SWODY1
SPC AC 201938
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0238 PM CDT SUN SEP 20 2009
VALID 202000Z - 211200Z
...NO SVR TSTM AREAS FORECAST...
THE ONLY CHANGE TO THE OUTLOOK IS TO ADD A 5 PERCENT HAIL
PROBABILITY ACROSS WCNTRL TX. CONVECTION CURRENTLY DEVELOPING SOUTH
OF MIDLAND IS DEVELOPING JUST AHEAD OF A PRONOUNCED DRY SLOT WHERE
LARGE-SCALE ASCENT MAY BE ENHANCED. MODERATE INSTABILITY...25-30 KT
OF VERTICAL SHEAR AND STEEP MID-LEVEL LAPSE RATES EVIDENT ON
FORECAST SOUNDINGS SHOULD SUPPORT A MARGINAL THREAT FOR HAIL WITH
THE STRONGER MULTICELLS LATE THIS AFTERNOON.
ELSEWHERE...WATER VAPOR IMAGERY CURRENTLY SHOWS A BAND OF MID-LEVEL
MOISTURE ASSOCIATED WITH A SHORTWAVE TROUGH FROM CNTRL WY SWD INTO
WRN CO. THUNDERSTORMS ARE ONGOING IN THIS BAND ALONG THE WRN EDGE OF
THE SEE TEXT AREA WHERE LAPSE RATES ARE QUITE STEEP. THIS ACTIVITY
SHOULD GRADUALLY MOVE AND EXPAND IN COVEARGE EWD ACROSS ERN CO AND
WRN NEB LATE THIS AFTERNOON. AS DESTABILIZATION CONTINUES TO TAKE
PLACE...WSR-88D VWPS ACROSS THE REGION SUGGEST ENOUGH VERTICAL SHEAR
EXISTS FOR MARGINALLY SEVERE HAIL AND ISOLATED STRONG WIND GUSTS. A
STRONG CAPPING INVERSION IS PRESENT IN THE PLAINS AND THE CONVECTION
SHOULD BECOME INCREASINGLY ELEAVATED THIS EVENING. HOWEVER...THE
HAIL THREAT MAY CONTINUE OVERNIGHT AS THE STORMS INTERACT WITH THE
LOW-LEVEL JET.
..BROYLES.. 09/20/2009
.PREV DISCUSSION... /ISSUED 1138 AM CDT SUN SEP 20 2009/
CORRECTED TO ADD 5% HAIL PROBABILITIES FOR NW CO
...SYNOPSIS...
PATTERN AMPLIFICATION IS UNDERWAY AS A PRONOUNCED MID LEVEL TROUGH
OVER MT BEGINS TO DIG SEWD AND EVENTUALLY SSEWD TOWARD THE CENTRAL
ROCKIES AND CENTRAL HIGH PLAINS BY EARLY MONDAY. DOWNSTREAM...THE
PRIOR SERIES OF WEAK/SLOW-MOVING VORTICITY CENTERS FROM NM TO THE
MID MS VALLEY ARE BEGINNING TO CONSOLIDATE INTO A PRIMARY TROUGH
THAT WILL EJECT NEWD OVER THE MID MS VALLEY TOWARD MI...IN RESPONSE
TO THE UPSTREAM HEIGHT FALLS/INCREASING MEAN FLOW.
...MS VALLEY AREA THIS AFTERNOON...
THE MS VALLEY MID LEVEL TROUGH IS ACCOMPANIED BY A WEAK SURFACE WAVE
IN SE MO/SRN IL ALONG A SURFACE WARM FRONT. THIS WAVE AND
BAROCLINIC ZONE WILL CONTINUE TO MOVE NNEWD IN CONJUNCTION WITH THE
EJECTING MID LEVEL WAVE...ALLOWING A DEEP MOISTURE PLUME TO ALSO
SPREAD NWD WITH A CONTINUING THREAT FOR THUNDERSTORMS. MOIST
PROFILES AND EFFECTIVE SRH OF 150-200 M2/S2 COULD SUPPORT WEAK
ROTATING STORMS NEAR THE WARM FRONT...BUT THE MARGINAL INSTABILITY
AND DEEP LAYER VERTICAL SHEAR SUGGEST THAT ANY TORNADO THREAT WILL
BE QUITE LIMITED. FARTHER S IN THE WARM SECTOR ACROSS
MS/AL/TN...THE THREAT FOR SEVERE STORMS WILL BE MARGINAL AT BEST
GIVEN LARGELY UNIDIRECTIONAL WIND PROFILES WITH ONLY 15-25 KT
LOW-MID LEVEL FLOW...AND MID LEVEL LAPSE RATES AOB 6 C/KM.
...CENTRAL PLAINS THIS EVENING INTO TONIGHT...
LOW-LEVEL MOISTURE /BOUNDARY LAYER DEWPOINTS IN THE MID 50S TO LOWER
60S/ IS RETURNING NWD ACROSS THE CENTRAL PLAINS IN ADVANCE OF THE
LEE TROUGH PRECEDING AMPLIFYING NRN ROCKIES MID LEVEL TROUGH. A
PLUME OF STEEP MID LEVEL LAPSE RATES ALREADY PRESENT OVER THE HIGH
PLAINS WILL COMBINE WITH THE RETURNING MOISTURE AND DAYTIME HEATING
TO SUPPORT A NARROW CORRIDOR OF MODERATE INSTABILITY THIS
AFTERNOON/EVENING. THE CAP WILL BE SUFFICIENT TO DELAY THUNDERSTORM
DEVELOPMENT UNTIL THIS EVENING ACROSS NEB/SD AS THE COLD FRONT
OVERTAKES THE LEE TROUGH...AND CONVECTION SHOULD PERSIST OVERNIGHT
IN ASSOCIATION WITH LOW-LEVEL WAA/FRONTOGENESIS. GIVEN THE STEEP
LAPSE RATES AND RATHER WEAK DEEP LAYER VERTICAL SHEAR IN ADVANCE OF
THE FRONT...THE STRONGEST STORMS MAY PRODUCE ISOLATED LARGE HAIL AND
STRONG OUTFLOW WINDS.
...NW CO AND VICINITY THIS AFTERNOON/EVENING...
THE STRONG MID LEVEL TROUGH AND ASSOCIATED COLD FRONT WILL MOVE SEWD
THROUGH TONIGHT...PROVIDING A FOCUS FOR THUNDERSTORM DEVELOPMENT.
REGIONAL 12Z SOUNDINGS FROM GJT AND SLC SHOW SOME POTENTIAL FOR WEAK
INSTABILITY THIS AFTERNOON AS SURFACE TEMPERATURES WARM WITHIN CLOUD
BREAKS...AND VERTICAL SHEAR WILL BE SUFFICIENT FOR ORGANIZED SEVERE
STORMS...INCLUDING SUPERCELLS. MARGINALLY SEVERE HAIL AND ISOLATED
DAMAGING GUSTS WILL BE THE MAIN THREATS THIS AFTERNOON/EVENING.
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