    Over a 50-year span from 1962 to 2011, the most violent (F/EF-4 and F/EF-5) tornadoes occurred over Oklahoma and Mississippi. The maximum grid-points reported 16 over that period, with two of those grid-points in central Oklahoma and one in central Mississippi.
    While a broad area from the Plains to the mid-South sees the most tornadoes overall across the United States, there are three sub-areas with the most violent tornadoes. Much of Oklahoma falls into that category, as it is widely considered to be in the heart of tornado alley. Further north into Iowa is another area that has seen the most violent tornadoes during the period. The third area falls across Mississippi, where the tornado season is relatively elongated from the heart of winter into mid-Spring. The tornado season in Oklahoma generally occurs in a narrower window in mid-Spring. The season in Iowa tends to fall from mid to late spring with a secondary peak in the fall. A smaller and less significant maximum for violent tornadoes can also be identified across the upper Ohio Valley. While the tornado season tends to peak there in mid-Spring, a few events have also occurred in the fall.
    An interesting tornado minimum occurs in a small portion of central Missouri. Among multiple factors is the unique geographical area Missouri falls into. The classic tornado setup in the Plains is driven largely by lee-side cyclogenesis and the dryline. As storm systems move across the Plains, the bulk of the violent tornadoes tend to occur west and northwest of Missouri. Also, the dryline tends to have trouble advancing enough east to penetrate far into Missouri. Likewise, typical tornado events in the mid-South tend to thrive off of moisture from the Gulf of Mexico, producing tornadoes east of Missouri. With that said, higher instances of violent tornadoes have been noted across northwestern Missouri (tornado alley), far southeastern portions of the state (Dixie alley) and the devestating EF-5 tornado in Joplin, in far southwestern Missouri in 2011.
    East of the Appalachians, violent tornadoes are fairly uncommon. However, two particular regions have reported three or more violent tornadoes from 1962 to 2011. Portions of the Carolinas fall into that category. Further north, grid-points in the mid-Hudson Valley into southwestern New England have reported four violent tornadoes in that same span.

  24. 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.
  25. Quincy
    Multiple variables are important to consider when it comes to supercell thunderstorm development. The Supercell Composite Parameter (SCP) was created to factor three important ingredients to determine how favorable an environment is to produce supercells. SCP focuses on instability, wind shear and helicity (potential for updraft rotation). SCP is a parameter that starts at zero and becomes higher as there is better overlap of parameters to favor the formation of supercells. For example, an SCP of zero suggests that supercells are unlikely to form, while a value of 10 suggests a much better likelihood of supercell formation. There will be more discussion later on as to what SCP values support supercells.
     
    Calculating SCP
    MUCAPE term * effective bulk shear term * effective SRH term
    instability term * wind shear term * helicity term
     
    The equation is more specifically
    (MUCAPE/1000 J•kg-1) * (EBWD/20 m•s-1) * (ESRH/50 m2s-2)
     
    This tells us that each variable has a “threshold” in the equation. Basically, instability of 1000 J•kg-1 will equal out to 1. For bulk shear, 20 m•s-1 (~39 knots) will bring that term to 1. With helicity, 50 m2s-2 is the threshold. (EBWD stands for effective bulk wind difference/shear) Effective shear is used instead of 0-6km shear, for example, since it takes into account storm depth. Effective SRH is used as opposed to standard SRH, as ESRH is tuned to discriminate against environments with considerable capping, that may prevent air from being able to rise and form a thunderstorm.
     
    An SCP value of 1 is relatively low and most surface-based supercells form when SCP is between 2 and 11. Values much higher are relatively uncommon, but when they do happen, there is a strong probability of supercell formation, assuming other ingredients come into play, such as forcing, breaching a cap and storm mode. The average value for SCP with supercells ends up being close to 6.
     
    There are some other considerations to make. Elevated supercells can and often form with "low" SCP values. In fact, some can form with an SCP value below 1 and the average value for elevated supercells is only about 2. An elevated supercell will form in an environment where there is elevated instability, but there may be a low-level inversion that prevents the supercell from routing itself at the surface. Likewise, marginal supercell structures that may be messy or disorganized, can also form with low SCP values.
     
    Also, an SCP value can be extremely high, such as 20+, but if there is no forcing mechanism to trigger thunderstorm initiation, there could easily be no supercells in such an environment. This seems to be most common on hot, summer days where there is extreme instability in place, but only modest wind shear/helicity.
     
    Below is an example from 2015 where SCP values were over 30 across parts of Kansas, largely due to extreme instability. However, a lack of forcing delayed supercell initiation for several hours.

    The parameter is also helpful because it will be set to a lower value if, for example, there is considerable instability, but little to no wind shear. As wind shear drops below the ~39 knot threshold, the term falls closer to 0, setting the entire equation to a lower resultant number.
     
    In summary, SCP is a helpful parameter to evaluate the favorability for supercell thunderstorm development. It takes into account instability, wind shear and helicity, outputting a larger value where there is the best overlap of supportive ingredients. For surface-based supercells, SCP is generally higher than 1. SCP should always be used in combination with other considerations, as a favorable SCP value may not yield any supercells, if there is nothing to trigger updraft (thunderstorm) formation.
     
    Read more here: http://www.spc.noaa.gov/publications/thompson/stp_scp.pdf
    Forecast models that show SCP: http://weather.cod.edu/forecast/
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