MESOSCALE DISCUSSION 2143
   NWS STORM PREDICTION CENTER NORMAN OK
   0802 PM CST SUN DEC 09 2012
   
   AREAS AFFECTED...ERN AR THROUGH WRN TN AND SWRN KY
   
   CONCERNING...TORNADO WATCH 677...
   
   VALID 100202Z - 100300Z
   
   THE SEVERE WEATHER THREAT FOR TORNADO WATCH 677 CONTINUES.
   
   SUMMARY...TORNADO WATCH 677 IS SCHEDULED TO EXPIRE AT 03Z. THE
   TORNADO THREAT IS EXPECTED TO GRADUALLY DECREASE THROUGH
   EVENING...BUT A NON ZERO TORNADO OR SEVERE THREAT WILL PERSIST
   THROUGH 05Z FROM ERN AR INTO WRN TN. AS A RESULT...DEPENDING ON
   CONVECTIVE TRENDS...WW 677 COULD BE LOCALLY EXTENDED IN TIME AND
   SPACE TO INCLUDE A PORTION OF WRN TN. SPC WILL CONTACT AFFECTED
   OFFICES SOON TO DISCUSS THE COURSE OF ACTION REGARDING THE
   EXPIRATION OF WW 677.
   
   DISCUSSION...STORMS HAVE EVOLVED INTO A PREDOMINANT LINEAR MODE FROM
   SWRN KY THROUGH ERN AR. TRENDS IN CONVECTIVE MODE...A STABILIZING
   BOUNDARY LAYER...INCREASINGLY MARGINAL INSTABILITY...AND A TENDENCY
   FOR THE LLJ TO GRADUALLY SHIFT EAST OF THE SFC MOIST AXIS SUGGEST
   THE TORNADO THREAT SHOULD GRADUALLY DECREASE WITH TIME.
   HOWEVER...LATEST VWP DATA FROM MEMPHIS STILL SHOW STRONG LOW LEVEL
   SHEAR WITH 0-1 KM STORM RELATIVE HELICITY AROUND 250 M2/S2. GIVEN
   THE FAVORABLE SHEAR ENVIRONMENT...EMBEDDED SUPERCELLS AND A TORNADO
   OR TWO CANNOT BE COMPLETELY RULED OUT NEXT 2-3 HOURS AS ACTIVITY
   CONTINUES THROUGH THE REMAINDER OF THE WEAK INSTABILITY AXIS.
   
   OTHERWISE...STORMS DEVELOPING FARTHER EAST WITHIN WARM ADVECTION
   REGIME OVER WRN TN ARE MORE DISCRETE IN CHARACTER...BUT ARE WITHIN A
   WEAK INSTABILITY ENVIRONMENT AND APPEAR TO BE AT LEAST SLIGHTLY
   ELEVATED.
   
   ..DIAL.. 12/10/2012
   
   
   ATTN...WFO...PAH...MEG...LZK...
   
   LAT...LON   34319237 35709026 36828848 36148845 35138934 34989022
               34239099 34319237