SPC AC 241951
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0251 PM CDT MON AUG 24 2009
VALID 242000Z - 251200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS ERN ND AND NWRN MN...
...NRN PLAINS...
SEVERAL MORE HOURS OF HEATING AND PERSISTENT SYNOPTIC SCALE FORCING
FOR ASCENT WILL BE NEEDED TO OVERCOME PRONOUNCED CONVECTIVE
INHIBITION ACROSS THE DAKOTAS AND NRN MN. FORCING NEAR THE SURFACE
LOW...AND ALONG ADVANCING FRONT...SHOULD THEN BE SUFFICIENT FOR TSTM
DEVELOPMENT WITHIN MOIST AND MODERATELY UNSTABLE AIRMASS. SHEAR
ACROSS THE REGION COULD RESULT IN A FEW MORE PERSISTENT STORMS WITH
POTENTIAL FOR HAIL/WIND. SEE PREVIOUS DISCUSSION FOR ADDITIONAL
DETAILS.
...CENTRAL HIGH PLAINS...
EXPANDED LOW PROBABILITY SEVERE WIND POTENTIAL ACROSS PARTS OF WRN
KS AND ERN/SERN CO WHERE PERSISTENT HEATING AND LOW LEVEL
CONVERGENCE MAY PROMOTE HIGH-BASED TSTM DEVELOPMENT WITHIN SURFACE
TROUGH AXIS NEXT FEW HOURS. NAM-WRF FORECASTS INDICATE A PERSISTENT
CORRIDOR OF CONVECTION ACROSS THESE AREAS THROUGH LATE EVENING WHERE
HOT AND DEEPLY MIXED BOUNDARY LAYER COULD SUPPORT ISOLATED DAMAGING
DOWNBURST WINDS.
...MID ATLANTIC...
LOW SEVERE STORM PROBABILITIES HAVE BEEN REMOVED FROM THE
CAROLINAS/ERN VIRGINIA AREAS FOR THE REMAINDER OF THIS FORECAST
PERIOD. WHILE CHANCE FOR A COUPLE OF STRONGER STORMS STILL
EXISTS...LACK OF MORE PRONOUNCED FORCING FOR ASCENT...AND GENERALLY
WEAKENING DEEP-LAYER FLOW REGIME...SHOULD LIMIT MORE ORGANIZED
DEVELOPMENT.
..CARBIN/COOK.. 08/24/2009
.PREV DISCUSSION... /ISSUED 1119 AM CDT MON AUG 24 2009/
...EXTREME ERN ND/NWRN MN...
UPPER TROUGH FROM SASKATCHEWAN INTO MT IS FORECAST TO MOVE EWD
ACROSS THE CANADIAN PRAIRIE PROVINCES AND ND THROUGH THE
PERIOD...WITH STRONGER MID LEVEL WINDS TRANSLATING ACROSS THE NRN
BORDER REGION. SURFACE LOW CURRENTLY OVER CENTRAL ND IS EXPECTED TO
MOVE ENEWD ACROSS NERN ND/EXTREME NWRN MN LATE THIS AFTERNOON AND
EVENING...CONTINUING TOWARD CENTRAL ONTARIO TONIGHT. TRAILING COLD
FRONT EXTENDING SWWD FROM THE LOW WILL MOVE EWD ACROSS THE NRN
PLAINS...REACHING A MN ARROWHEAD/SWRN MN/CENTRAL NEB/ERN CO BY
25/12Z.
MOISTURE IS EXPECTED TO GRADUALLY INCREASE NNEWD ACROSS THE ERN DAKS
INTO MN AHEAD OF THE ADVANCING COLD FRONT...IN ASSOCIATION WITH A
30-40 KT LOW LEVEL JET. SURFACE DEW POINTS ARE EXPECTED TO RISE
INTO THE MID-UPPER 60S THIS AFTERNOON...BENEATH A LAYER OF STEEP
LAPSE RATES/ELEVATED MIXED LAYER THAT HAS SPREAD EWD ACROSS THE NRN
PLAINS INTO THE UPPER MS VALLEY. WEAK ELEVATED CONVECTION MOVING
ACROSS ERN ND IS EXPECTED TO DIMINISH THIS AFTERNOON WITH A CAPPING
INVERSION INHIBITING DEVELOPMENT OF SURFACE-BASED CONVECTION THROUGH
MUCH OF THE AFTERNOON.
