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  1. blog-0942525001461109466.png

    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.

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    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.

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    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:

    1. 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.
    2. 4/25 – 4/29/1994: 124 tornadoes in five days, including an outbreak from the 25th into the 26th.
    3. 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. 4/17/1995: AR/OK/TX outbreak with 21 tornadoes.
    5. 4/26/1991: Plains outbreak with 53 tornadoes, including the Andover, KS EF-5 tornado.
    6. 5/2/1994: The only analog in this bunch that didn’t have notable tornado activity.
    7. 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)
    8. 5/8/1979: 21 tornadoes, though mostly in Florida, two were reported in Iowa, including an EF-3.
    9. 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.
    10. 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.
    11. 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…

  2. For far western areas along I-81 between I-64 and OKV

    An area of low pressure will move out of the northern Gulf of Mexico and up the east coast tomorrow and Wednesday. As it does, light precipitation will begin to overspread the area early tomorrow morning and will increase in intensity throughout the day … before beginning to taper during the overnight and possibly end as a period of snow Wednesday.

    Initially, there will be just enough cold air in place for some spotty flurries during the predawn hours Tuesday morning. Precipitation type will quickly shift towards freezing rain and sleet by 7:30am. As warmer air continues to move in with heavier precipitation through the day, freezing rain will transition to a plain cold rain. As typical, deeper and sheltered valleys will hold to freezing rain the longest … possibly into the late morning hours.

    Steady rain will decrease in intensity after midnight Tuesday night as colder air begins to filter in on the back side of the storm. Any leftover precipitation will begin to mix with sleet and snow again early Wednesday morning and eventually end as light snow/snow showers, as precipitation fully shuts off Wednesday afternoon.

    Wintry accumulations Tuesday will likely not amount to much at all … up to a coating of snow/sleet and up to 0.1” of ice accretion in the very coldest valleys (where freezing rain will hold on the longest). I do not expect roads to be that much of a problem tomorrow … icy spots should be limited to bridges and overpasses I think. This is not a great setup for a high impact freezing rain event … but that does not mean that slick spots cannot develop.

    Snow/sleet accumulations that occur Wednesday will likely total an inch or less for most. This part of the storm remains the most uncertain in terms of sensible weather. If colder air filters in more quickly than expected, then we will see a change over to snow earlier. There is also a question as to how much moisture will still be available and how efficient snowflake production will be aloft … both factors will have a big impact on the end result.

  3. The Quasi-Biennial Oscillation (QBO) is a cycle of zonal wind in the equatorial stratosphere with a period that varies between 24 and 30 months. This oscillation is a product of downward propagating alternating wind regimes. The current method of monitoring this oscillation is through an index, calculated by the zonal wind anomaly at 30hPa averaged along the equator. This method excludes information on the vertical structure of winds in the stratosphere, and presents the QBO as a one-dimensional temporal oscillation.

    Presented here is a new framework for monitoring the QBO. This framework incorporates both the oscillation in time and in space. The two leading principle components (PCs) of equatorial stratospheric zonal winds are calculated from NCEP/NCAR reanalysis monthly mean data. These PCs are then standardized and used as X and Y coordinates in a 2-dimensional phase space. A phase angle can then be calculated from the coordinates, forming a new index that is much more representative of the state of the stratosphere. The results show a very clear pattern of zonal wind and temperature regimes in the stratosphere.

    Furthermore, from each of these phases, there are physical connections to characteristics in the troposphere on a monthly to seasonal basis. These include the distribution of tropical cyclone activity, the El Niño / Southern Oscillation (ENSO), and the extratropical pattern. This new framework for monitoring the QBO is shown to be much more applicable to seasonal forecasting.

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    Compare the EOF-constructed oscillation to a 52-month reanalysis segment

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  4. As we move through August and into September strong/severe thunderstorm possibilities across New England begin to decrease very rapidly. This is due to the fact that as we move through this time of year we begin to transition from a summer weather pattern toward an Autumn weather pattern and it's much more difficult to get higher amounts of instability to develop across the region. While it becomes much more difficult, it certainly isn't impossible and for it to happen you need a really special setup. For the past several days now, medium/long range computer forecast models have been very consistent with this "special setup". When referring to this "special setup" we are talking about the elevated-mixed layer.

