From the ABC 7 Weather team

Tornado Watch until 8 p.m., here's why and what you can expect

April 27, 2011 - 04:46 PM
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We know a tornado watch is in effect, but why is it in effect for our region and what should we really be expecting?

With such a severe outbreak occurring for the Deep South and Tennessee Valley yesterday and today, local residents are probably a bit on edge with the talk of a Tornado Watch for our area. Should this really be the case though?

As of 4 p.m. today, I see one storm that we should be concerned of moving just east of Charlottesville on our Doppler Radar. Otherwise, nothing else has popped up yet. A Tornado Watch has been placed in effect for our entire area until 8 p.m., and here's why.

Our area has plenty of instability, as noted in the forecast sounding for Reagan National above. There is more than enough instability to create thunderstorms, and many of the indicies that meteorologists usually take into account are at good levels to indicate a severe threat. The forecast soundings we have been looking at today show 1000-2000 j/kg CAPE, LI's of -4 to -6, K-Index's in the 30's, BRN in the 20's and SWEAT values in the 300's. Combine that with the clearing the region received through the day, enhancing the surface heating, and the moisture available with dewpoints in the 60s, the potential for severe storms continues to increase.

What about the environment makes it more favorable for tornadoes and tornadogenesis? Well if you take a glance at the Tornado-Warning-strewn Tennessee Valley, there they have the perfect environment for tornadoes and potentially strong, long-track tornadoes. Deep-layer wind shear is needed, with winds veering with height and increasing in speed with height. This is the directional shear and speed shear that meteorologists talk about in the atmosphere. In those areas, winds are around 70 m.p.h. only 1,000 feet above the ground. Winds then scream out of the west a few thousand feet above that around 100 m.p.h., ensuring a twist in the atmosphere and leaving the region perfect for rotating thunderstorms. Combine that with the perfect lifting mechanism in a cold front, and that potential goes up even more.

Here, the winds aren't quite as impressive, with winds out of the south around 30 m.p.h. at 1,000 feet and around 50 m.p.h. out of the southwest in the mid-levels. We also do not really have much of a lifting mechanism other than day-time heating and the available moisture. With that being said, there is still enough wind shear for a an isolated spin-up tornado, which is why the Storm Prediction Center issued a watch. I think the potential exists until the sun sets and we lose the instability from daytime heating.

Our region may not even see anything, but the potential still exists, and with the recent severe weather through the Carolinas and now through the Missouri and Tennessee valleys, it is good to error on the side of caution.

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