SWODY2
SPC AC 201728
DAY 2 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1228 PM CDT THU SEP 20 2007
VALID 211200Z - 221200Z
..THERE IS A SLGT RISK OF SVR TSTMS ACROSS PORTIONS OF THE UPR MS
VLY AND WRN GRTLKS REGION...
..UPR MS VLY/UPR GRTLKS REGION...
POTENT MID-LEVEL WAVE CURRENTLY DIGGING SEWD ACROSS THE CANADIAN
ROCKIES WILL REACH THE NRN PLAINS TONIGHT...THEN TRANSLATE EWD INTO
THE UPR GRTLKS REGION BY FRIDAY NIGHT. THIS SYSTEM WILL BE
ACCOMPANIED BY 90+ METER/12 HR HEIGHT FALLS AND 75+ KT H5 WINDS. A
SFC LOW OVER ERN ND AT 12Z FRI WILL DEEPEN INTO A SUB-990 MB LOW AS
IT MOVES INTO NRN ONTARIO BY 00Z SAT. TRAILING THIS LOW...A STRONG
COLD FRONT WILL SWEEP SEWD ACROSS THE UPR MS VLY AND UPR GRTLKS FRI
AFTN/NIGHT. THE FRONT WILL BE A FOCUS FOR POSSIBLE SVR TSTMS.
PRE-EXISTING STRONG SLY LOW-LEVEL FLOW IN ADVANCE OF THE APPROACHING
TROUGH WILL MAINTAIN A HEALTHY ZONE OF WARM ADVECTION ACROSS THE UPR
MS VLY REGION TONIGHT INTO EARLY FRIDAY. THIS SHOULD SUPPORT
SEVERAL TSTM CLUSTERS OVER A BROAD AREA FROM NRN/CNTRL MN EWD INTO
NRN WI...SOME WITH LARGE HAIL...FRIDAY MORNING. THIS ACTIVITY
SHOULD SHIFT ENEWD TOWARD THE LAKE SUPERIOR AREA BY AFTN.
AIR MASS WILL LIKELY DESTABILIZE S OF THE MORNING ACTIVITY ACROSS
THE UPR MS VLY...THOUGH THERE IS SOME UNCERTAINTY HOW MORNING
ACTIVITY WILL MODULATE THE NWD EXPANSION OF THIS INSTABILITY AXIS.
CURRENT ANTICIPATION IS FOR AN AXIS OF 1000-2500 J/KG MLCAPE TO
EXIST AHEAD OF THE COLD FRONT FROM NERN IA INTO NWRN WI BY EARLY
AFTN. AS STRONGER ASCENT OVERSPREADS THE WARM SECTOR...TSTMS SHOULD
ERUPT ALONG THE FRONT. FCST SOUNDINGS SUGGEST THAT BOTH LOW AND
DEEP LAYER SHEARS WILL BE SUPPORTIVE FOR SUPERCELLS WITH LARGE HAIL.
IF CELLS CAN REMAIN DISCRETE FOR A SUFFICIENT TIME...TORNADOES MAY
OCCUR. BUT...ORIENTATION OF THE 2-8KM SHEAR WITH RESPECT TO THE
BOUNDARY AND MAGNITUDE OF THE FORCING SHOULD YIELD A SQUALL LINE
WITH PRIMARILY DAMAGING WINDS. THE STORMS SHOULD PEAK IN INTENSITY
BY LATE AFTN...THEN WEAKEN DURING THE EVENING BEFORE REACHING LAKE
MI.
..CNTRL/ERN GULF COAST...
NON-TROPICAL UPR LOW WEST OF THE FL PENINSULA MAY ACQUIRE
SUB-TROPICAL CHARACTERISTICS AND DRIFT WWD JUST OFF THE CNTRL/ERN
GULF COAST ON FRIDAY. RATHER STRONG BELT OF SLY LOW-LEVEL
FLOW/SHEAR WILL EXIST ALONG THE ERN QUADRANT OF THE SYSTEM. COUPLED
WITH LOW LCL/S AND THREAT FOR CONVECTIVE BANDS MOVING
ONSHORE...THREAT FOR ISOLD TORNADOES WILL EXIST.
..LWR CO RVR VLY NWD INTO THE SRN NV...
BROAD WARM CONVEYOR WILL EVOLVE WELL AHEAD OF THE UPR LOW OFF THE
SRN CA COAST FRIDAY AFTN/NIGHT. INCREASING MID-LEVEL MOISTURE WILL
LIKELY CONTRIBUTE TO SPORADIC HIGH-BASED TSTMS...PARTICULARLY OVER
THE HIGHER TERRAIN STRADDLING THE LWR CO RVR. ISOLD STORMS MAY
PRODUCE GUSTY/SVR WINDS.
..SRN CA COAST INCLUDING THE CHANNEL ISLANDS...
UPR LOW DRIFTING S OFF THE SANTA BARBARA COAST TODAY WILL BEGIN TO
REVERSE COURSE AND EJECT NEWD INTO SRN CA BY 12Z SATURDAY. COLDEST
TEMPERATURES ALOFT /H5 AOB MINUS 20 DEG C/ ASSOCD WITH THE LOW WILL
SPREAD NEWD FRIDAY NIGHT/EARLY SATURDAY...CONTRIBUTING TO STEEPENING
LAPSE RATES AND INCREASING TSTM PROBABILITIES. STRONGEST FORCING
FOR ASCENT WILL LIKELY ARRIVE LATE IN THE PERIOD AND SUPPORT BANDS
OF TSTMS MOVING NEWD THROUGH THE CHANNEL ISLANDS AND APPROACHING THE
SRN CA COASTLINE BY 12Z SATURDAY. ISOLD STORMS COULD PRODUCE
MARGINALLY SVR HAIL AND PERHAPS WATERSPOUTS/TORNADOES.
.RACY.. 09/20/2007
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