Strong To Severe Storms Possible Today

There is a threat, however small, of some strong to severe thunderstorms today! This threat will only extend through the western portions of Middle Tennessee. However, there will be plenty of rain for the rest of you! The cold front is still digging its way in, and is just now reaching the outer edges of Eastern Tennessee. Here is the Hazardous Weather outlook the NWS has issued.

THIS HAZARDOUS WEATHER OUTLOOK IS FOR MIDDLE TENNESSEE.

.DAY ONE...TODAY AND TONIGHT

A FEW STRONG TO SEVERE STORMS ARE POSSIBLE OVER WESTERN PORTIONS
OF THE MID STATE AROUND MID AFTERNOON.

.DAYS TWO THROUGH SEVEN...SATURDAY THROUGH THURSDAY

NO HAZARDOUS WEATHER IS EXPECTED AT THIS TIME.

.SPOTTER INFORMATION STATEMENT...

DUE TO THE BRIEF AND ISOLATED NATURE OF POSSIBLE SEVERE WEATHER...
SPOTTER ACTIVATION IS NOT ANTICIPATED.

The areas that are shaded also have a very small 2% tornado risk, as seen in this image. Unfortunately, the discussion does not mention anything that would involve Tennessee, but I will post it in here anyway.

...THERE IS A SLGT RISK OF SVR TSTMS OVER PARTS OF MS AND WESTERN
   AL...

   A DEEP UPPER TROUGH CURRENTLY OVER THE CENTRAL UNITED STATES WILL
   SLOWLY WEAKEN AND TRACK EASTWARD INTO THE MS VALLEY TODAY...WHILE
   THE PRIMARY SURFACE LOW OCCLUDES AND LIFTS NORTHWARD INTO MN.  THE
   MAIN COLD FRONT CURRENTLY EXTENDS FROM CENTRAL MO INTO EASTERN
   AR...AND IS THE FOCUS FOR NUMEROUS SHOWERS AND THUNDERSTORMS.
   SEVERAL CONVECTIVE ELEMENTS ALONG THE FRONT HAVE SHOWN TRANSIENT
   BOW/LEWP CONFIGURATIONS AND OCCASIONAL SUPERCELL STRUCTURES...BUT
   DAMAGING WINDS REACHING THE SURFACE HAVE BEEN RARE.  THIS TREND WILL
   LIKELY CONTINUE THROUGH THE MORNING AS 12Z JAN RAOB CONFIRMS VERY
   STRONG LOW LEVEL AND DEEP LAYER VERTICAL SHEAR BUT WEAK INSTABILITY.
   THE POTENTIAL FOR WIDESPREAD SEVERE WEATHER APPEARS LOW...BUT AN
   ISOLATED DAMAGING WIND GUST OR TORNADO IS CERTAINLY POSSIBLE.

   LATER THIS AFTERNOON...POCKETS OF DAYTIME HEATING WILL HELP TO
   DESTABILIZE THE AIR MASS ACROSS PARTS OF MS/AL...DESPITE WEAK MID
   LEVEL LAPSE RATES.  MLCAPE VALUES AROUND 500 J/KG MAY BE SUFFICIENT
   FOR A FEW STRONG STORMS TO INTENSIFY ALONG THE FRONT OVER MS AND
   TRACK INTO WESTERN AL.  MODEL GUIDANCE SUGGESTS THAT THE PRIMARY
   UPPER TROUGH AND LOW LEVEL JET WILL BE MOVING AWAY FROM THE GULF
   COAST STATES BY THAT TIME...INDICATING THAT THE COVERAGE OF SEVERE
   STORMS WILL REMAIN ISOLATED AT BEST.

Looking at the SKEW-T plot, there is some amazing shear going on! Shear is a rate in change of wind at different levels. This change can be both speed and directional, as seen in the plot below. Why do we care about shear? Well, my understanding is limited, but try and follow. Shear helps a thunderstorm separate its updraft from its downdraft, which is needed for long lasting severe storms. It does this because once precip has reached the top of the thunderstorm, it gets blown downwind and fall into the downdraft. Anyway, here is the SKEW-T plot. Note that at this hour there was no instability, which is what is inhibiting a larger severe threat. Remember, this was for that hour only, so it does not apply to now.

(click for a larger image)

Here is the 500 mb wind map. To put it in refrence, 500 MB is roughly 5850 m today. This map came from twisterdata.com

Note the trough (the white area between the purple) where the low-pressure system is. This trough is what is producing the cold front, and is also what was producing some supercells and tornadoes last night. Here is another shear map from twisterdata.com

I will be keeping an eye on this system as it roars on through! Stay tuned to NOAA Weather Radio and your local news if you are feeling that it is getting dangerous. For those not in the risk outline, prepare to be wet