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The Storm Prediction Center has taken a relatively rare step for this time of the year and issued a moderate risk for severe weather for a large section of the Ohio Valley and northeastern United States for Thursday. The SPC states that a potentially intense derecho and tornadoes are possible in parts the moderate risk zone. If a derecho forms tomorrow, it shouldn't be as bad as the one that hit on June 30th, but it'll be stronger than the one that happened yesterday.

Severe weather outlook for Thursday, July 26, 2012, issued on Wednesday.

Here is their severe weather risk probabilities map, showing the potential for severe weather within 25 miles of any point in the shaded area. They have a very large bullseye of a 45% risk for significant severe weather extending from Ohio to Connecticut, reflecting the possibility of a damaging derecho and tornadoes forming during the day on Thursday.

Severe weather probabilities outlook for Thursday, July 26, 2012.

Jump the fold for some more analysis and graphics...

The forecast tomorrow poses a challenge pinpointing exactly where severe weather will happen. The SPC (and most forecasters) issue their forecasts based on what the models show. Surprisingly, Mother Nature tends to do her own thing without consulting humans first. The major inhibiting factor in Thursday's forecast will be early morning clouds and precipitation. The longer and more widespread clouds and precipitation hinder daytime heating, the lower the severe risk will be. That will be the major wildcard tomorrow.

The large-scale setup is that a center of low pressure will move into southern Ontario during the day on Thursday, and two fronts -- a cold front extending from the Great Lakes into the Midwest, and a warm front extending across the northeast into the Atlantic Ocean -- will serve as foci for storms to devlop.

As the cold front pushes into the very hot, humid, and therefore unstable airmass, a line of storms is expected to develop along the front from Indiana to New York. This line of storms, given the instability and favorable deep-layer shear, will quickly turn severe. This severe line of storms is expected to turn into what's called a serial derecho. A serial derecho is a bit different from the derecho that happened on June 30th, but it's dangerous just the same.

Somewhat separate from the derecho threat is the threat for tornadoes across parts of New York and southern New England. The SPC puts a big asterisk on this threat, however, due to two factors: 1) how quickly the warm front draws in unstable air from the south, and 2) how much cloud cover/stability exists in the area to inhibit storm development. At this time, if discrete (individual) storms are able to develop, there will be enough rotation in the atmosphere available to them for tornadoes to develop.

For the weather geeks among us, here's a forecast hodograph from southern Vermont from the 12z NAM run:

That's how much helicity there is in the atmosphere. If the warm front is able to fire off storms across southern NY and southern New England, and they can tap into that helicity, it could get hairy for a few hours.

At this time, big megalopolis cities like Philadelphia, New York City, and Boston could be at risk for some rough weather tomorrow. Baltimore and Washington DC are on the southern edge of the risk. Things can and will change as the event gets closer, so if you live anywhere from Dayton OH to Washington DC to Portland ME and anywhere in between, you need to keep a very close watch on how this severe weather event develops.

The next update from the SPC won't come out until overnight (200AM EDT), so I won't have another diary on it until tomorrow. I'll post more updates on Facebook and Twitter as they come in.

If you have any questions, please ask. The purpose of this diary isn't to panic you, but to let you know of the potential for severe weather tomorrow so you can take preparations to protect yourself if bad storms threaten.

Severe weather safety tips from Ready.gov

National Weather Service Main Page
National Weather Service -- Indianapolis, IN
National Weather Service -- Wilmington, OH
National Weather Service -- Cleveland OH
National Weather Service -- Pittsburgh PA
National Weather Service -- State College PA
National Weather Service -- Baltimore/Washington DC
National Weather Service -- New York City
National Weather Service -- Albany NY
National Weather Service -- Boston MA
National Weather Service -- Burlington VT
National Weather Service -- Portland ME

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

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

NOAA Weather Models

TwisterData.com's excellent GFS/NAM/RAP model website.

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

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