With strong westerly to northwesterly mid-level winds present across the OH/TN Valleys and Southeast forecast to increase further with the arrival of the mid-level jet max, effective bulk shear of 50-60+ kt will support supercell structures with initial discrete development. Greater boundary-layer moisture remains confined along/near the Gulf Coast owing to prior convection, but residual moisture characterized by mid 50s to lower 60s dewpoints should support sufficient instability, with MLCAPE generally ranging from 500-1250 J/kg across the Slight Risk area. Large hail and damaging straight-line winds appear to be the main threats with this convective activity, with the wind threat possibly becoming dominant with eastward extent into the Carolinas as convection may attempt to grow upscale in a broken line. A brief tornado may also occur with the mainly discrete supercell development across KY/TN/MS/AL, even though low-level winds are not expected to be overly strong, which should tend to limit effective SRH values to less than 200 m2/s2. The Slight Risk area has been expanded southwestward across middle TN and northern/central AL, as clearing across this area may support convective initiation a little sooner than previously forecast, and southward across parts of central GA where a line of storms may pose an organized damaging wind threat this evening into the early overnight hours.
Mesoscale Discussion 0160
NWS Storm Prediction Center Norman OK
0302 PM CST Wed Feb 08 2017
Areas affected...Middle Tennessee...northern Alabama...and far
Concerning...Severe potential...Watch possible
Valid 082102Z - 082300Z
Probability of Watch Issuance...40 percent
SUMMARY...Surface-based thunderstorms, some becoming severe, should
develop from north to south across middle Tennessee into northeast
Mississippi and northern Alabama this afternoon and evening, and
spread to the east. Locally strong winds and hail will be the
primary severe-weather threats.
DISCUSSION...Trends in mesoscale analyses showed an ill-defined low
pressure area tracking from western into northern middle TN this
afternoon along a northeast-southwest oriented surface trough.
Clearing of earlier clouds across the discussion area allowed for
destabilization with MLCAPE around 500 J/kg already present in
middle TN. A strong 60-70 kt midlevel jet now nosing into the Mid
South and TN Valley is resulting in bulk shear of 50-60 kt. Forcing
for ascent within the exit region of this jet appears to be
supporting recent elevated storm development across portions of
western KY into northern middle TN. Meanwhile, visible satellite
imagery showed cumulus continuing to develop, expanding in coverage
across western middle TN with greater vertical development.
Surface-based storms are expected to develop within the cumulus
field as the environment undergoes further destabilization, and
convection spreads east, with locally strong winds and hail being
possible with the stronger storms. 19Z HRRR continues to favor this
scenario for a few sustained storms in middle TN to northeast MS
developing by 23Z.
Severe Thunderstorm Warning for...
southwestern Madison County in north central Alabama...
southeastern Limestone County in north central Alabama...
central Morgan County in north central Alabama...
east central Lawrence County in northwestern Alabama...
* until 730 PM CST
* at 642 PM CST, a severe thunderstorm was located near Trinity, or 8
miles west of Decatur, moving east at 40 mph.
Hazard... 60 mph wind gusts and quarter size hail.
Source... radar indicated.
Impact... hail damage to vehicles is expected. Expect wind damage
to roofs, siding, and trees.
* Locations impacted include...
Decatur, Hartselle, Redstone Arsenal, Priceville, Trinity, Triana,
Huntsville international Airport, Somerville, Valhermoso Springs