thunderman

Severe weather outbreak looking more and more possible across portions of the lower Mississippi Valley and deep south tomorrow and tomorrow night. This evening the culprit low is located across western Kansas with a warm front stretching along the Kansas border and into northern Missouri. The dryline is located from west of KICT down through western Oklahoma and Texas … roughly along a line from Altus, OK to Sweetwater, Texas and then southwest from there. The cold front is lagging behind in far western Kansas and eastern New Mexico. A fairly large moist warm sector has developed thanks to an extended period of southerly winds off the Gulf. Dew points in the 50s extend almost up to the Missouri/Iowa line ahead of dryline/cold front and will generate instability on the order of around 750j/kg despite clouds and lack of strong radiational heating of the boundary layer. As forcing increase aloft and steeper lapse rates move in, atmosphere will likely pop along/ahead of the dryline as it punches into eastern Texas. Models hinting at two potential dryline bulges north and south of Austin, Texas. This will have to monitored for possible initiation areas as the morning/afternoon progresses. Low level storm relative winds on the order of 20-40 knts and upper/mid level storm relative winds parallel to dry line/initiation axis will promote uniform gust front lifting and upscale growth into a squall line/QLCS structures with damaging winds being the primary threat. Convection will move East with the evening and overnight hours. As it does, shear will increase across Arkansas, Louisiana, Western Tennessee, and Mississippi and hodographs are expected to become open and elongated. Thus, the tornado threat will likely increase as we approach dark and persist into the overnight … especially if more QLCS type structures can be maintained though the time period. Couple this with low LCLs under 500 meters and there could be a strong tornado somewhere tomorrow night … again if semi-discrete QLCS structures persist. Folks across southern Arkansas, northern Louisiana, and western Mississippi definitely need to monitor this situation closely.

An area of low pressure with move out of the plains and up to our Northwest. As it passes by, it will drag a fairly healthy cold front across the area Wednesday afternoon/evening. It is only January but strong winds just off the surface leading to strong shear, a good quality warm sector, and linear forcing with the front may be enough to trigger a strong to severe line of forced convection along the cold front. The primary threat would be damaging winds and heavy rainfall with PWATS possibly exceeding 1.5".

I eluded to the severe weather threat briefly in my more board centered post on the freezing rain/sleet potential a bit ago, but I thought that it deserved its own post given that it may be first real large (on an area scale) event. Area of interest is to the right of a line from the Akrlatex region to southern Indiana/Illinois to Western TN to the Gulf. The same synoptic trough out west that helps to setup our boundary will also be responsible for driving a low from the central plains up towards the Great Lakes. As this low develops and moves East, flow in the warm sector/around the high to East will setup a funnel of moisture inland off the Gulf, with a large area of the Mississippi river valley and Deep South seeing sufficient moisture return to support deep moist convection. Accent along the cold front and ahead of the upper level trough, sufficient moisture return, and open/lengthy hodographs will work to produce an environment conducive to squall line/QLCS development. Given the track of the low and the factors above, a widespread damaging wind event is possible … along with some tornadoes, given the open and spread out nature of hodographs ahead of the front and low LCLs. KMEM sounding for 00z Wednesday just as a sample of environment ahead/along front:

A warm front associated with the next system out west will cross the area Sunday night/Monday. The associated precipitation is expected to be very light ... with well under 0.25" very likely. Cold air at the surface looks marginal, with upper 20s at the lowest and with southerly flow around high pressure to the East, that will be kicked out fairly quickly IMO. Any precipitation that does fall will be in the form of sleet and freezing rain initially, with precipitation after roughly 13z likely falling as just plain rain. Soundings are also hinting at some dryness around/below h85, so that may end up being a limiting factor early on (when the ice threat is). I wouldn't expect too much from this event ... though given the potential for freezing rain early in the event, many of us may get a freezing rain/winter wx advisory out of it. This would be for elevated surfaces primarily, 80% of the roads should be fine. Sorry guys, not feeling this one. Out west though across the MS River Valley/parts of the deep south could get interesting on the severe weather side of things ... maybe even a few 'naders! 12z GFS for early Monday morning: Sterling, VA GFS Sounding: KSHD (Staunton/Harrisonburg, VA) GFS Sounding:

Yeah the 50 dbz rule works well.  I use it often as well when trying to to determine severity.

