SWODY1
SPC AC 211242
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0742 AM CDT MON MAY 21 2007
VALID 211300Z - 221200Z
..THERE IS A SLGT RISK OF SVR TSTMS FOR THE DAKOTAS/NW MN SWD
ACROSS MUCH OF THE WRN GREAT PLAINS...
..SYNOPSIS...
A MID LEVEL LOW OVER ERN ORE THIS MORNING WILL MOVE EWD TO ERN
ID/WRN WY BY EARLY TUESDAY...WHILE THE LARGER SCALE TROUGH AMPLIFIES
OVER THE GREAT BASIN. SURFACE PRESSURES WILL FALL IN A BROAD AREA
OF THE HIGH PLAINS DOWNSTREAM FROM THE AMPLIFYING MID LEVEL
TROUGH...AS MID LEVEL SWLY FLOW OVER THE FRONT RANGE INCREASES. ONE
SURFACE CYCLONE IN NW SD WILL DEVELOP SLOWLY NEWD TOWARD CENTRAL ND
BY THIS EVENING AS AN ASSOCIATED WARM FRONT LIFTS NWD ACROSS ND/MN.
FARTHER S...A LEE CYCLONE WILL BEGIN TO DEVELOP ACROSS NE CO.
MEANWHILE...A SEPARATE SRN STREAM WILL PERSIST FROM NRN MEXICO EWD
ACROSS THE GULF OF MEXICO.
..NRN PLAINS TODAY INTO TONIGHT...
THE SURFACE PRESSURE FALLS IN THE LEE OF THE CENTRAL/NRN ROCKIES
WILL INDUCE STRENGTHENING SLY FLOW TODAY INTO TONIGHT ACROSS THE
PLAINS. BOUNDARY LAYER DEWPOINTS CURRENTLY RANGE FROM 56-60 F FROM
SD TO N TX...WITH MID 60S CONFINED TO S TX. THEREFORE... EXPECT
ONLY SMALL MOISTURE INCREASES DURING THE DAY AS A RESULT OF
HORIZONTAL ADVECTION AND MODEST EVAPOTRANSPIRATION. MEANWHILE...MID
LEVEL LAPSE RATES OF 7-8 C/KM HAVE ALREADY OVERSPREAD THE NRN PLAINS
SURFACE WARM SECTOR...WHERE AFTERNOON TEMPERATURES IN THE LOW-MID
80S WILL SUPPORT MLCAPE VALUES OF 1500-2000 J/KG.
IN THE WAKE OF MORNING ELEVATED THUNDERSTORMS WITH WAA OVER NRN
ND/MN...EXPECT ADDITIONAL SURFACE-BASED THUNDERSTORM DEVELOPMENT BY
MID-LATE AFTERNOON FROM CENTRAL ND SSWWD INTO WRN SD ALONG THE PATH
OF THE SURFACE LOW AND AN ASSOCIATED BAROCLINIC ZONE THAT WILL DRIFT
EWD FROM MT/WY. VERTICAL SHEAR WILL BE SUFFICIENT FOR A FEW
SUPERCELLS AND/OR BOWING SEGMENTS ALONG THE SYNOPTIC FRONT...THOUGH
THE MODEST DEEP LAYER FLOW AND SHEAR LARGELY PARALLEL TO THE
BOUNDARY SUGGEST THAT CONVECTION WILL TEND TO MERGE INTO A LARGER
CONVECTIVE BAND BY LATE AFTERNOON/EVENING. AN ISOLATED TORNADO
CANNOT BE RULED OUT...BUT LARGE HAIL AND DAMAGING WINDS SHOULD BE
THE PRIMARY SEVERE STORM THREATS.
AN MCV THAT EMANATED FROM OVERNIGHT CONVECTION WILL MOVE NEWD FROM
ERN SD TO CENTRAL MN TODAY. THIS LOW-MID LEVEL VORTICITY CENTER
WILL INTERACT WITH THE SURFACE WARM FRONT...WHERE ADDITIONAL
THUNDERSTORM DEVELOPMENT MAY OCCUR. WILL EXTEND THE SLGT RISK AREA
EWD TO ACCOUNT FOR A MARGINAL SEVERE STORM THREAT WITH THIS SYSTEM.
..CENTRAL AND SRN HIGH PLAINS THIS AFTERNOON/EVENING...
A GRADUAL PHASING OF THE POLAR AND SUBTROPICAL JETS IS EXPECTED
THROUGH TOMORROW AS HEIGHTS FALL OVER THE SRN ROCKIES. THE INCREASE
IN MEAN FLOW OVER THE SRN ROCKIES WILL ALSO HELP EJECT A SERIES OF
SMALLER SCALE WAVES THAT NOW EXIST OVER ERN CO AND NERN NM.
SOME INCREASE IN LOW-LEVEL MOISTURE IS EXPECTED TODAY ACROSS THE TX
PANHANDLE AND WRN OK INTO WRN KS AS LOW-MID 60S DEWPOINTS IN
CENTRAL/S TX SPREAD NWD ON INCREASING SLY FLOW...WHICH WILL
CONTRIBUTE TO MODERATE INSTABILITY WITH DAYTIME HEATING. AT THE
SAME TIME...LOW-MID LEVEL WIND PROFILES ARE FORECAST TO BECOME
FAVORABLE FOR SUPERCELLS THIS AFTERNOON/EVENING FROM THE TX
PANHANDLE INTO SW KS IN ASSOCIATION WITH INCREASINGLY SLY LOW-LEVEL
WINDS AND WLY MID LEVEL FLOW WITH THE EJECTING WAVE FROM E/NE NM
TODAY. HOWEVER...STRONGLY BACKED AND RELATIVELY WEAK FLOW IN THE
UPPER TROPOSPHERE MAY TEND TO LESSEN THE THREAT FOR PERSISTENT
SUPERCELL STRUCTURES AND INCREASE THE POTENTIAL FOR STORM
INTERACTIONS AND COLD POOL PRODUCTION. GIVEN THE COMPLICATED NATURE
OF THE WIND PROFILES...WILL OUTLOOK ONLY LOW TORNADO PROBABILITIES.
OTHERWISE...THE STRONGER STORMS WILL BE CAPABLE OF PRODUCING LARGE
HAIL AND DAMAGING WINDS INTO THE LATE EVENING HOURS.
..S TX TODAY INTO THIS EVENING...
A SMALL SCALE SHORTWAVE TROUGH/VORTICITY CENTER S OF THE BIG BEND
WILL MOVE EWD OVER S TX TODAY INTO EARLY TONIGHT. RICHER BOUNDARY
LAYER MOISTURE /MID-UPPER 60S DEWPOINTS/ AND STRONGER FLOW ALOFT IN
PROXIMITY TO THE SUBTROPICAL JET COULD SUPPORT A FEW ORGANIZED
THUNDERSTORM CLUSTERS...AS WELL AS SOME EMBEDDED SUPERCELL
STRUCTURES. LARGE HAIL AND DAMAGING WINDS WILL BE THE PRIMARY
SEVERE STORM THREATS THROUGH THIS EVENING.
.THOMPSON/TAYLOR.. 05/21/2007
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