SWODY1
SPC AC 061246
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0746 AM CDT MON SEP 06 2010
VALID 061300Z - 071200Z
...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTERNOON INTO TONIGHT
OVER PARTS OF MN...IA AND WI...
...UPPER MIDWEST/UPPER MS VALLEY INTO THE LOWER MO VALLEY...
A COMPACT BUT POTENT SHORT WAVE TROUGH CURRENTLY OVER WY WILL
TRANSLATE RAPIDLY EWD TODAY FORMING A CLOSED MIDLEVEL CIRCULATION
OVER ERN SD INTO MN BY TONIGHT. THIS SYSTEM WILL BE ACCOMPANIED BY
A MIDLEVEL JET STREAK WHICH IS FORECAST TO STRENGTHEN TO 80-90 KT
AND A CORRIDOR OF SIGNIFICANT HEIGHT FALLS --I.E. 100-150 M/12 H--
SPREADING EWD ACROSS SD/NEB TODAY AND MN/IA/WI TONIGHT. AT THE
SURFACE...CYCLONE CURRENTLY ALONG THE NEB/KS BORDER E OF MCK WILL
CONTINUE TO DEEPEN WHILE ACCELERATING NNEWD INTO W-CNTRL MN BY LATE
AFTERNOON. THIS LOW WILL UNDERGO OCCLUSION TONIGHT WHILE
DECELERATING OVER CNTRL MN. MEANWHILE...STRONG COLD FRONT CURRENTLY
FROM THE WRN DAKOTAS SWD THROUGH WRN NEB INTO NWRN KS AND E-CNTRL CO
WILL SURGE EWD THROUGH THE MID/LOWER MO VALLEY AND UPPER MIDWEST BY
TUESDAY MORNING. THIS BOUNDARY WILL LIKELY SERVE AS THE FOCUS FOR
STRONG TO SEVERE TSTM DEVELOPMENT LATER THIS AFTERNOON INTO TONIGHT.
NW-SE ORIENTED BAND OF ELEVATED STRONG TO MARGINALLY SEVERE TSTMS IS
ONGOING THIS MORNING OVER WI...DRIVEN BY WAA/ISENTROPIC ASCENT IN
EXIT REGION OF 45-60 KT SWLY...NOCTURNAL LLJ AND DEEPER-LAYER
FORCING ATTENDANT TO A LOW AMPLITUDE PERTURBATION PROGRESSING ESEWD
THROUGH MN/WI. GIVEN THE PRESENCE OF MODEST INSTABILITY AND
RELATIVELY STRONG CLOUD-BEARING SHEAR...THE MOST INTENSE STORMS WILL
BE CAPABLE OF MARGINALLY SEVERE HAIL THIS MORNING. ACTIVITY SHOULD
GRADUALLY WEAKEN WITH THE CESSATION OF THE NOCTURNAL LLJ.
MEANWHILE...12Z UPSTREAM OBSERVED SOUNDINGS INDICATE THE PRESENCE OF
A STRONG CAP ASSOCIATED WITH A WELL-DEFINED EML WHICH HAS BEEN
ADVECTED EWD INTO THE PLAINS IN ADVANCE OF THE NRN/CNTRL ROCKIES
UPPER TROUGH. BENEATH THE EML...THE 12Z SOUNDING DATA AND
SATELLITE-DERIVED PW IMAGERY SHOW SIMILAR MOISTURE CONTENT EXTENDING
SWWD INTO NRN TX. THIS WOULD SUGGEST THAT THE HORIZONTAL ADVECTION
OF MOISTURE IN THE LOWER TROPOSPHERE WILL CONTRIBUTE LITTLE TO THE
DESTABILIZATION PROCESS WITHIN THE WARM SECTOR TODAY. AS SUCH...IT
APPEARS THAT THE CAP WILL REMAIN ROBUST AND LIKELY INHIBIT
SURFACE-BASED STORM DEVELOPMENT THROUGH MUCH OF THE DIURNAL HEATING
CYCLE.
LATEST MESOSCALE AND CONVECTION-ALLOWING MODEL GUIDANCE INDICATES
THAT TSTMS WILL BECOME INCREASINGLY LIKELY BY 07/00Z FROM VICINITY
OF SURFACE LOW SEWD ALONG BENT BACK PORTION OF COLD FRONT OVER
W-CNTRL INTO SRN MN AND NRN IA AS STRONG DYNAMIC FORCING FOR ASCENT
ASSOCIATED WITH THE APPROACHING UPPER WAVE BEGINS TO RAPIDLY ERODE
THE CAP. FARTHER SWD/SWWD ALONG THE FRONT --I.E. CNTRL/SRN IA INTO
NERN KS/NRN AND WRN MO-- STORM INITIATION AND SUSTENANCE BECOME MORE
UNCERTAIN DUE TO THE COMPACT NATURE OF THE UPPER TROUGH AND TENDENCY
FOR LARGE-SCALE FORCING TO RAPIDLY DIMINISH ON THE ANTICYCLONIC SIDE
OF THE ATTENDANT MIDLEVEL JET STREAK.
AMBIENT WARM SECTOR ENVIRONMENT --PARTICULARLY OVER SRN MN INTO IA--
WILL BE QUITE SUPPORTIVE OF ORGANIZED STORMS WHERE MLCAPE OF
1000-1500 J/KG WILL COINCIDE WITH 150-300 M2/S2 0-1 KM SRH AND 50-70
KT OF DEEP-LAYER SHEAR. A LARGELY NORMAL ORIENTATION OF THE
DEEP-LAYER SHEAR VECTOR TO THE SURFACE FRONT SUGGESTS THAT INITIAL
STORM MODE WOULD BE SUPERCELLULAR WITH THE THREAT FOR
HAIL...DAMAGING WINDS AND A FEW TORNADOES. SHOULD ACTIVITY GROW
UPSCALE INTO A LINEAR CONVECTIVE SYSTEM...THEN A RISK FOR DAMAGING
WIND GUSTS WILL CONTINUE AFTER DARK...MAINLY ALONG AND AHEAD OF THE
MIDLEVEL JET STREAK. CURRENTLY...UNCERTAINTIES IN THE DAYTIME
INITIATION OF STORMS WITHIN THE ZONE OF MOST FAVORABLE PARAMETERS
PRECLUDES AN INCREASE IN SEVERE WEATHER PROBABILITIES. HOWEVER...AN
UPGRADE MAY BE NECESSARY IN SUBSEQUENT OUTLOOKS.
...TX...
RECENTLY NAMED TS HERMINE IS FORECAST TO MOVE NWWD THROUGH THE WRN
GULF OF MEXICO TODAY AND INTO DEEP S TX TONIGHT. AN ASSOCIATED
INCREASE IN LOWER-TROPOSPHERIC SHEAR AND MOISTURE WILL ACCOMPANY
THIS SYSTEM INLAND...AND AT LEAST A MARGINAL RISK FOR A BRIEF
TORNADO OR TWO WILL EXIST ALONG THE LOWER TO MIDDLE COASTAL PLAIN
DURING THE LATTER HALF OF THE FORECAST PERIOD.
..MEAD/GRAMS.. 09/06/2010
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