SWODY1
SPC AC 231256
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0756 AM CDT TUE SEP 23 2008
VALID 231300Z - 241200Z
...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTERNOON INTO EARLY
TONIGHT FOR ERN NEB/WRN IA INTO MN AND SE ND...
...NRN PLAINS/UPPER MS VALLEY REGION THROUGH EARLY TONIGHT...
AN INITIAL SHORTWAVE TROUGH WILL LIFT NEWD FROM WRN SD/CENTRAL ND
AND WEAKEN WHILE A STRONGER UPSTREAM WAVE OVER CENTRAL MT/NRN WY
PROGRESSES EWD OVER THE DAKOTAS THIS AFTERNOON AND MN TONIGHT. A
WEAKENING SURFACE COLD FRONT ACCOMPANIES THE LEAD SHORTWAVE TROUGH
FROM THE ERN DAKOTAS INTO CENTRAL NEB THIS MORNING. THIS FRONT WILL
CONTINUE TO MOVE SLOWLY EWD/SEWD INTO CENTRAL KS/ERN NEB/WRN IA/WRN
MN BY THIS EVENING.
THE OPERATIONAL NAM/GFS...AS WELL AS THE 4KM NMM AND ARW VERSIONS OF
THE WRF...MAINTAIN A PRE-FRONTAL CONVECTIVE BAND THROUGH THE DAY AND
SHOW AN INCREASE IN STORM COVERAGE/INTENSITY DURING THE AFTERNOON
FROM WRN IA INTO ERN MN. MODIFIED 12Z SOUNDINGS SUPPORT AFTERNOON
MLCAPE VALUES OF ONLY 500-750 J/KG ACROSS MN WHERE CLOUDS WILL
LIKELY LINGER THROUGH MUCH OF THE DAY...AND PERHAPS 1000-1500 J/KG
FARTHER SW INTO WRN IA AND CENTRAL KS WHERE SURFACE HEATING AND MID
LEVEL LAPSE RATES WILL BE GREATER. THE STRONGER DEEP LAYER VERTICAL
SHEAR IS FORECAST TO REMAIN ALONG AND W OF THE PRE-FRONTAL
CONVECTIVE BAND...WHILE LOW-LEVEL SHEAR WILL WEAKEN LATER THIS
MORNING AS THE LLJ CORE DEVELOPS NEWD INTO ONTARIO WITH THE LEAD
SHORTWAVE TROUGH. THE NET RESULT WILL BE A RATHER MODEST
COMBINATION OF VERTICAL SHEAR AND INSTABILITY E OF THE WEAKENING
COLD FRONT THIS AFTERNOON/EVENING. A FEW DAMAGING GUSTS AND
ISOLATED LARGE HAIL MAY OCCUR WITH THE MORE INTENSE/ORGANIZED STORMS
WITHIN THE BAND OF CONVECTION...BUT THE OVERALL SEVERE THREAT
APPEARS SOMEWHAT MARGINAL FOR A SLIGHT RISK.
A SEPARATE BAND OF CONVECTION WILL LIKELY DEVELOP THIS AFTERNOON
ACROSS CENTRAL ND/N CENTRAL SD AND SPREAD EWD THROUGH THE EVENING IN
ASSOCIATION WITH THE STRONGER UPSTREAM SHORTWAVE TROUGH NOW OVER MT.
THE BELT OF ASCENT AND THUNDERSTORM DEVELOPMENT WILL BE LOCATED W
OF THE COLD FRONT...THUS LOW-LEVEL MOISTURE WILL BE SOMEWHAT LIMITED
/BOUNDARY LAYER DEWPOINTS IN THE UPPER 40S/. STEEP LOW-MID LEVEL
LAPSE RATES AND COOLING MID LEVEL TEMPERATURES WILL COMPENSATE
SOMEWHAT FOR THE MARGINAL MOISTURE...SUCH THAT AFTERNOON SBCAPE
COULD APPROACH 500 J/KG. GIVEN THE RELATIVELY COOL TEMPERATURE
PROFILES AND STEEP LAPSE RATES...THE STRONGEST STORMS COULD PRODUCE
MARGINALLY SEVERE HAIL AND GUSTY WINDS THIS AFTERNOON INTO EARLY
THIS EVENING.
..THOMPSON/SMITH.. 09/23/2008
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