SWODY1
SPC AC 291621
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1121 AM CDT SUN APR 29 2007
VALID 291630Z - 301200Z
..THERE IS A SLGT RISK OF SVR TSTMS THROUGH TONIGHT FOR SW TX...
..SW TX THROUGH TONIGHT...
CLOUDS AND CONVECTION HAVE BEEN RATHER WIDESPREAD OVERNIGHT INTO
THIS MORNING ACROSS SW TX IN ADVANCE OF THE MID LEVEL LOW OVER NW
MEXICO WHICH IS BEGINNING TO MOVE ENEWD. IN THE WAKE OF THIS
CONVECTION...THERE WILL BE SOME POTENTIAL FOR SURFACE
HEATING/DESTABILIZATION WITH CLOUD BREAKS IN THE MID LEVEL DRY SLOT
SPREADING NEWD OVER SW TX. THE MORE PROBABLE AREAS FOR
DESTABILIZATION AND STORM INITIATION WILL BE NEAR AND JUST NE OF THE
HIGHER TERRAIN FROM JUST W OF DRT INTO THE AREA IMMEDIATELY N OF THE
BIG BEND THIS AFTERNOON. BOUNDARY LAYER DEWPOINTS IN THE 60S
SPREADING WNWWD UP THE RIO GRANDE AND LOWER PECOS VALLEYS...ALONG
WITH GRADUALLY INCREASING SWLY MID LEVEL FLOW...SHOULD SUPPORT THE
FORMATION OF A COUPLE OF SUPERCELLS CAPABLE OF PRODUCING WIDELY
SCATTERED LARGE HAIL/DAMAGING WINDS...AND PERHAPS A TORNADO OR TWO.
..MT AREA LATE THIS AFTERNOON INTO EARLY TONIGHT...
A MID LEVEL SHORTWAVE TROUGH OVER NW WA WILL MOVE THROUGH THE MEAN
RIDGE AXIS AND OVER MT AND SRN ALBERTA/SASKATCHEWAN BY TONIGHT.
ASCENT WITH THIS WAVE WILL BE FOCUSED ALONG THE ACCOMPANYING COLD
FRONT THAT WILL MOVE ACROSS MT LATE THIS AFTERNOON INTO TONIGHT.
LOW-LEVEL MOISTURE IS QUITE LIMITED IN ADVANCE OF THIS SYSTEM...AND
ANY INSTABILITY THIS AFTERNOON/EVENING WILL BE DRIVEN PRIMARILY BY
WARM SURFACE TEMPERATURES AND RESULTANT STEEP LAPSE RATES/DEEP MIXED
LAYERS. GIVEN THE FOCUSED ASCENT...STEEP LAPSE RATES...AND STRONG
VERTICAL SHEAR...A FEW HIGH-BASED STORMS COULD PRODUCE ISOLATED
DAMAGING WINDS LATE THIS AFTERNOON INTO EARLY TONIGHT.
..UPPER MS VALLEY...
A STRONG MID LEVEL SHORTWAVE TROUGH OVER WRN ONTARIO WILL AMPLIFY
AND MOVE ESEWD TOWARD QUEBEC BY TONIGHT. IN THE WAKE OF THIS MID
LEVEL WAVE AND AN ASSOCIATED SURFACE CYCLONE...A COLD FRONT WILL
MOVE SEWD FROM CENTRAL MN INTO NRN IA/SRN WI BY LATE EVENING.
RECENT FORECASTS FROM THE NAM/GFS HAVE SHOWN UNREALISTIC MOISTENING
IN THE LOW-MID TROPOSPHERE FROM ERN NEB INTO SRN WI BY LATE
AFTERNOON...WHERE SURFACE DEWPOINTS ARE FORECAST TO INCREASE TO THE
UPPER 50S AND EVEN LOW-MID 60S. REGIONAL 12Z SOUNDINGS AND SURFACE
OBSERVATIONS SHOW NO SUPPORT FOR THIS SCENARIO BASED ON A VERY
SHALLOW NOCTURNAL BOUNDARY LAYER WITH ONLY UPPER 40S DEWPOINTS AND
MUCH DRIER AIR ALOFT...AND PW VALUES AROUND 0.60 INCHES. THE 12Z
SOUNDINGS DO SHOW THE POTENTIAL FOR AFTERNOON TEMPERATURES INTO THE
MID 80S WITH A DEEP MIXED LAYER...AND MLCAPE AOB 500 J/KG BASED ON
MEAN DEWPOINTS NEAR 50 F. THEREFORE...ISOLATED/HIGH-BASED STORMS
MAY FORM ALONG THE FRONT THIS EVENING FROM NRN IA INTO SRN
WI...WHERE WIND PROFILES WILL BE FAVORABLE FOR SUPERCELLS.
HOWEVER...CONFIDENCE IN STORM INITIATION/COVERAGE IS LOW...AND WILL
MAINTAIN ONLY LOW CONDITIONAL PROBABILITIES FOR DAMAGING WINDS AND
HAIL.
.THOMPSON/GRAMS.. 04/29/2007
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