BREAKS IN CLOUD COVER/THINNING OF CLOUDS IS EXPECTED TO SPREAD NEWD
OVER ERN ND AND NWRN MN AHEAD OF THE COLD FRONT THIS AFTERNOON. THIS
WILL PERMIT LOCALIZED AREAS OF STRONGER DIABATIC HEATING TO OCCUR
WHICH WILL GRADUALLY WEAKEN THE CAP. THIS WILL RESULT IN
DESTABILIZATION WITH MLCAPE OF 1500-2000 J/KG LATER THIS AFTERNOON
AND EVENING. FOCUSED MESOSCALE ASCENT ALONG THE COLD FRONT COUPLED
WITH DYNAMIC FORCING ALOFT WITHIN THE RIGHT ENTRANCE REGION OF THE
UPPER LEVEL JET ARE EXPECTED TO BE SUFFICIENT FOR THUNDERSTORMS TO
DEVELOP LATE THIS AFTERNOON OR EVENING OVER EXTREME ERN ND OR NWRN
MN. WIND PROFILES ARE EXPECTED TO VEER AND INCREASE WITH HEIGHT
WITH RESULTANT 35-45 KT DEEP LAYER SHEAR SUFFICIENT FOR ORGANIZED
STRONG/SEVERE STORMS TO DEVELOP...INCLUDING POTENTIAL FOR ISOLATED
SUPERCELLS. STRONGER CELLS WILL BE CAPABLE OF PRODUCING LARGE HAIL
AND DAMAGING WIND GUSTS. ACTIVITY IS LIKELY TO SPREAD EWD DURING
THE OVERNIGHT HOURS...ALTHOUGH SEVERE POTENTIAL IS EXPECTED TO
DIMINISH BY 03-6Z AS BOUNDARY LAYER COOLING OCCURS.
...ERN WY INTO SD AND NEB...
WATER VAPOR IMAGERY SHOWS A MID/UPPER LEVEL TROUGH LIFTING NEWD FROM
THE FOUR-CORNERS REGION TOWARD THE CENTRAL ROCKIES...AND MODEL
GUIDANCE INDICATES THIS TROUGH MOVE INTO THE CENTRAL PLAINS TONIGHT.
THE DEEP LAYER MOISTURE PLUME AHEAD OF THIS TROUGH OVER THE ROCKIES
IS EXPECTED TO INTERACT WITH THE COLD FRONT MOVING SWD INTO THE
CENTRAL HIGH PLAINS LATER THIS AFTERNOON. LOW LEVEL MOISTURE MAY
ALSO BE ENHANCED BY INCREASING SURFACE DEW POINTS ALONG/NORTH OF THE
FRONT. SCATTERED THUNDERSTORMS ARE EXPECTED TO INCREASE FROM ERN WY
EWD INTO THE HIGH PLAINS OF WRN NEB AND SWRN SD BY THIS EVENING.
STORMS ARE LIKELY TO SPREAD EWD ACROSS PARTS OF NEB AND SD TONIGHT
AS A SECONDARY LOW LEVEL JET DEVELOPS OVER THE CENTRAL
PLAINS...ENHANCING CONVERGENCE AND WARM ADVECTION OVER THE AREA.
THE CONVECTIVE ACTIVITY WILL BE SOUTH OF THE STRONGER WINDS
ALOFT...AND GENERALLY MARGINAL SHEAR IS EXPECTED TO LIMIT THE
OVERALL COVERAGE OF THE SEVERE THREAT.
...NC/VA...
THE MAIN UPPER TROUGH AXIS WILL TRACK EASTWARD ACROSS THE MID
ATLANTIC AND CAROLINAS TODAY. RELATIVELY COOL TEMPERATURES ALOFT
/-11C AT 500 MB/ COUPLED WITH BREAKS IN CLOUD COVER/STRONG HEATING
WILL RESULT IN MLCAPE VALUES OF 1000-1500 J/KG. SCATTERED
THUNDERSTORMS ARE EXPECTED TO DEVELOP LATER TODAY FROM EAST TN INTO
MUCH OF VA/NC. A FEW STRONG MULTICELL STORMS ARE POSSIBLE WITH A
RISK OF HAIL AND GUSTY WINDS...BUT WEAK VERTICAL SHEAR SUGGEST THAT
AN ORGANIZED SEVERE STORM THREAT IS UNLIKELY TO DEVELOP.
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