SPC AC 081928
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0228 PM CDT TUE SEP 08 2009
VALID 082000Z - 091200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS PARTS OF THE NORTHERN
AND CENTRAL PLAINS...
...CNTRL/NRN PLAINS...
AS PREVIOUSLY FORECAST...STRONGEST BOUNDARY LAYER HEATING IS
OCCURRING AHEAD OF COLD FRONT FROM SCNTRL SD...SWWD INTO THE CNTRL
HIGH PLAINS. AS A RESULT THUNDERSTORMS ARE BEGINNING TO DEVELOP
OVER THE HIGHER TERRAIN FROM SERN WY...SWD ALONG THE FRONT RANGE OF
CO TO THE PALMER DIVIDE. THIS ACTIVITY IS EXPECTED TO GRADUALLY
INCREASE IN BOTH AREAL COVERAGE/INTENSITY AS CONVECTION SPREADS INTO
INCREASING AXIS OF MOISTURE/INSTABILITY. FARTHER NORTH...WHILE
STRONGER FORCED LINES OF CONVECTION PERSIST FROM NCNTRL SD...NEWD
INTO NWRN MN...THIS ACTIVITY MAY STRUGGLE TO ATTAIN SEVERE
LEVELS...PRIMARILY DUE TO A BIT WEAKER INSTABILITY/LAPSE RATE
ENVIRONMENT...ESPECIALLY ACROSS ERN ND/NWRN MN. FOR THIS REASON
HAVE LOWERED SEVERE PROBABILITIES NEAR THE CANADIAN BORDER.
OTHERWISE...EARLIER FORECAST APPEARS TO BE ON TRACK.
...SRN PLAINS...
MUCH OF KS REMAINS UNDER THE INFLUENCE OF LONG-LIVED THUNDERSTORM
COMPLEX. OUTFLOW FROM THIS ACTIVITY HAS EASED INTO NRN OK AND
STRETCHES FROM JUST NORTH OF TUL...WWD TO NE OF GAG. A CONSIDERABLE
AMOUNT OF BOUNDARY LAYER CONVECTION IS DEVELOPING ACROSS MUCH OF THE
WARM SECTOR OVER OK WHERE LOW LEVEL LAPSE RATES ARE QUITE
STEEP...SFC-3KM IN EXCESS OF 9 C/KM...WITH SFC TEMPERATURES AOA 90F.
AFOREMENTIONED BOUNDARY MAY SERVE AS THE FOCUS FOR POTENTIALLY
ROBUST CONVECTION...ESPECIALLY GIVEN THE WEAK MCV...NEAR CDS AT
19Z...APPROACHING SWRN OK.
..DARROW.. 09/08/2009
.PREV DISCUSSION... /ISSUED 1142 AM CDT TUE SEP 08 2009/
...TRANSMISSION DELAYED BY COMPUTER PROBLEMS...
...SYNOPSIS...
MT SHORTWAVE TROUGH WILL MOVE E INTO WRN ND THIS EVE...BEFORE
TURNING NE INTO WRN ONTARIO EARLY WED AS PARENT UPR LOW MOVES SLOWLY
E FROM NRN AB TO NRN MB. OTHERWISE...WEAK MID/UPR LVL FLOW WILL
PERSIST OVER THE REMAINDER OF THE LWR 48. IND CLOSED LOW SHOULD
CONTINUE SLOWLY SE INTO ERN KY BY THE END OF THE PERIOD. IN
RESPONSE...UPR IMPULSE NOW OVER ERN NC LIKELY WILL PIVOT NNEWD...
TAKING ATTENDANT SFC LOW NOW NEAR HATTERAS TO OFF THE MD/VA CST.
BACK WEST...OBSERVED AND FORECAST WIND FIELDS SUGGEST THAT KS MCV
WILL MOVE ONLY VERY SLOWLY E/ESE TODAY...AND MAY REFORM SOMEWHERE
NEAR ITS CURRENT POSITION EARLY WED.
...CNTRL/NRN PLNS THIS AFTN/TONIGHT...
PRESSURE RISES NOW OVER THE NRN HI PLNS SHOULD KEEP COLD FRONT
ASSOCIATED WITH MT IMPULSE MOVING STEADILY E ACROSS THE DAKOTAS
TODAY...AND INTO ERN SD/NW MN TONIGHT. THE FRONT WILL ALSO PROGRESS
SE ACROSS WRN NEB/NE CO/NW KS LATER TODAY/TONIGHT... PRECEDED BY A
LEE TROUGH.
