Severe Weather Threat Increasing to the West

The Storm Prediction Center has posted a MODERATE RISK of severe storms for parts of the Midsouth on Tuesday:

This storm system is very potent, and the reason the risk is higher to the west of the Tennessee Valley is pretty simple: instability and shear will be more balanced near the Mississippi River Tuesday afternoon through Tuesday night than it will be as the system advances into North Alabama and Southern Middle Tennessee. Here are the severe weather probabilities produced by SPC. That 45% hatched area is significant.

Quote from SPC’s discussion:

  ...SRN PLAINS THROUGH LOWER MS VALLEY REGION...

   POTENTIAL EXISTS FOR A SIGNIFICANT SEVERE WEATHER EVENT TUESDAY WITH
   DAMAGING WIND AND TORNADOES THE PRIMARY THREATS...ESPECIALLY DURING
   THE EVENING AND OVERNIGHT IN MODERATE RISK AREA. OBSERVATIONS SHOW A
   RESERVOIR OF MIDDLE TO UPPER 60S DEWPOINTS FROM CNTRL INTO SRN TX.
   SLY BOUNDARY LAYER FLOW ALREADY EXISTS FROM THE SRN PLAINS INTO THE
   LOWER MS VALLEY AND WILL STRENGTHEN TUESDAY IN ASSOCIATION WITH AN
   INTENSIFYING LLJ. WIDESPREAD MULTI-LAYER CLOUDS WILL LIMIT BOUNDARY
   LAYER WARMING DURING THE DAY. HOWEVER...MID 60S DEWPOINTS SHOULD
   ADVECT AS FAR NORTH AS SERN OK INTO CNTRL AR AND NRN MS BENEATH A
   PLUME OF 6.5 C/KM MID LEVEL LAPSE RATES RESULTING IN AT LEAST
   500-1000 J/KG MLCAPE.

   STORMS ARE EXPECTED TO DEVELOP WITHIN PRE-FRONTAL CONFLUENCE BAND
   ACROSS CNTRL/ERN OK INTO NCNTRL/NERN TX EARLY TUESDAY AND SHIFT EWD
   AND NEWD DURING THE DAY. OTHER STORMS MAY DEVELOP FARTHER SOUTH
   THROUGH ERN TX DURING THE AFTERNOON ALONG MERGING PACIFIC FRONT AND
   SHIFT EWD THROUGH THE LOWER MS VALLEY. THE BOUNDARY LAYER WILL
   MOISTEN SUFFICIENTLY FOR THIS ACTIVITY TO BECOME SFC BASED. STORMS
   WILL BE EMBEDDED WITHIN STRONG DEEP LAYER FLOW WITH A 50+ KT SLY LLJ
   AND MID LEVEL WINDS INCREASING TO 80 KT. STORMS ARE EXPECTED TO
   EVOLVE INTO A PREDOMINANT QLCS WITH EMBEDDED SUPERCELL AND BOWING
   SEGMENTS POSING A RISK FOR DAMAGING WIND AND TORNADOES. TORNADO
   THREAT MAY INCREASE OVERNIGHT...ESPECIALLY IN MODERATE RISK AREA
   WHERE LLJ INTENSIFYING TO IN EXCESS OF 70 KT WILL BE COLLOCATED WITH
   NRN EDGE OF MID 60S DEWPOINTS.

   ...UPPER MS VALLEY THROUGH TN VALLEY REGION...

   OTHER STORMS WILL EXPAND NEWD THROUGH THE MID MS VALLEY AND SWRN
   PARTS OF OH VALLEY WHERE INSTABILITY WILL BE MORE LIMITED...BUT
   SUFFICIENT FOR A THREAT FOR DAMAGING WIND WITH PRIMARY MODE BEING
   LINES AND BOWING SEGMENTS WITH MESO VORTICES. A FEW EMBEDDED
   SUPERCELLS WILL ALSO BE POSSIBLE GIVEN SIZE OF HODOGRAPHS AND
   STRENGTH OF DEEP LAYER FLOW/SHEAR SUGGESTING A FEW TORNADOES CANNOT
   BE RULED OUT.

As this system moves northeast (rapidly) into the Tennessee Valley on Wednesday morning, our primary risk will be strong, straight line winds in excess of 60-70 MPH; however, given the nature of the system and the strength of the wind field in the lower atmosphere, there is at least some tornado threat between 2 AM and 10 AM Wednesday. (The graphic above only represents the storm risk through 6 AM Wednesday; the risk is SLIGHT for North Alabama and Southern Tennessee during the day Wednesday.)

This is a forecast for a threat of storms over a broad area; we cannot be specific about an individual town, community, or county because the thunderstorms happen on a different time and space scale than the bigger system producing them.

The storm system is like an eighteen wheeler traveling down the interstate. It’s easy to pick out the path and time the system, but it’s a lot harder to know how the freight in the trailer will shift when that truck speeds up, slows down, or hits a pot hole. In other words, the small-scale things that enhance or interfere with individual thunderstorm development are tracked and forecast on a scale of minute to an hour or two. That’s why you should be ready in case a warning is issued for your community. Have a NOAA weather radio handy tomorrow night into Wednesday morning, and there are a number of other ways to stay informed as well:

WHNT.com’s Maps & Radar Page
Mobile Applications
Baron Saf-T-Net

We’ll be here to keep you updated!

-Jason
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