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.
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.
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