    When talking about the potential for severe weather and higher event significant severe weather events, having an elevated mixed-layer in place is extremely critical in order to achieve a major severe weather outbreak consisting of high end significant severe weather. High end significant severe weather refers to widespread wind damage, typically with winds in excess of 65-70 mph, widespread large hail reports exceeding 1.5''-2'', and tornadoes, typically with at least one strong/violent tornado (EF2 or greater).

    Elevated mixed-layers are areas of the atmosphere, typically between 800-600mb of extremely warm and dry air. This lead to very steep mid-level lapse rates on order of at least 7.5-8 C/KM and can even exceed 8.5-9.5 C/KM. This combination, especially when coupled with very high surface temperatures and lots of low-level moisture (high dewpoints) leads to extreme amounts of instability which is the main source of fuel for t'storms. Elevated mixed-layers are located across the Inter-mountain west region across the southwestern United States and can/usually gradually shift east into the Great Plains region. While it's very rare for elevated mixed-layers to advect all the way to New England, they can do so given special atmospheric setups. If you happen to remember June 1st, 2011, the day of the Springfield tornado, that was associated with an elevated mixed-layer.

    When looking for elevated mixed-layer advection into New England, what you want to see is a very strong 700mb ridge centered across the southeastern US with 700mb temperatures in excess of +10 to +12C over your region. You also want ridging at 500mb and to be on the crest or top of the ridge along with an amplifying trough to your northwest. This allows you to be closer to the cooler 500mb temperatures and also allows you to be closer to the much strong mid-level jet. Here are some images from today's 12z GFS run for 48 hours out, meaning the following images are for 8:00 AM eastern time Tuesday morning.

    500mband700mbheights12z8GFS48HR_zps869a92e3.jpgDuring the morning hours of Tuesday a warm front will be lifting northward through the region as well, and in fact, will be the leading edge of the elevated mixed-layer air which will move into the region. As the warm front lifts northward humidity will be on the sharp increase as dewpoints will climb to near 70F and the low-level airmass will warm as well and to the point to where surface temperatures could reach the mid-80's to even near 90F in spots if full heating potential is realized.

    On Tuesday, severe weather could be a definite possibility across northern New England first in the AM associated with the warm front and then again during the afternoon as a cold front slowly slides east and a piece of short-wave energy moves through aloft. Late Tuesday evening/overnight then could be interesting for central/southern New England as we could be looking at the potential for strong to severe t'storms during the overnight hours.

    As we move into Wednesday and perhaps even Thursday, as a cold front slides east severe weather may be a possibility both days as well. What we will have to see is whether we can hold the elevated-mixed layer or at least steeper lapse rates both days as if we can, both days could be extremely active with severe weather.

    At this point it's a bit too early to get into specifics for Wednesday or Thursday but as far as Tuesday is concerned it could be very active across central/northern NY and then into VT/western NH/northwestern MA during the AM/PM. We could be looking at the possibility for damaging winds, large hail, and perhaps the threat for isolated tornadoes depending on whether or not the low-level jet is strong enough and hodographs can become enlarged.

    As far as Tuesday night is concerned for the rest of MA and down into CT/RI, this will all depend on how much forcing is present. We definitely will have to watch the Tuesday night period for some severe weather across these areas with large hail/damaging winds being more of a threat.

    As we move through the next 24-48-60 hours, details should continued to be ironed out.

  5. Same general theme, with a slight shift south along the southern edges and a more pronounced southern shift towards the upper part of the map. Risks are generally to the higher side on the southern fringe areas if the models are to be believed (especially if the overnight front-end thump is fairly wet).

