Currently, the majority of the eastern US is under a tornado watch:
|Fig 1 -- SPC tornado watches (red) and severe thunderstorm watches (blue) as of 0041Z, April 27, 2011.|
|Fig 2 -- SPC tornado, wind, and hail reports for April 27, 2011 as of 0041Z, April 28, 2011.|
|Fig 3 -- Birmingham, AL, sounding from 00Z, April 28, 2011.|
That's a practically uncapped environment with a surface-based CAPE calculate at 3458 J/kg--an enormous amount of convective potential energy from the surface layer. The entire lower half of the sounding is potentially unstable--any significant lift is going to fire off storms. And when those updrafts go up, what kind of wind shear are they running into? Winds go from southerly at 15 knots at the surface immediately to 65 knots out of the southwest at 850mb. This results in a 0-1km storm-relative helicity of 470 m^2/s^2. That's an incredible amount of low-level wind shear, meaning a high potential for rotation in storms. This is an environment with strong tornadic potential.
Everything just seems to be coming together for this event. A deep, neutrally-titled trough in the 500-250mb layer aloft over the Mississippi Valley and a strong jet stream around its base are providing divergence aloft over much of the eastern US. With surface dewpoints in the mid 60s or greater all the way from Tennessee up through Maryland, juicy air with lots of energy has nowhere to go but up. Violently.
If you live anywhere where there's a tornado watch, please be extra vigilant. We've already seen strong tornadoes move through places like Tuscaloosa, Birmingham, and even near Washington, DC. To quote the SPC, this is a particularly dangerous situation. Stay safe.