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The environment is setting itself up for a dangerous one-two punch across the Mid-Atlantic this afternoon, with the greatest threat being dangerous tornadoes, strong straight-line winds, and some large hail.

I'm uploading a YouTube severe weather update right now that will explain it in more detail, but it's taking a while to upload, so I'll post it when it's ready.

The first round of severe weather will come in the form of supercell thunderstorms forming over the NC/VA area, and as they move into an area of large instability and rotation over Eastern VA and the Washington DC/Northern Virginia area, these storms will quickly start rotating and will bring a high risk of tornadoes to these densely populated areas. Some of the tornadoes could be strong, but even the tornadoes that aren't strong can be extremely dangerous if they hit a populated area.

The second round of severe weather will come from a cold front draped west of the Appalachian Mountains right now, along which a powerful line of thunderstorms will develop this afternoon. As this line of thunderstorms develops and crosses the mountains, they will become severe with damaging winds posing the main threat. They will have free reign of the land from the eastern slopes of the Appalachians to the Atlantic Ocean, so anyone in the red 30% damaging wind zone seen in the wind risk map below needs to take extreme caution.

Severe weather outlook:

Tornado outlook:

Wind outlook:

Hail outlook:

A new tornado watch was just issued for a huge chunk of real estate from Pennsylvania to North Carolina, effective until 900PM EDT:

Today has all the makings of a day that could be very dangerous, simply because of the population density of the areas affected. I will have solid liveblogging all afternoon once the storms start going and the tornado threat ramps up.

If you live in any of these areas, begin going over what you need to do to keep yourself safe. Figure out where you can take shelter if you're out on the road when a tornado warning is issued. Ask your place of work what their tornado precautions are. Start scoping out your house now for a safe place to take shelter if need be, keeping in mind the heavy furniture around and above you.

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 -- Greenville SC
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)

10:44 AM PT: Updating the diary with the weather geek info because I wanted to get the important stuff out there first. All the parameters are coming together for a dangerous situation across the DC Metro/Eastern VA today. The models are predicting 0-3 km helicities of almost 350 m2/s2 in some areas, and the hodographs in parts of VA and MD are about as clockwise-curved as it gets. Some meteorologists have been emulating Chicken Little over the last few days (and especially last night), and I've been trying to stay as reserved as possible. It's hard walking that thin line between sounding alarmist and keeping calm and stating the facts in a non-hysterical way. Even a small tornado during DC rush hour could spell disaster, which is why as many people need to know about this dangerous situation as possible. I grew up in Lake Ridge, VA where the population density is something like 4,000/square mile. This area is a sitting duck, and all the more so given the fact that 90% of these people don't pay attention to tornado warnings anyway. Let as many people know as you can that the environment is setting up for a dangerous day that can produce tornadoes across the DC area.

10:59 AM PT: I'll start the liveblog once the storms start threatening the I-95/66 areas. For intermediate updates, follow me on:

Facebook: http://www.facebook.com/...
Twitter: https://twitter.com/...

11:30 AM PT: Rotation is wrapping up on the storm in Montgomery County right now. Tornado Warning for Montgomery, Frederick. Howard and Carroll Counties MD. It'll pass just west of Damascus towards Green Valley and Mount Airy.

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