Quiet Last Night, but Severe Weather Erupts Later Today

Thankfully last night was quiet, but today promises to be an active day for much of Alabama, Tennessee, Georgia and the Carolinas as a powerful storm system west of us accelerates east through the day. We still expect the primary threat to be from damaging straight-line winds in excess of 60 to 70 MPH, but tornadoes can and do occur in environments like this.

A lot has to change throughout the day in order for us to get that severe threat, though. The air is relatively dry in the low-levels this morning, but the higher dewpoints (63-68º F) are not far off; that more humid air is just now starting to stream north into Northwest Alabama. Dewpoints are forecast to rise to at least 64º by Noon today just as the heavier storms begin intensifying near The Shoals:

Another factor that has to change is the shear in the lower atmosphere. Shear is a change in wind speed and direction with height, and we are now seeing a trend toward more significant shear in the lower levels through the afternoon. We’re now seeing model forecasts of 0-3 km helicity in the 300-400 m2/s2 range. This is the RAP (Rapid Update Cycle) forecast for 7 PM – significant low-level shear over Northeast and East-Central Alabama is especially concerning:

Screen Shot 2013-04-11 at 6.40.39 AM

Because of the very slow eastward progression of this, we are adjusting the timing to be even later than earlier expected; here’s the rough outline:

11 AM to 2 PM: Showers and storms move into the The Shoals by late morning, but they will not move east very quickly. This morning’s rain over North Mississippi is only sliding east at about 10 MPH, and that forward speed is not expected to increase for several hours. Severe storms are possible in this time frame in Marion, Winston, Franklin (AL), Lawrence (AL), Colbert, Lauderdale, Wayne, and Lawrence (TN).

2 PM to 5 PM: Storms will grow stronger as they move east through Cullman, Morgan, Limestone, Madison, Giles and Lincoln Counties. A small-scale feature called a mesolow developing along the line of storms could enhance the severe weather threat (increasing the tornado threat) in this area through early afternoon.

5 PM to 8 PM: Intense thunderstorms will start moving a little faster through Blount, Marshall, DeKalb, Jackson, Etowah, Cherokee, Lincoln, and Franklin (TN) Counties. This is likely where the highest severe weather threat will be considering that low-level shear is increasing in the warmest, most unstable region of the state.

Be alert today; these times do not indicate a specific severe weather threat to an individual town or community. That’s what warnings are for. Make sure you have a way to get specific information about watches and warnings through NOAA Weather Radio or the following offerings from WHNT News 19:

WHNT.com’s Maps & Radar Page
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