CLOUDS/LOW LVL OUTFLOW ASSOCIATED WITH OVERNIGHT QLCS LIKELY WILL
HINDER LOW LVL DESTABILIZATION OVER ERN ND AND WRN MN TODAY. MORE
SUBSTANTIAL DESTABILIZATION SHOULD OCCUR FARTHER S OVER SD/NEB
...WHERE STG HEATING AND LOW-MID 60S SFC DEWPOINTS WILL EXIST
BENEATH EML. COMBINATION OF HEATING AND ASCENT ALONG COLD FRONT/
TROUGH SHOULD REMOVE CIN TO SUPPORT TSTM DEVELOPMENT BY MID-LATE
AFTN...WITH A BROKEN BAND OF STORMS LIKELY ALONG THE LENGTH
OF THE FRONT EVE.
ORIENTATION OF FRONT WITH RESPECT TO UPR JET WILL KEEP STRONGER FLOW
ALOFT N AND W OF THE FRONT...ESPECIALLY IN THAT PORTION OF WARM
SECTOR LIKELY TO UNDERGO GREATEST DESTABILIZATION. NEVERTHELESS...
SUFFICIENT...LARGELY UNIDIRECTIONAL DEEP SHEAR SHOULD EXIST FOR
BOWING SEGMENTS AND MARGINAL SUPERCELL STRUCTURES AS SBCAPE
APPROACHES 2000-2500 J/KG. THE PRIMARY SVR THREATS WILL BE ISOLD
DMGG WIND AND HAIL THIS AFTN INTO AT LEAST EARLY TONIGHT.
...SRN KS/NRN OK TODAY THROUGH TONIGHT...
KS MCV APPEARS TO HAVE A WELL-DEFINED PATTERN OF UPR LVL OUTFLOW
ATTM...SUGGESTING THAT IT MIGHT PERSIST IN AT LEAST A WEAKENED STATE
THROUGH THE DAY. COMBINATION OF ENHANCED UPR DIVERGENCE AND RELATED
ASCENT ON PERIPHERY OF MID LVL CIRCULATION LIKELY WILL INFLUENCE
DIURNAL STORM DEVELOPMENT OVER PARTS OF KS AND PERHAPS NRN OK/WRN
MO/NW AR THROUGH THIS EVE. SOME OF THOSE STORMS COULD YIELD ISOLD
DMGG WIND ANDS PERHAPS SVR HAIL GIVEN CONSIDERABLE SBCAPE /2000 J
PER KG/ AND HIGH PW.
CONTINUED EWD ADVANCE OF MT/DAKOTAS IMPULSE...AND NOCTURNAL
DECOUPLING...SHOULD STRENGTHEN/DEEPEN SSWLY LLJ OVER THE CNTRL PLNS
TONIGHT. THIS COULD SUPPORT REGENERATION OF EXISTING MCV...OR THE
DEVELOPMENT OF A NEW ONE...RELATIVELY CLOSE PRESENT CIRCULATION.
THUS...COULD SEE ANOTHER ROUND OF HEAVY RAIN AND LOCALLY DMGG WIND
OVER MUCH THE SAME REGION OVERNIGHT.
...W TX...
A FEW STG PULSE STORMS MAY AFFECT PARTS OF W TX TODAY...IN AREA OF
STRONGLY HEATED/MODERATELY MOIST SLY FLOW AHEAD OF FAR W TX UPR
VORT.
...TN/KY/OH/SE INDIANA TODAY...
HEATING ON ERN/SRN FRINGE OF IND UPR LOW AND RELATIVELY COOL MID LVL
TEMPERATURES WILL YIELD AFTN SBCAPE OF 500-1000 J/KG...AND SCTD
DIURNAL TSTMS. WEAK VERTICAL SHEAR MARGINAL INSTABILITY SHOULD
LIMIT SVR RISK.
...NC OUTER BANKS/FAR SE VA...
SVR THREAT APPEARS TO BE DIMINISHING OVER THE NC OUTER BANKS AS SFC
LOW AND ASSOCIATED UPR IMPULSE CONTINUE NNEWD. MOIST AND MODERATELY
SHEARED WARM SECTOR...AND ASSOCIATED CONVECTION WITH THE POTENTIAL
FOR LOW LVL ROTATION...SHOULD THEREFORE REMAIN OFFSHORE THROUGH THE
PERIOD.
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