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    WxWeiks
    Latest Entry

    Greetings Everyone,

    My first post will just provide a little background information about myself and what my future plans for the blog are. My name is Erik Weiksner and I am a recent meteorology graduate (May, 2012) of Millersville University. Just like most people in this forum, I've been obsessed with the weather ever since I can remember. I grew up, and still currently live, in Lancaster, PA (where Millersville is located) while I search for a full-time job that utilizes my degree. I never knew what I exactly wanted to do with my degree once I graduated but after taking mesoscale and radar during my last semester, it became very obvious to me that operational forecasting is what excited me the most. This summer I began a blog using google's blogspot so that I may further my forecasting skills by having an outlet for my thoughts. I have not been a member of American Weather for too long but after learning about the new blog system I decided this would be a better platform for my discussions. I am hoping to receive constructive criticism and advice so please don't hold anything back.

    I mostly blog about PA weather with a focus on South-Central PA. My goal is to write informative pieces about upcoming weather events in a way so that anyone can read them without having an AMS glossary on-hand. I share my posts with friends and family that are weather-enthusiasts, so if you are looking for an extremely technical blog, this may not be the one to read. However, I do hope you will stop by from time to time and see whats happening next in the Keystone State.

    Thanks for reading.

    - Erik

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    blog-0791228001355077027.gifMuch of Georgia and the Carolinas stand a good shot at some much needed rain mid week. The first front, like many others this Fall, will basically fade out, as dynamics pull north, so the best lift and rain amounts again will be centered on the Tennessee Valley and Gulf States side.

    But immediately this front will stall out, and create some southwest flow aloft. In Winter, southwest flow can be a sneaky thing, causing a quick weather event (sometimes snow or ice) in what was just days before forecast to be only partly cloudy. The GFS showed this a week ago, then it was come and go, now last nights ECMWF also showed it.

    Here it is on Wednesday as the s/w coming out of the Lower Mississippi Valley takes on a neutral tilt. Some times, these flat waves can be great precip producers (but also, sometimes convection in the Fl. panhandle robs areas north). Notice the confluent flow in the Northeast, providing the flow to stabilize a High Pressure in Pennsylvania and hold it in place as cool rains fall in the Deep South, from eastern Alabama northeast ward through the Carolinas to the southeast Virginia Coast.

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    The GFS produces a nice swath of .50" to .75" amounts from southern Alabama, FL panhandle through central GA and into the central and eastern Carolinas during the day Wednesday and Wed. Night. Further west, there may be just enough cold air at the 850 level for some snow or wet snow/mix type of precip around the northern Mountains of NC, but mostly above 3000 feet. The airmass doesn't look quite cold enough for lower elevations than this, and right now the models don't bring much moisture that far west, but it's something to watch , since southern stream systems like this are notoriously sneaky, and with no big blocking high to the north, this southern system could amp up just a little more than shown, but it's only worth watching. I'm not expecting a big snow event for the mountains from this, but the rain would be the first significant rain this Fall for parts of GA, SC, NC.

    GFS qpf from second wave:

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    Damming will also be rather weak, but once again after several CAD events this fall, it fits the pattern of how things have gone a few times this season, but this one has much more moisture than the last few. A rather benign 1025mb high will sit in PA under the confluence there, and wedge down northeast winds, holding surface temps down into the 40s during the afternoon even into parts of GA and SC and NC. The cold damming is actually classic and strong on this event, but the cold air is just not very deep, and the high pressure not that strong. If the airmass were colder following the front, and if the high were probably stronger, we'd be seeing the first Wintry mess in northeast GA through the Piedmont of Carolinas, but temps just aren't low enough.

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    After this system, the high slides offshore and we resume southwest flow, and warming temps as the next system digs into the Southwest states, so temperatures will once again go above normals starting Friday and through next weekend. The good news is more rain will be coming, so we'll take the good with the bad, and I think the chances of rain are going up with so many systems slated to move in from the Pacific and some will be taking a suppressed type of track with Gulf Taps.

  6. Wow
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    By Wow,

    Some non-mets have asked for blogs recently and we're aware there are plenty of members here who are just as informative as a degreed meteorologist. Therefore, we'll allow access to blogs for non-mets by request and allowed on a case-by-case basis. We're stilling figuring exactly how to go about doing this but you can still reserve your place once we get a plan in place. Just PM one of the admins.

    Thanks.

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