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Given the large amount of hype racing through the Facebook/Twitterverse about the possibility of another "d word" Tuesday, I figured I would weigh in and lay out a few scenarios about what could happen. Just so you don't have to read through the whole thing to get the main ideas, here they are:

  1. It is naturally hard for a derecho to form, so more likely than not, there won't be one.
  2. The atmosphere Tuesday afternoon will be supportive of severe weather across the Ohio Valley, Mid-Atlantic, and southeast, and one or more lines of storms aren't out of the question.
  3. Severe weather is likely tomorrow, but the scale and intensity is undetermined right now.
  4. This diary is based off of the 18z run of the models. In other words, these forecast models were run at 200PM EDT.
  5. The next run of the models won't come out (to the public, anyway) until after Midnight EDT, and Tuesday's first severe weather forecast from the SPC doesn't come out until 2AM EDT. We will get a clearer picture of what will happen as we get closer.
  6. I will write another diary tonight based on the overnight model runs/forecasts, and schedule it to post at 9:00AM EDT Tuesday morning.
  7. Update: The 00z (800PM EDT) NAM run still has a line of storms coming through the same general IN-OH-WV-VA-NC vicinity. Just waiting on the SPC's update in a few hours.

Regardless of what happens, be prepared for severe weather no matter where you live. Stay alert for rapidly changing weather by monitoring the Storm Prediction Center and your local National Weather Service office.

As is typical for the middle of July in the Untied States, a large ridge of high pressure is sitting over the middle of the country. This ridge (commonly called a "dome of heat" on the news) acts to deflect major weather systems around it. As I explained in deeper detail a few weeks ago, the edge of the ridge that serves as a boundary between heat and instability tends to act as the focus for severe storm development.

Daytime heating and moisture will rise across the eastern Ohio Valley during the late morning hours on Tuesday, and CAPE (Convective Available Potential Energy...essentially the atmospheric "fuel" for thunderstorms, the higher the better) approaching 3500-4000 j/kg in some areas:

18z NAM model run showing CAPE at 15z on Tuesday July 24, 2012.
July 23rd 18z NAM run showing max CAPE at 15z/noon on Tuesday, July 24.
A few hours later (18z, or 200PM EDT), a mid-level jet stream is expected to nose its way into the Ohio Valley, bringing 500 millibar (about 18,000 feet up) winds of 60-70 MPH. The amount of vertical wind shear (winds changing speed and direction with height) should be sufficient to organize thunderstorms into a line.
18z run of the NAM model showing 500 millibar heights and winds at 18z Tuesday July 24, 2012.
July 23rd 18z NAM run showing the forecast 500 millibar height/wind chart at 18z on Tuesday July 24th.
The NAM model's simulated radar output shows a batch of thunderstorms develop over northern Indiana, and quickly form into a line as it races towards the Appalachian Mountains. It's important to keep in mind that this is a model forecast, and that the storms that do form will differ from what is shown.
18z NAM simulated composite reflectivity showing a possible MCS approaching the Appalachian Mountains at 18z Tuesday July 24, 2012.
July 23rd 18z NAM run showing the simulated composite reflectivity showing a possible MCS approaching the Appalachians at 18Z/200PM EDT on Tuesday July 24th.
On the other side of the coin, the GFS model is showing a much less aggressive atmosphere in this area. At 18z/200PM Tuesday afternoon, the GFS shows minimal amounts of CAPE across the eastern parts of the Ohio Valley, along with a weaker 500 millibar wind.

The lower amount of instability is largely due to the model forecasting precipitation during the morning hours across the region, the clouds from which will act to inhibit daytime heating.

Given the large amount of discrepancy between the two models, anything definitive will have to wait until overnight or tomorrow to pinpoint.

The Storm Prediction Center, in a forecast issued at 130PM Monday afternoon, noted an elevated risk for severe weather across parts of Kentucky, West Virginia, Virginia, and North Carolina:

The 1230PM Monday forecast severe weather probabilities for Tuesday July 24th, from the Storm Prediction Center. The 30% red shaded area indicates a heightened risk for severe weather. This image will be switched out at 200AM EDT when the new forecast is issued.
NAM GUIDANCE SUGGESTS SURFACE TEMPERATURES WILL REACH AROUND 100 F IN PARTS OF VA/NC. THIS STRONG HEATING COMBINED WITH INCREASING 850-500 MB NWLYS IN THE AFTERNOON/EARLY EVENING MAY PROVE FAVORABLE FOR A FEW ORGANIZED CLUSTERS PRODUCING PRIMARILY DAMAGING WIND.
This forecast was issued before the 18z/200PM model runs came out, so they won't have anymore input until the 200AM forecast comes out.

I will update this diary with any further information that comes out this afternoon, and I will write a diary overnight and schedule it to post at 900AM EDT Tuesday morning. I might not be around to tend to it (I enjoy sleeping until the crack of noon), but I'll try to be there.

Follow me on Facebook and Twitter for more updates between diaries, and use the following links to keep track of official forecasts from teh gummint:

National Weather Service Main Page
National Weather Service -- Northern Indiana
National Weather Service -- Indianapolis, IN
National Weather Service -- Wilmington, OH
National Weather Service -- Charleston WV
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

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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