Hail can be a weather enthusiast delight during the Summer time (perhaps it reminds them of snow?). Unfortunately, forecasting hail size can be a bit of challenge. One method that I use is VIL Of the Day (VOTD). It was developed by the NWS back in the 90s. It purely relies on temperatures aloft and while simple, does a surprisingly good job. Obviously there are some limitations, but I will get to those in a minute. The calculation is straight forward and is as follows: VOTD = 750 / [(h5T+h4T) / 2] where h5T and h4T are the absolute values of the 500mb and 400mb temperatures respectively. The VIL value given from this equation is the approximate value at which you can expect 0.75” diameter hail stones to fall. 0.75” was chosen because this used to be the hail criteria for a severe thunderstorm. For operational purposes, the lower the expected VOTD the higher the large hail potential you can expect. I like to use 40 g/m^2 as a rough baseline in the Summer time. For those that are not familiar with VIL (Vertically Integrated Liquid), it is a radar product that operational meteorologists use to locate areas of heavy precipitation and hail. It is available in real time in all GR-LevelX products and some places on the internet (Weather Underground, etc). See the wiki page for a brief overview. Now back to the limitations of the VOTD approximation. It is best used for regular thunderstorms and not those with established mesocyclones (i.e. supercells) … although honestly it still works fairly well on most supercells we see around here. In storms with well-organized and established mesocyclones, there are additional mesoscale lifting mechanisms at work that can work to produce huge hail more efficiently than what you would tend to find in your average non-supercellular severe thunderstorm. Keep in mind that the VOTD can change as the thermal profile aloft changes. Check forecast soundings (remember, there is more than 1 model) for the afternoon ahead instead of relying solely on the 12z observed soundings. Lastly, VOTD gives you the VIL value for 0.75” hail stones and tells you nothing about maximum hailstone size, etc. I have attached a calculator that I wrote to help me compute VOTD quickly. I wrote it a while back, it’s ugly, it’s written in FORTRAN, but it does the job! votd.zip

Sorry I have been MIA over the last couple of weeks ... I spent the holiday with fiancee in PA (btw, the 12/29 storm was fun!) and then caught some kind of crud/flu that I have been fighting since the first of the year. This upcoming "event" is actually the most I have looked at anything weather-wise since the new year. Anyways, there are some rumblings of a potential snow or mix event for the Monday night and Tuesday time frame. Precip is expected Monday Night and Tuesday with a secondary wave behind a cold front that will cross the region Sunday Night. The air mass ahead of the front is obviously not very supportive of wintry weather ... with highs progged to creep into the lower 70s in some spots. Even behind the front though, really cold air fails to make it into the Western VA/NOVA areas ... therefore I wouldn't get to excited about this "event". Low level thicknesses across most of western and northern VA would support frozen precipitation, however a warm layer above that (h8-h7) would support an IP/zr/rasn type of event. Marginal surface temps should help to mitigate any onset-zr threat. North of I-66 looks to be the best for enough cold air for snow, however decent precipitation may struggle to make it this far north. Overall, I would expect this event to be characteristic of Early November or early spring than mid January. GFS sounding near Leesburg, VA: GFS sounding around Harrisonburg, VA:

Low status/fog has been hanging out all day across some portions of MD, Extreme Northwest VA, and Eastern PA. The result has been a near 20 degree temperature gradient across west-central VA and southeast PA! Some current temps as of 4PM across the area: KSHD 67 (High unofficially 69) KOKV 55 (High unofficially 61) KIAD 60 (High unofficially 62)

I just want to take a few minutes outline the areas that I will be posting about and to introduce myself as I do not post much in general and rarely really post outside of the Mid Atlantic forum. I have been a member since the Eastern era of the board and have been tracking the weather as an enthusiast since the early 2000's. I moved to Southeastern PA to go school at Millersville in August 2008 and graduated from there with a B.S. in Meteorology in 2011. I enjoy forecasting in general and especially mesoscale forecasting. I try to get out and do some local chases, though lately I haven't gotten out as much as I would like too. I hope to keep this blog updated at least weekly with a focus on general forecasting. Obviously during slow times you may not here much out of me. On the other hand, during severe weather days I may use this blog to voice my opinions on the very short term as the event evolves and update several times a day. My "area of interest" (or whatever you want to call it) will generally be the Shenandoah Valley of Virginia and the more western suburbs of DC. I may post outside of this area occasionally if there is something overly interesting happening and things are dull here. As a computer/technology, don't be surprised to see the occasional technology oriented post either!

I had to retake ODE in the spring b/c of this storm......