Posted by JAC on 3/28/2009, 2:49 pm, in reply to "Noth AL/GA, East TN Area"

MESOSCALE DISCUSSION 0318 NWS STORM PREDICTION CENTER NORMAN OK 1258 PM CDT SAT MAR 28 2009 AREAS AFFECTED...PORTIONS OF THE LOWER OH AND TN RIVER VALLEYS CONCERNING...SEVERE POTENTIAL...TORNADO WATCH LIKELY VALID 281758Z - 281930Z SCATTERED TSTMS SHOULD DEVELOP BY 20-21Z ALONG A COLD FRONT PROGRESSING EWD ACROSS THE LOWER OH AND TN RIVER VALLEYS. VERY STRONG DEEP-LAYER SHEAR AMIDST MODEST SURFACE-BASED INSTABILITY WILL RESULT IN SUPERCELLS PRODUCING TORNADOES...LARGE HAIL...AND DAMAGING WINDS. A TORNADO WATCH WILL LIKELY BE ISSUED PRIOR TO 19-20Z. 17Z SURFACE ANALYSIS PLACED A 993 MB LOW NEAR SGF WITH AN ARCING COLD FRONT FROM CGI TO TUP. BREAKS IN LOW-LEVEL STRATOCUMULUS AND PRESENCE OF MID-LEVEL DRY SLOT HAS ALLOWED SURFACE TEMPERATURES TO WARM TO AROUND 70 F ALONG AND AHEAD OF THE BOUNDARY...WHILE DEW POINTS HAVE RISEN TO AROUND 60 F GIVEN DOWNWARD MIXING/ADVECTION. WITH CONTINUED INSOLATION...MODIFIED RUC FORECAST SOUNDINGS SUGGEST MLCAPES OF 600 TO 1200 J/KG WILL BE COMMON. AS THE CYCLONE OVER THE OZARKS PIVOTS NEWD...LARGE-SCALE FORCED ASCENT WILL INCREASE AS AN INTENSE /90 KTS AT 500 MB/ S-SWLY MID-LEVEL JET OVERSPREADS THE REGION. TSTM INITIATION SHOULD OCCUR BY 21Z...WHICH IS SUPPORTED BY 00Z 4-KM WRF-NSSL AND 12Z GFS. WITH BACKED S/SELY LOW-LEVEL FLOW...DEEP-LAYER SHEAR WILL BE QUITE STRONG /AOA 60 KTS/ AND WILL PROMOTE SUPERCELL DEVELOPMENT. FAVORABLE LOW-LEVEL HODOGRAPHS WITH 0-1 KM SRH AOA 200 M2/S2 WILL SUPPORT POTENTIAL FOR TORNADOES. WHILE RELATIVELY STEEP MID-LAPSE RATES /AS SAMPLED UPSTREAM BY 12Z LZK RAOB/ AND THE VERY STRONG SHEAR WILL PROMOTE HAIL DEVELOPMENT AND LOCALLY DAMAGING WINDS. ..GRAMS.. 03/28/2009