A large area of the eastern United States is under a tornado watch until 1000PM as numerous storms form and move into an atmosphere ripe for severe weather. The risks this afternoon and evening are limited to tornadoes and damaging winds. Hail won't be much of a threat today due to warmer temperatures aloft.

Here are the watches as of 230PM, in effect until 1000PM. They include the Washington DC and Philadelphia metro areas.

The atmosphere over the Mid-Atlantic is ripe for at least one or two tornadoes this afternoon. Ample instability is firing off thunderstorms over the western Carolinas and Virginia right now, and as these storms move into the area of enhanced rotation over the tornado watch area -- especially the DC metro area -- some of these storms are expected to rotate and produce tornadoes. The tornadoes won't be exceptionally strong, but even a weak tornado in a densely populated area is dangerous.

CAPE values across the watch area range from 2000 j/kg to 4000 j/kg, sufficient for strong storms to develop.

Here's the significant tornado parameter (STP) as of diary publication. The yellow shading is indicative of an atmosphere conducive to tornadoes, and the orange shading (values greater than 1) indicate an enhanced risk for tornadoes. The SPC notes that almost all significant (EF-2 or greater) tornadoes are associated with an STP value greater than 1, but the risk for significant (EF-2 or greater) tornadoes across the watch area is low.

Here are the surface winds and 500 millibar winds overlaid on top of each other, with the STP map as a background to help you see the wind direction arrows. The more parallel the two arrows are with each other, the more unidirectional the atmospheric winds are. This isn't favorable for tornadoes to form.

The wind direction arrows that are nearly perpendicular to each other show an atmosphere conducive for tornadic development. This helps create horizontal rotation in the atmosphere, which can be tilted upright by stronger storms and cause the storm to rotate, potentially leading to a tornado.

If things start to look hairy near major cities/metro areas, I'll update this liveblog as needed. For everything else, follow my Facebook page for all your weathery needs.


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)

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Originally posted to El Blogo de Weatherdudeo on Mon Jun 10, 2013 at 12:09 PM PDT.

Also republished by Philly Kos and DKos Pennsylvania.

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