SWODY1
SPC AC 131959
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0259 PM CDT SUN SEP 13 2009
VALID 132000Z - 141200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS ERN MT...
...THERE IS A SLGT RISK OF SVR TSTMS OVER THE CENTRAL GULF COAST
REGION...
...PARTS OF E/SE TX TO CENTRAL GULF COAST...
STRONG SURFACE HEATING WITHIN A DRY SLOT ROTATING NWD THROUGH SERN
TX...ESE OF THE N TX CLOSED LOW...HAS RESULTED IN A MODERATELY
UNSTABLE AIR MASS...DESPITE WEAK MID LEVEL LAPSE RATES. TSTMS HAVE
FORMED IN THE VICINITY OF A SURFACE BOUNDARY EXTENDING ESEWD THROUGH
SERN TX TO SWRN LA. LOW LEVEL DIRECTIONAL SHEAR IN VICINITY OF THIS
BOUNDARY WITH AMBIENT SURFACE VORTICITY ATTENDANT TO THE UPPER LOW
SUGGESTS AN ISOLATED BRIEF TORNADO IS POSSIBLE. THUS...LOW TORNADO
PROBABILITIES HAVE BEEN EXTENDED WWD INTO SERN TX.
FARTHER E...THE EARLY AFTERNOON SURFACE ANALYSIS SHOWED THE PRIMARY
WARM FRONT HAD MOVED FARTHER INLAND FROM NRN LA TO CENTRAL MS AND
SRN AL...WITH SURFACE DEWPOINTS IN THE LOWER 70S ALONG AND S OF THIS
BOUNDARY. MODELS SUGGEST THIS BOUNDARY SHOULD TEND TO WEAKEN AS IT
MOVES NWD...WITH STRONGER LOW LEVEL CONVERGENCE BECOMING REFOCUSED
ALONG A COASTAL BOUNDARY EXPECTED TO STRETCH FROM SRN LA TO FAR SWRN
MS THIS EVENING THROUGH TONIGHT.
...ERN MT...
INCREASING DEEP LAYER AND LOW LEVEL SHEAR INTO THIS EVENING SUGGESTS
POTENTIAL WILL EXIST FOR SUPERCELLS PRODUCING HAIL...STRONG WIND
GUSTS AND AN ISOLATED TORNADO. SEVERE WEATHER PROBABILITIES HAVE
BEEN ADJUSTED TO REFLECT THESE SEVERE WEATHER THREATS. EARLY
AFTERNOON SURFACE ANALYSIS INDICATED AN AREA OF LOW PRESSURE WAS
LOCATED OVER NORTH CENTRAL WY...WITH STRONGEST SURFACE PRESSURE
FALLS EXTENDING NNEWD THROUGH ERN MT/WRN DAKOTAS. THIS SURFACE LOW
ATTENDANT TO A COMPACT UPPER LOW WILL TRACK NWD INTO ERN MT WHILE
THE UPPER FEATURE REACHES SWRN SASKATCHEWAN BY 12Z MONDAY.
OBJECTIVE ANALYSES SHOWED AIR MASS IS UNDERGOING DESTABILIZATION
WITH THE RETURN OF LOW LEVEL MOISTURE /SURFACE DEWPOINTS AROUND 50
F/...SURFACE HEATING BENEATH STEEPENING MID LEVEL LAPSE RATES.
ADDITIONAL DESTABILIZATION IS EXPECTED AS STRENGTHENING SSELY LLJ
ADVECTS SURFACE DEWPOINTS IN THE LOWER 50S INTO ERN MT BY EARLY
EVENING AND COLD MID LEVEL TEMPERATURES SPREAD NWD ACROSS ERN MT.
FORECAST FOR SEVERE THREAT IS ON TRACK...WITH POTENTIAL FOR SURFACE
BASED DEVELOPMENT BY LATE AFTERNOON. FOR ADDITIONAL MESOSCALE
DETAILS...PLEASE REFER TO THE SPC MESOSCALE DISCUSSION 2011.
..PETERS.. 09/13/2009
.PREV DISCUSSION... /ISSUED 1100 AM CDT SUN SEP 13 2009/
...ERN MT...
COMPACT UPPER LOW OVER SCENTRAL MT ATTM IS HEADING BACK N AGAIN INTO
CANADA BY 12Z MON. INITIALLY ENVIRONMENT NOT CONDUCIVE FOR STRONG
CONVECTION...BUT BY THIS EVENING COMBINATION OF UPPER COLD
POOL...40KT SSELY LLJ TRANSPORTING LOW LEVEL MOISTURE INTO ERN
MT...AND STRONG SURFACE HEATING E OF UPPER LOW WILL PUT IN PLACE
PARAMETERS FOR A FEW SEVERE THUNDERSTORMS. WITH LAPSE RATES IN
EXCESS OF 7C/KM AND MLCAPES APPROACHING 1000 J/KG BY LATER THIS
AFTERNOON...SURFACE BASED CONVECTION SHOULD INCREASE NEWD IN
FAVORABLE SHEAR ENVIRONMENT FOR NOT ONLY ORGANIZED
THUNDERSTORMS...BUT POTENTIALLY A FEW SUPERCELLS. PRIMARY THREAT
GIVEN THE COLD AIR ALOFT AND MARGINAL LOW LEVEL MOISTURE WILL BE
LARGE HAIL. HOWEVER WITH ANY SUPERCELL DEVELOPMENT THERE WILL BE
POTENTIAL FOR A BRIEF TORNADO AND LARGER HAIL. ACTIVITY SHOULD
DEVELOP NWD BY LATE EVENING INTO CANADA WITH THREAT ENDING OVER ERN
MT.
...CENTRAL GULF COAST...
SETUP OVER THE WRN/CNTRL GULF CST REGION WILL REMAIN SIMILAR TO THAT
ON SAT. HOWEVER A MORE DECIDED NWD ADVANCE OF THE W/E WARM
FRONT NOW OVER THE NRN GULF RELATIVE TO SAT. BY LATE THIS AFTN THE
BOUNDARY SHOULD EXTEND FROM NEAR LFK EWD THROUGH CNTRL LA INTO S
CNTRL MS. ADVANCE OF THE FRONT WILL ALLOW VERY RICH MOISTURE /SFC
DEWPOINTS IN THE MID 70S F AND PW AOA 2.00 INCHES/ TO OVERSPREAD
AREAS ALONG AND N OF THE I-10 CORRIDOR.
MID LVL SSWLY WINDS WILL STRENGTHEN THROUGH THE DAY...WITH 700-500
MB FLOW EXPECTED TO REACH 25-30 KTS. LOW-LVL WINDS...HOWEVER...
SHOULD REMAIN FAIRLY WEAK AS ANY DEEPENING OF SFC LOW NOW OVER SE TX
IS EXPECTED TO BE MINIMAL. NEVERTHELESS...COMBINATION OF SUSTAINED
WAA WITH RICH MOISTURE AND LOW-LVL VEERING WIND PROFILES WILL FAVOR
THE DEVELOPMENT OF NUMEROUS STORM CLUSTERS NEAR WARM FRONT. THESE
MAY EXHIBIT PERIODIC LOW LVL ROTATION...AND COULD POSE A THREAT FOR
A FEW TORNADOES AND/OR STG WIND GUSTS.
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