SPC AC 190517
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1217 AM CDT SUN APR 19 2009
VALID 191200Z - 201200Z
...THERE IS A SLGT RISK OF SVR TSTMS FROM PARTS OF THE LOWER OH/MS
VALLEYS EWD INTO THE TN VALLEY AND CNTRL GULF STATES...
...SYNOPSIS...
MIDLEVEL TROUGH INITIALLY OVER THE OZARK PLATEAU WILL TRANSLATE
ENEWD THROUGH THE OH/TN VALLEYS IN RESPONSE TO UPSTREAM JET
STREAK/VORTICITY MAXIMUM DIGGING SEWD THROUGH THE UPPER GREAT LAKES.
THIS LARGE-SCALE PATTERN EVOLUTION WILL RESULT IN STRONG DYNAMIC
FORCING/HEIGHT FALLS SPREADING EWD ACROSS THE MID/LOWER MS AND OH/TN
RIVER VALLEYS. THESE INCREASED HEIGHT FALLS AND GRADUAL OPENING OF
MIDLEVEL WAVE WILL SERVE TO DEEPEN ATTENDANT SURFACE LOW WHICH WILL
DEVELOP FROM SERN MO NEWD TO NEAR THE IND/OH BORDER BY MONDAY
MORNING. AS THIS OCCURS...TRAILING COLD FRONT WILL PUSH EWD THROUGH
THE LOWER MS AND TN VALLEYS AND CNTRL GULF STATES.
...LOWER OH/MS VALLEYS EWD INTO THE TN VALLEY AND CNTRL GULF
STATES...
REMNANTS OF MCS CURRENTLY PRESENT OVER PORTIONS OF MS/LA ARE
EXPECTED TO BE ONGOING AT THE ONSET OF THE FORECAST PERIOD...FUELED
BY INFLUX OF LOWER/MID 60S BOUNDARY LAYER DEWPOINTS ALONG 40-50 KT
LLJ. THIS LLJ IS EXPECTED TO MIGRATE NEWD INTO THE APPALACHIANS
TODAY...THOUGH SWRN EXTENSION OF THIS FEATURE WILL MAINTAIN MODEST
LOW-LEVEL SHEAR IN ADVANCE OF MCS MOVING EWD ACROSS PARTS OF MS/AL
AND THE FL PNHDL. LAPSE RATES WILL NOT BE PARTICULARLY STEEP...BUT
THE RELATIVELY MOIST BOUNDARY LAYER AND DIABATIC HEATING WILL
CONTRIBUTE TO MLCAPE OF 500-1000 J/KG. THIS INSTABILITY IN
CONJUNCTION WITH 40-50 KT OF DEEP WLY SHEAR SUGGESTS THE POTENTIAL
FOR EMBEDDED SUPERCELL AND/OR BOWING STRUCTURES WITH THE THREAT FOR
MAINLY STRONG WIND GUSTS AND PERHAPS A TORNADO OR TWO.
MEANWHILE...MORNING CLOUDS AND TSTMS WILL GRADUALLY CLEAR FROM W-E
THROUGH SERN MO/ERN AR AND EVENTUALLY WRN PARTS OF KY/TN INTO NRN MS
WITH ARRIVAL OF MIDLEVEL DRY SLOT FROM THE W/SW. BREAKS IN THE
CLOUDS COUPLED WITH CONSIDERABLE MIDLEVEL COOLING/HEIGHT FALLS WILL
FOSTER RAPID AIR MASS DESTABILIZATION BY EARLY AFTERNOON FROM THE
VICINITY OF THE SURFACE LOW SWD ALONG PACIFIC FRONT.
EXPECT TSTMS TO RE-DEVELOP OR RE-INTENSIFY BY LATE MORNING OR EARLY
AFTERNOON OVER PARTS OF SERN MO AND ERN AR AS THE STRONG DEEP-LAYER
ASCENT OVERSPREADS THE SURFACE FRONTAL ZONE. STORMS SHOULD INCREASE
IN BOTH COVERAGE AND INTENSITY THROUGH THE AFTERNOON WHILE
MOVING/DEVELOPING EWD. LOW-LEVEL WIND FIELDS WILL REMAIN RATHER
WEAK OWING TO EWD DISPLACEMENT OF LLJ AXIS. BUT..THE PRESENCE OF
THE STEEP LAPSE RATES ALONG WITH 30-40 KT EFFECTIVE BULK SHEAR WILL
SUPPORT THE DEVELOPMENT OF A FEW ORGANIZED STORMS CAPABLE OF MAINLY
SEVERE HAIL. A BRIEF TORNADO OR TWO IS POSSIBLE WITH ANY STORMS
DEVELOPING NEAR THE SURFACE LOW WHERE THE STEEP LAPSE RATES WILL BE
COLOCATED WITH A CONCENTRATED ZONE OF VERTICAL VORTICITY.
..MEAD/SMITH.. 04/19/2009
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