SPC AC 231958
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0258 PM CDT THU JUL 23 2009
VALID 232000Z - 241200Z
...THERE IS A SLGT RISK OF SVR TSTMS OVER ERN ND AND PARTS OF
NRN/CNTRL MN...
...THERE IS A SLGT RISK OF SVR TSTMS OVER PART OF WRN NEB...
...PARTS OF WRN/CNTRL NEB...
CLOUD-FREE SKIES HAVE ALLOWED FOR STRONG DIABATIC HEATING TO OCCUR
TODAY WITH TEMPERATURES WARMING THROUGH THE 80S TO LOWER 90S. WHEN
COUPLED WITH STEEP MIDLEVEL LAPSE RATES AND A RATHER MOIST BOUNDARY
LAYER /NAMELY ALONG AND E OF A ANW-LBF-MCK LINE/...AIR MASS HAS
BECOME MODERATELY UNSTABLE WITH MLCAPE OF 1500-2000 J/KG.
LATEST VISIBLE IMAGERY INDICATES A GROWING CUMULUS FIELD OVER
N-CNTRL NEB...NEAR A PRE-FRONTAL TROUGH EXTENDING FROM WEAK SURFACE
LOW OVER N-CNTRL SD SSWWD INTO THE NEB PNHDL. WHILE TSTMS HAVE
ALREADY FORMED OVER THE HIGHER TERRAIN OF ERN WY/CO WITHIN A DRIER
AND DEEPER-MIXED BOUNDARY LAYER...IT REMAINS UNCERTAIN AS TO WHETHER
STORMS WILL DEVELOP FARTHER E WHERE CAPPING WILL REMAIN MORE
PROBLEMATIC. GIVEN CURRENT VISIBLE SATELLITE TRENDS AND LATEST
SHORT TERM MODEL GUIDANCE...THERE APPEARS TO BE A SUFFICIENT THREAT
FOR AT LEAST ISOLATED STORM FORMATION THAT HIGHER SEVERE
PROBABILITIES AND A CATEGORICAL SLIGHT RISK WILL BE ADDED.
CURRENT MERRIMAN NEB PROFILER INDICATES VEERING WINDS THROUGH THE
3-6 KM AGL LEVEL WITH AROUND 35 KT OF DEEP-LAYER SHEAR.
THUS...ENVIRONMENT WILL BE SUPPORTIVE OF ORGANIZED STORMS INCLUDING
SUPERCELLS GIVEN STORM INITIATION AND SUSTENANCE. LARGE HAIL AND
DAMAGING WINDS WILL BE THE PRIMARY SEVERE THREATS...THOUGH AN
ISOLATED TORNADO IS POSSIBLE.
FOR ADDITIONAL NEAR TERM GUIDANCE ON THIS AREA...SEE MCD 1650.
ELSEWHERE...LOW WIND/HAIL PROBABILITIES WHERE ADJUSTED SWD/SEWD
THROUGH NRN IL TO ACCOMMODATE ONGOING STORMS OVER THAT AREA.
OTHERWISE...PREVIOUS FORECAST APPEARS TO BE IN GOOD SHAPE WITH ONLY
SMALL CHANGES MADE.
..MEAD/ROBINSON.. 07/23/2009
.PREV DISCUSSION... /ISSUED 1105 AM CDT THU JUL 23 2009/
...UPPER MS VALLEY,..
S/WV TROUGH DROPPING SEWD FROM SASKATCHEWAN/MANITOBA INTO UPPER MS
VALLEY TONIGHT. SYSTEM PRECEEDED BY WARM ADVECTION AND STEEPENING
LAPSE RATES AND SOME MOISTENING ACROSS UPPER MS VALLEY THIS
AFTERNOON. WHILE THE MLCAPES AHEAD OF TROUGH AND ASSOCIATED COLD
FRONT WILL GENERALLY BE LESS THAN 1000 J/KG GIVEN LIMITED LOW LEVEL
MOISTURE...I.E. DEWPOINTS NO MORE THAN 60F...THE COMBINATION OF
STEEP LAPSE RATES...8C/KM...AND IMPRESSIVE VEERING SHEAR
PROFILES...WILL SUPPORT POTENTIAL FOR ROTATING STORMS ALONG/AHEAD OF
COLD FRONT BY THIS EVENING. THUNDERSTORMS SHOULD DEVELOP BY LATE
THIS AFTERNOON AS SFC TEMPS RISE THROUGH THE 80S AND DISSIPATE THE
CINH. PRIMARY THREAT WILL BE LARGE HAIL AND DOWNBURST
WINDS...PARTICULARLY WITH ANY SUPERCELL. ACTIVITY SHOULD TRACK SEWD
WITH SEVERE THREAT DIMINISHING BY LATE EVENING WITH LOSS OF DAYTIME
HEATING.
...CENTRAL HIGH PLAINS...
SIMILAR PATTERN TO WED THIS AREA WITH NWLY FLOW PROVIDING BOTH STEEP
LAPSE RATES AND 30-35 KT OF EFFECTIVE SHEAR. WHILE SCATTERED
THUNDERSTORMS EXPECTED TO DEVELOP BY MID/LATE AFTERNOON...IT APPEARS
TODAY THAT COVERAGE SHOULD BE LESS GIVEN GRADUAL MID LEVEL WARMING
AND RATHER WEAK MID LEVEL FLOW. HOWEVER PARAMETERS STILL SUPPORT
ISOLATED ROTATING STORMS BY LATER THIS AFTERNOON WITH PRIMARY THREAT
LARGE HAIL. FOR NOW WILL KEEP RISK BELOW SLIGHT...BUT WILL
RECONSIDER A POSSIBLE UPGRADE IN AFTERNOON OUTLOOK IF INSTABILITY
PROVES TO BE GREATER THAN NOW EXPECTED.
...INTERIOR PAC NW...
COMPACT UPPER LOW NOW MOVING INLAND WA COAST AND BY END OF PERIOD
SHOULD BE LOCATED VICINITY NERN WA. E OF CASCADES AIRMASS INITIALLY
RATHER DRY WITH STEEP LAPSE RATES. COMBINATION OF MID LEVEL COOLING
AND ASCENT WITH UPPER LOW WILL SUPPORT INCREASING THREAT OF
THUNDERSTORMS FROM CASCADES EWD TO ID PANHANDLE LATER THIS AFTERNOON
AND EVENING. EVEN WITH THE APPROACHING UPPER LOW...INSTABILITY
EXPECTED TO REMAIN SOMEWHAT MARGINAL FOR A SEVERE THREAT GIVEN THE
LIMITED MOISTURE. PRIMARY THREAT THIS REGION WILL BE A FEW STRONG
STORMS WITH HAIL POTENTIAL AND LOCALLY DAMAGING WIND THREAT.
...AZ...
EXTENSIVE MID CLOUDINESS DESERT VALLEY OF SRN AND WRN AZ THIS AM ARE
IN THE PROCESS OF THINNING/DISSIPATING. HOWEVER SURFACE HEATING
WILL BE SLOWED. EVEN WITH THE NOW RATHER MOIST AIR MASS ACROSS AZ
DESERTS VALLEYS...POTENTIAL FOR SEVERE THUNDERSTORMS SEEMS SMALL AS
REQUIRED LAPSE RATES FOR DOWNBURSTS WINDS WILL BE LESSENED GIVEN THE
DELAYED HEATING.
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