SPC AC 051732
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1232 PM CDT TUE MAY 05 2009
VALID 061200Z - 071200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS A LARGE PORTION OF THE
SERN CONUS AND INTO THE RED RIVER VALLEY...
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE UPPER MS
VALLEY...
...SYNOPSIS...
A PAIR OF MID-LEVEL SHORT-WAVE TROUGHS -- ONE IN THE NRN STREAM
CROSSING THE N CENTRAL U.S./S CENTRAL CANADA AND A SECOND CROSSING
THE MID MS/TN/OH VALLEYS -- WILL BE THE MAIN FEATURES ALOFT WITH
RESPECT TO INFLUENCING THE CONVECTIVE FORECAST THIS PERIOD.
AS THESE TWO FEATURES PROGRESS EWD...WEAK SHORT-WAVE RIDGING
SPREADING ACROSS THE PLAINS AND ASSOCIATED LARGE-SCALE SUBSIDENCE
SHOULD ACT TO LARGELY HINDER CONVECTIVE ACTIVITY FROM THE DAKOTAS
INTO KS AND SWD ACROSS THE SRN HIGH PLAINS.
AT THE SURFACE...A WEAK FRONTAL WAVE INITIALLY INVOF WRN TN SHOULD
MOVE ENEWD TOWARD THE UPPER OH VALLEY THROUGH THE AFTERNOON...AS THE
SRN STREAM MID-LEVEL TROUGH PROGRESSES EWD. THE TRAILING SURFACE
FRONT SHOULD LINGER FROM THE TN VALLEY WSWWD INTO CENTRAL TX.
FARTHER N...A WEAK FRONT ASSOCIATED WITH THE MORE NRN UPPER TROUGH
SHOULD MOVE EWD OUT OF THE DAKOTAS AND ACROSS THE MO/UPPER MS
VALLEYS...AND INTO THE UPPER GREAT LAKES REGION.
...CAROLINAS WWD INTO THE LOWER MS VALLEY...
AN EWD-MOVING UPPER TROUGH CROSSING THIS REGION AND
MOIST/DESTABILIZING AIRMASS AHEAD OF THE WEAK SURFACE FRONT SHOULD
SUPPORT THE DEVELOPMENT OF WIDESPREAD SHOWERS AND THUNDERSTORMS
ACROSS A LARGE PORTION OF THE SOUTHEAST CONUS. WEAK LAPSE RATES AND
GENERALLY LIMITED DEEP-LAYER WIND FIELD SHOULD LIMIT OVERALL STORM
INTENSITY. HOWEVER...OVERALL CAPE/SHEAR COMBINATION WILL LIKELY
SUPPORT OCCASIONAL LOCALLY-SEVERE STORMS ACROSS A LARGE REGION FROM
THE CAROLINAS WSWWD ACROSS PARTS OF AR/LA. GREATEST THREAT WILL
OCCUR DURING THE AFTERNOON...WITH THE OVERALL POTENTIAL DIMINISHING
DIURNALLY THROUGH THE EVENING.
...RED RIVER VALLEY/ARKLATEX REGION...
WIDESPREAD THUNDERSTORMS SHOULD BE SHIFTING E OF THIS REGION THROUGH
THE MORNING...AS THE WWD BRANCH OF THE LOW-LEVEL JET VEERS/SHIFTS
EWD WITH TIME INTO THE MID/LOWER MS VALLEY REGION.
HOWEVER...WITH MOIST AIRMASS IN PLACE NEAR/S OF A WEAK SURFACE FRONT
AND WITH THE ONSET OF DIURNAL HEATING AS LINGERING CLOUDS SHIFT
EWD...MODERATE DESTABILIZATION IS FORECAST. WHILE CAPPING INVERSION
IS FORECAST ACROSS A LARGE PORTION OF THE SRN PLAINS AS WEAK
SHORT-WAVE RIDGING SPREADS ACROSS THE AREA...LOCALIZED ENHANCEMENT
IN LOW-LEVEL UVV -- MAINLY INVOF THE WEAK SURFACE FRONT AND ANY
LINGERING CONVECTIVE BOUNDARIES -- SHOULD SUPPORT ISOLATED TO
SCATTERED STORM REDEVELOPMENT DURING THE AFTERNOON. WEAK/SELY
LOW-LEVEL FLOW VEERING/INCREASING TO NEAR 30 KT FROM THE WNW AT MID
LEVELS SHOULD BE SUFFICIENT FOR AT LEAST WEAK ORGANIZATION...LIKELY
ALLOWING A FEW STRONGER/SEVERE STORMS TO EVOLVE. MAIN THREATS WILL
BE HAIL AND/OR LOCALLY-DAMAGING WINDS BEFORE STORMS DIMINISH DURING
THE EVENING HOURS.
...UPPER MS VALLEY REGION...
MARGINAL DESTABILIZATION /AOB 1000 J/KG MIXED-LAYER CAPE/ IS
FORECAST THROUGH THE AFTERNOON ACROSS PARTS OF THE MID MO/UPPER MS
VALLEYS AHEAD OF THE APPROACHING/WEAK SURFACE FRONT...AS DAYTIME
HEATING OF A MODESTLY-MOIST BOUNDARY LAYER OCCURS BENEATH THE EWD
ADVECTION OF COOLER MID-LEVEL AIR/STEEPER LAPSE RATES.
WITH MODERATE/WEAKLY-VEERING FLOW WITH HEIGHT EXPECTED ACROSS THIS
REGION...DEVELOPING AFTERNOON STORMS INVOF THE FRONT SHOULD BECOME
LOCALLY-ORGANIZED/SEVERE. GIVEN LINEAR FORCING FAVORING LINE
SEGMENTS AND WNWLY FLOW ALOFT...PRIMARY SEVERE THREAT -- IN ADDITION
TO HAIL -- WILL LIKELY BE LOCALLY-DAMAGING WINDS WITHIN SEWD-MOVING
STORM CLUSTERS. THREAT SHOULD DIMINISH INTO THE EVENING AS HEATING
DIMINISHES AND THE LOW-LEVEL AIRMASS STABILIZES.
...SERN OH/WRN WV INTO ERN KY/TN...
ZONE OF ENHANCED DEEP-LAYER FLOW IS FORECAST TO SPREAD ACROSS THIS
REGION AS THE MID-LEVEL SHORT-WAVE TROUGH AND AN ASSOCIATED SURFACE
FRONTAL WAVE CROSS THE TN/MID OH VALLEYS. POTENTIAL FOR
DESTABILIZATION APPEARS LIMITED W OF THE APPALACHIANS FROM ERN TN
NWD...AS WIDESPREAD CLOUDS/PRECIPITATION OVERSPREAD THIS REGION AS
LOW-LEVEL JET AND ASSOCIATED/BROAD ZONE OF WARM ADVECTION SHIFTS
INTO THIS AREA. WHILE STRONG/VEERING FLOW WITH HEIGHT WOULD SUPPORT
SEVERE/SUPERCELL STORMS...SEVERE THREAT LIKELY IS TO BE TEMPERED BY
AN OVERALL LACK OF FAVORABLE INSTABILITY. THUS...WILL MAINTAIN ONLY
LOW-END SLIGHT RISK/15% SEVERE PROBABILITY ACROSS THIS REGION FOR A
FEW STRONGER STORMS WHICH MAY EVOLVE IN CONJUNCTION WITH THE
APPROACH OF THE SURFACE SYSTEM.
..GOSS.. 05/05/2009
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