SPC AC 120538
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1238 AM CDT WED AUG 12 2009
VALID 121200Z - 131200Z
...THERE IS A SLGT RISK OF SVR TSTMS ALONG THE SERN ATLANTIC
COAST...
...THERE IS A SLGT RISK OF SVR TSTMS OVER PARTS OF THE CNTRL AND SRN
HIGH PLAINS...
...SYNOPSIS...
SHORT WAVE TROUGH CURRENTLY MOVING THROUGH THE GREAT LAKES AND
MID/LOWER MS VALLEY WILL WEAKEN AND UNDERGO ELONGATION OVER THE
APPALACHIANS LATER TODAY INTO TONIGHT. UPSTREAM...A MIDLEVEL
IMPULSE WILL TRANSLATE EWD FROM THE CNTRL HIGH PLAINS TOWARD THE MID
MO VALLEY IN ADVANCE OF STRONGER TROUGH DIGGING SEWD THROUGH WRN
CANADA INTO THE PACIFIC NW.
AT THE SURFACE...FRONTAL ZONE ALONG THE SERN ATLANTIC COASTAL PLAIN
WILL SERVE AS THE FOCUS FOR SCATTERED TO NUMEROUS TSTMS TODAY. OVER
THE HIGH PLAINS...MORE ISOLATED STORM DEVELOPMENT IS ANTICIPATED BY
LATE AFTERNOON OR THIS EVENING ALONG/E OF INTENSIFYING LEE TROUGH.
OTHER ISOLATED DIURNAL STORMS WILL BE POSSIBLE LATER TODAY ALONG
COLD FRONT STRENGTHENING OVER WRN MT.
...SERN ATLANTIC COAST...
12/00Z SOUNDING DATA INDICATE THAT A VERY MOIST AIR MASS IS IN PLACE
OVER THE COASTAL PLAINS WITH PW VALUES OF 2.00-2.25 INCHES. WHEN
COUPLED WITH DAYTIME HEATING...EXPECT ENVIRONMENT TO BECOME
MODERATELY UNSTABLE BY AFTERNOON WITH MLCAPE APPROACHING 2000-2500
J/KG. ABSENCE OF ANY APPRECIABLE CAPPING COUPLED WITH FRONTAL
CONVERGENCE AND DEEP ASCENT INTO ENTRANCE REGION OF UPPER JET STREAK
OVER THE MID ATLANTIC STATES SHOULD SUPPORT FAIRLY WIDESPREAD TSTM
DEVELOPMENT BY AFTERNOON. VERTICAL SHEAR WILL REMAIN MARGINAL...BUT
THE PRESENCE OF THE MODERATE INSTABILITY AND HIGH PW VALUES WILL
FAVOR THE DEVELOPMENT OF ORGANIZED STORM CLUSTERS CAPABLE OF WET
MICROBURST ACTIVITY.
...CNTRL/SRN HIGH PLAINS...
STORM INITIATION ALONG LEE TROUGH REMAINS SOMEWHAT UNCERTAIN TODAY
OWING TO THE PRESENCE OF EML AND RESULTANT CAP. WHILE LARGE-SCALE
FORCING FOR ASCENT IS FORECAST TO REMAIN NEGLIGIBLE...DIABATIC
HEATING AND SUSTAINED CONVERGENCE ALONG LEE TROUGH MAY BE SUFFICIENT
FOR ISOLATED STORM DEVELOPMENT LATER THIS AFTERNOON OR EVENING.
AMBIENT AIR MASS WILL BECOME MODERATELY UNSTABLE /MLCAPE OF
1500-2500 J PER KG/ BY AFTERNOON OWING TO THE PRESENCE OF THE STEEP
LAPSE RATES AND BOUNDARY LAYER DEWPOINTS IN THE 50S TO LOWER 60S.
PROXIMITY TO UPPER HIGH CENTERED OVER THE SRN ROCKIES WILL RESULT IN
RATHER WEAK NWLY MIDLEVEL FLOW ACROSS SURFACE TROUGH.
BUT...FORECAST HODOGRAPHS INDICATE CONSIDERABLE VEERING OF WINDS
WITH HEIGHT WITH AROUND 30 KT OF DEEP-LAYER SHEAR. THIS SETUP WILL
FAVOR THE DEVELOPMENT OF ORGANIZED/ROTATING STORMS CAPABLE OF HAIL
AND LOCALLY DAMAGING WIND GUSTS. AN ISOLATED WIND/HAIL THREAT WILL
LIKELY CONTINUE INTO TONIGHT ALONG NOCTURNAL LLJ AXIS WHERE STORMS
WILL BE MAINTAINED BY THE ENHANCED INFLOW OF A MOIST/UNSTABLE AIR
MASS.
...WRN/CNTRL MT...
DESPITE MARGINAL MOISTURE AND RESULTANT CONVECTIVE INSTABILITY
/MLCAPE BELOW 1000 J PER KG/...THE PRESENCE OF STEEP LAPSE RATES AND
INCREASED FORCING FOR ASCENT /ASSOCIATED WITH LOW AMPLITUDE
PERTURBATION/ WILL SUPPORT ISOLATED/SCATTERED STORM DEVELOPMENT
ALONG COLD FRONT LATE THIS AFTERNOON INTO EVENING. THE COMBINATION
OF 40-45 KT OF DEEP WLY SHEAR AND THE STEEP LAPSE RATES INDICATES
SOME POTENTIAL FOR MARGINAL SUPERCELL AND/OR BOWING STRUCTURES
CAPABLE OF ISOLATED DAMAGING WINDS AND HAIL.
..MEAD/JEWELL.. 08/12/2009
CLICK TO GET WUUS01 PTSDY1 PRODUCT
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1300Z
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