SPC AC 121239
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0739 AM CDT TUE MAY 12 2009
VALID 121300Z - 131200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE NORTH-CENTRAL PLAINS
AND MISSOURI VALLEY...
...THERE IS A SLGT RISK OF SVR TSTMS OVER PARTS OF WEST TX AND
WESTERN OK...
WESTERLY FLOW ALOFT WILL PREVAIL OVER MOST OF THE NATION AGAIN
TODAY...WITH A LOW-AMPLITUDE UPPER TROUGH SLOWLY DEEPENING OVER THE
NORTHWEST US. SEVERAL SMALL-SCALE FEATURES WILL TRAVERSE THE
CENTRAL PLAINS AND AID IN THE DEVELOPMENT OF SCATTERED THUNDERSTORMS
DURING THE PERIOD.
...OK/KS/MO/IA...
AMPLE LOW LEVEL MOISTURE IS IN PLACE THIS MORNING OVER MUCH OF TX.
THIS AIRMASS WILL SPREAD SLOWLY NORTHWARD THROUGH THE DAY AS
WARM/MOIST ADVECTION TAKES PLACE. THIS WILL RESULT IN AN INCREASING
SHIELD OF SHOWERS AND THUNDERSTORMS OVER OK/KS THROUGH THE
AFTERNOON...SPREADING ACROSS MO/IA THIS EVENING. ELEVATED CAPE IN
THIS AREA WILL PROMOTE A RISK OF HAIL IN STRONGER CELLS THROUGHOUT
THE REGION.
...WEST TX/WRN OK...
A DRYLINE IS FORECAST TO FOCUS THIS AFTERNOON FROM THE TX PANHANDLE
TO NEAR MAF. STRONG HEATING IN THIS AREA WILL RESULT IN STEEP
LOW/MID LEVEL LAPSE RATES AND A VERY UNSTABLE AIRMASS /AFTERNOON
MLCAPE OF 3000+ J/KG/. FORECAST SOUNDINGS INDICATE A PRONOUNCED CAP
WILL KEEP CONVECTIVE INITIATION QUITE SPARSE. HOWEVER...A
COMBINATION OF WEAK CONVERGENCE ALONG THE CAP...SLIGHT OROGRAPHIC
FORCING ALONG THE CAPROCK ESCARPMENT...AND ENHANCED FORCING ALONG
THE RETREATING SURFACE BOUNDARY LIFTING NORTHWARD ACROSS TX MAY
RESULT IN ISOLATED SEVERE STORMS OVER THE EASTERN TX PANHANDLE.
THESE STORMS WILL POSE A RISK OF LARGE HAIL...DAMAGING WINDS...AND
ISOLATED TORNADOES. FARTHER SOUTH...MODELS SUGGEST DEEP MIXING OF
THE BOUNDARY LAYER EAST OF THE DRYLINE WHICH WILL RESULT IN HIGH
BASES OF THOSE STORMS THAT CAN DEVELOP FROM LBB SOUTHWARD.
...NORTHERN/CENTRAL PLAINS...
CONSIDERABLE UPPER TROUGH/ASSOCIATED COLD FRONT WILL DEVELOP
EASTWARD OVER THE NORTHERN INTERMOUNTAIN REGION TODAY...WITH
DOWNSTREAM HEIGHT FALLS AND LEE CYCLOGENESIS ACROSS THE NORTHERN
HIGH PLAINS. MODEST BOUNDARY LAYER MOISTURE /SURFACE DEWPOINTS
MAINLY 40S F TO LOWER 50S/ WILL TEND TO BE A LIMITING FACTOR...
ALTHOUGH A VERY STEEP LAPSE RATE ENVIRONMENT/STRONG SHEAR AMIDST
STRENGTHENING FLOW ALOFT /50+ KT AT 500 MB/ WILL SUPPORT A SEVERE
RISK THIS AFTERNOON/TONIGHT. STORMS SHOULD INITIALLY DEVELOP THIS
AFTERNOON ACROSS PORTIONS OF SOUTHEAST MT/ADJACENT NORTHEAST WY...AS
WELL AS THE WESTERN PORTIONS OF SD/NEB...AND SUBSEQUENTLY DEVELOP
EASTWARD THROUGH EARLY/MID EVENING. GIVEN A WELL-MIXED/ STEEP LAPSE
RATE ENVIRONMENT...DAMAGING WIND GUSTS AND SEVERE HAIL WILL BE THE
PRIMARY HAZARDS.
...SOUTHERN NEW ENGLAND...
MODEST SURFACE HEATING...COMBINED WITH COLD MID LEVEL TEMPERATURES
/-25C TO -27C AT 500 MB/ IN ASSOCIATION WITH AN OPEN MID LEVEL
WAVE...WILL RESULT IN THE DEVELOPMENT OF MODEST SBCAPE AND THE
POTENTIAL FOR ISOLATED SHOWER AND THUNDERSTORM DEVELOPMENT THIS
AFTERNOON ACROSS SOUTHERN NEW ENGLAND/PERHAPS LONG ISLAND VICINITY.
WITH POTENTIAL FOR SBCAPE UP TO 500 J/KG AND A STEEP LAPSE RATE
ENVIRONMENT...SOME HAIL MAY BE POSSIBLE. WEAK LOW TO MID
TROPOSPHERIC FLOW WILL LIKELY KEEP THE HAIL THREAT MARGINAL/SPORADIC
IN NATURE...AND ANY SUCH THREAT WILL QUICKLY WANE AFTER DARK.
..HART/HURLBUT.. 05/12/2009
CLICK TO GET WUUS01 PTSDY1 PRODUCT
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1630Z
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