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I've decided to try my hand at forecasting tomorrow's severe weather in the Mid-Atlantic. The atmosphere is setting up for a pretty interesting afternoon for areas stretching from central South Carolina northward up through central Pennsylvania. All modes of severe weather will be possible -- tornadoes, large hail, and especially damaging winds.

The severe weather will occur in two rounds. The first round will happen early in the afternoon as supercell thunderstorms develop over North Carolina and Virginia in the warmth and instability well out ahead of the cold front. Given a favorable shear profile in parts of central North Carolina and southwestern Virginia (link to a NAM forecast sounding from SW VA tomorrow afternoon), supercell thunderstorms are possible. The supercells will carry the threat of all modes of severe weather -- tornadoes, large hail, and damaging winds.

The second, and most serious, round of severe weather will occur late in the afternoon through the evening hours. A cold front will sweep across the Appalachian Mountains and help provide focused lift -- in addition to the instability already present -- to fire off a line of thunderstorms. Jump the squiggle for my personal forecast, as well as mouse-made weather maps.

Disclaimer to cover my ass: The following maps and forecast show an approximate position of severe weather based on my interpretation of current model data. The actual location of the severe thunderstorms will vary based on ever-changing environmental conditions. Always rely on the National Weather Service, Storm Prediction Center, and Doppler radar for the most current warnings, watches, and storm positions. Do not use this forecast alone to make major decisions. Meteorology is like medicine -- always get a second opinion.

Update: The latest run of the NAM model just came in, and it's moving everything a lot slower than the run this morning. If this run is accurate, the storms may be delayed a few hours from what I've got below.

First off, a look at what the experts have to say. The Storm Prediction Center has issued a slight risk for severe weather for much of the east tomorrow, with a heightened 30% risk of severe weather stretching from central SC to central PA:

The heightened risk is due to the aforementioned double-whammy expected tomorrow afternoon. Clockwise-curved hodographs and instability between 1500-2000 j/kg will allow any thunderstorms that develop out ahead of the cold front in NC/VA the chance to become supercellular. Modest helicities and an EHI between 1 and 2 in some locations would allow the supercells to gain the extra oomph needed to become a real nuisance, with damaging winds, large hail, and possibly a few tornadoes if they're able to get their act together and strengthen enough. These supercells will likely move east or northeast towards the I-95 corridor throughout the day, provided they're able to hold together.

The real threat will come from the squall line expected to form just ahead of the cold front as it moves over the mountains. The focused lift from the front will hit the already-unstable air east of the Appalachians and fire off storms along the boundary. There will be plenty of vertical wind shear in the area to allow the storms that do form to organize into a squall line as they move east. The line will quickly become severe with damaging winds being the main threat. The line will move east towards the coast, where it should begin to weaken a bit as daytime heating and instability begin to erode.

I based the following forecast maps off of the 12z NAM and GFS models, which seem to be in pretty good agreement of the frontal position and precipitation. I moved the squall line a bit faster than the NAM had it, since mesoscale convective systems tend to start moving at a pretty good clip once they mature, and the NAM had the line fall back behind the front towards the end of the period on Friday (which, as far as I know, doesn't happen in reality).

Given the population density of the area under the threat tomorrow, I do plan on starting a liveblog once the storms form and begin to threaten Virginia/Maryland. I'll announce the liveblog -- as well as other weather updates -- on my Facebook page.

National Weather Service Main Page
National Weather Service -- Pittsburgh PA
National Weather Service -- State College PA
National Weather Service -- Baltimore/Washington DC
National Weather Service -- Blacksburg, VA
National Weather Service -- Wakefield, VA
National Weather Service -- Raleigh, NC

National Weather Service -- Charleston WV
National Weather Service -- State College, PA
National Weather Service -- Philadelphia, PA

Storm Prediction Center Main Page
Storm Prediction Center -- Current Severe Weather Watches
Storm Prediction Center -- Convective (Severe Weather) Outlooks
Storm Prediction Center -- Mesoscale Discussions
Storm Prediction Center -- Storm Reports
Storm Prediction Center -- Mesoscale Analysis Pages

National Hurricane Center

Wunderground's Detailed Radar (click the + nearest to you to see your local radar)

NOAA Weather Models

iMapWeather Radio App for iPhone/iPod Touch (costs $9.99 but well worth it)

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