SPC AC 201629
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1129 AM CDT THU AUG 20 2009
VALID 201630Z - 211200Z
...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTERNOON/EVENING FROM THE
UPPER OH VALLEY SWWD TO THE RED RIVER VALLEY...
...OH VALLEY TO THE SRN PLAINS THIS AFTERNOON/EVENING...
THE DEEP OCCLUDED LOW OVER THE UPPER MS VALLEY WILL MOVE SLOWLY EWD
TO THE UPPER GREAT LAKES THROUGH TONIGHT. AN ASSOCIATED SURFACE
COLD FRONT WILL ALSO MOVE SLOWLY SEWD ACROSS THE SRN PLAINS/MID MS
VALLEY TO INDIANA/MI. THE COLD FRONT IS PRECEDED BY A
TROUGH/CONFLUENCE BOUNDARY ACROSS CENTRAL/NE INDIANA...AND AN
RESIDUAL OUTFLOW BOUNDARY FROM OVERNIGHT CONVECTION THAT NOW EXTENDS
FROM WRN OH SWWD ACROSS CENTRAL KY/WRN TN/NW MS...THEN WWD ACROSS
SRN AR/OK. WIDESPREAD CONVECTION IS ONGOING N OF THE BOUNDARY FROM
CENTRAL OK EWD INTO AR...AND IT APPEARS THAT THE STABILIZING IMPACTS
OF WIDESPREAD CLOUDS AND CONVECTIVE OVERTURNING WILL LIMIT
INSTABILITY ALONG THE SYNOPTIC FRONT FARTHER N IN OK/AR/MO. THE
THREAT FOR SEVERE STORMS SHOULD BE CONFINED PRIMARILY TO THE
SWD-MOVING OUTFLOW BOUNDARY FROM THE RED RIVER VALLEY EWD TO SRN
AR/NW MS THIS AFTERNOON. THE OUTFLOW BOUNDARY CORRIDOR WILL BE JUST
S/SE OF THE BELT OF STRONGER MID-UPPER FLOW AND ASSOCIATED VERTICAL
SHEAR...THUS MULTICELL STORMS WITH ISOLATED LARGE HAIL AND DAMAGING
DOWNBURST WINDS WILL BE THE MAIN THREATS.
BETTER PHASING OF SURFACE HEATING AND THE STRONGER LOW-MID LEVEL
FLOW IS EXPECTED THIS AFTERNOON ALONG THE WEAKENING OUTFLOW BOUNDARY
FROM NRN KY INTO CENTRAL OH...AND FARTHER W ALONG THE CONFLUENCE
BOUNDARY FROM CENTRAL INDIANA NEWD INTO SE MI/NW OH. MLCAPE WILL
INCREASE TO THE 1500-2000 J/KG RANGE BY EARLY AFTERNOON AS SURFACE
TEMPERATURES REACH 80-85 F AND DEWPOINTS REMAIN NEAR 70 F...DESPITE
RATHER MODEST MID LEVEL LAPSE RATES. THE COMBINATION OF MODERATE
INSTABILITY WITH LARGELY UNIDIRECTIONAL LOW-MID LEVEL FLOW OF 35-50
KT ON THE ERN PERIPHERY OF THE LARGE MID LEVEL LOW...AND EFFECTIVE
BULK SHEAR NEAR 35 KT...WILL SUPPORT STORMS WITH SOME SUPERCELL
STRUCTURES AND PERSISTENT LINE SEGMENTS CAPABLE OF PRODUCING
DAMAGING WINDS AND ISOLATED LARGE HAIL.
...CENTRAL/ERN PA AND NY THIS AFTERNOON...
A MOIST ENVIRONMENT PERSISTS ACROSS MUCH OF SRN/WRN NY AND PA TO THE
SW OF A RETREATING WARM FRONT. ASIDE FROM WRN NY/PA...MUCH OF THIS
AREA IS REMOVED TO THE EAST OF THE STRONGER DEEP LAYER
FLOW/SHEAR...AND RATHER POOR MID LEVEL LAPSE RATES WILL LIMIT THE
MAGNITUDE OF INSTABILITY. ISOLATED DAMAGING WINDS WILL BE POSSIBLE
THIS AFTERNOON WITH WARM SECTOR STORMS...BUT THE OVERALL THREAT
APPEARS TO WARRANT NO MORE THAN LOW SEVERE STORM PROBABILITIES.
..THOMPSON/GARNER.. 08/20/2009
CLICK TO GET WUUS01 PTSDY1 PRODUCT
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 2000Z
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