SWODY1
SPC AC 221250
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0750 AM CDT MON SEP 22 2008
VALID 221300Z - 231200Z
...THERE IS A SLGT RISK OF SVR TSTMS LATE THIS AFTERNOON INTO
TONIGHT FROM WRN NEB TO THE DAKOTAS/NW MN...
...CENTRAL AND NRN PLAINS THIS AFTERNOON INTO TONIGHT...
A PRONOUNCED SHORTWAVE TROUGH NOW OVER NRN NV WILL EJECT NEWD TO
CENTRAL/ERN WY THIS EVENING AND THE DAKOTAS OVERNIGHT. LEE
CYCLOGENESIS WILL OCCUR DOWNSTREAM FROM THIS MID LEVEL TROUGH INVOF
THE NEB/WY/SD BORDER THIS AFTERNOON. THIS CYCLONE WILL THEN DEVELOP
NNEWD TOWARD NE ND LATE TONIGHT...WHILE A COLD FRONT MOVES EWD
ACROSS NEB AND THE DAKOTAS. THE SURFACE FRONT AND LEE CYCLONE WILL
HELP FOCUS WIDELY SCATTERED THUNDERSTORM DEVELOPMENT LATE THIS
AFTERNOON FROM SW NEB INTO THE CENTRAL DAKOTAS...WHILE MORE
WIDESPREAD CONVECTION IS EXPECTED TONIGHT ACROSS THE DAKOTAS WITH
THE SURFACE LOW AND MID LEVEL TROUGH.
A GRADUAL INCREASE IN LOW-LEVEL MOISTURE HAS OCCURRED DURING THE
PAST 24-48 HOURS...AND BOUNDARY LAYER DEWPOINTS NOW RANGE FROM 60-64
F FROM SE ND SWD TO ERN KS AND OK. RELATIVELY STEEP MID LEVEL LAPSE
RATES AND AN ASSOCIATED CAP WILL CONTINUE TO OVERSPREAD THE MOIST
AXIS FROM THE W...PRIOR TO MORE WIDESPREAD CONVECTIVE OVERTURNING
THAT IS EXPECTED TONIGHT AS THE CAP WEAKENS WITH THE ARRIVAL OF THE
MID LEVEL TROUGH AND FOCUSED ASCENT. EXPECT MLCAPE VALUES TO
INCREASE TO AROUND 1500 J/KG WITH DAYTIME HEATING FROM THE CENTRAL
DAKOTAS INTO NEB BY MID-LATE AFTERNOON. MEANWHILE...A SLY LLJ OF
35-45 KT WILL BE MAINTAINED DURING THE DAY FROM THE CENTRAL PLAINS
TO THE ERN DAKOTAS/WRN MN...AND THIS LLJ WILL LIKELY STRENGTHEN SOME
TONIGHT. MID LEVEL FLOW WILL ALSO INCREASE WITH THE APPROACH OF THE
MID LEVEL TROUGH FROM WY...LEADING TO SUFFICIENT VERTICAL SHEAR FOR
SUPERCELLS THIS AFTERNOON/EVENING FROM NEB INTO THE DAKOTAS.
BOUNDARY LAYER TEMPERATURE-DEWPOINT SPREADS WILL BECOME RELATIVELY
LARGE DURING THE AFTERNOON TO THE E OF THE LEE CYCLONE IN WRN
NEB...WHICH SUGGESTS THAT LARGE HAIL AND ISOLATED DAMAGING GUSTS
WILL BE THE MAIN SEVERE THREATS. SOMEWHAT RICHER MOISTURE AND
COOLER SURFACE TEMPERATURES ARE EXPECTED ALONG THE SURFACE BOUNDARY
LATE THIS AFTERNOON/EVENING ACROSS THE CENTRAL DAKOTAS...WHERE THE
THREAT FOR A TORNADO OR TWO SHOULD BE GREATER AS LOW-LEVEL SHEAR
INCREASES THIS EVENING. OTHERWISE...LARGE HAIL AND DAMAGING WINDS
WILL CONTINUE TO BE POSSIBLE INTO THE OVERNIGHT HOURS ACROSS CENTRAL
SD AND ERN ND AS CONVECTION LIKELY GROWS UPSCALE INTO AN MCS/SQUALL
LINE...THOUGH DIMINISHING INSTABILITY WILL BEGIN TO LIMIT THE SEVERE
THREAT BY ABOUT 06Z.
..THOMPSON/SMITH.. 09/22/2008
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