DAY1SVR: Day 1 Convective Outlook

ACUS01 KWNS 172002
SWODY1
SPC AC 172000

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0300 PM CDT MON SEP 17 2012

VALID 172000Z - 181200Z

...THERE IS A SLGT RISK OF SVR TSTMS OVER PORTIONS OF ERN AL INTO
WRN AND CNTRL GA AND SERN TN...

...ERN AL...SERN TN THROUGH WRN AND CNTRL GA...

PRIMARY CHANGE TO THIS OUTLOOK IS TO EXPAND THE SLIGHT RISK AREA
FARTHER NORTH IN RESPONSE TO EXPECTED INCREASE IN 0-2 KM HODOGRAPHS
SIZE AND DEEP SHEAR AS LOW LEVEL JET AND MID LEVEL FLOW STRENGTHEN
IN RESPONSE TO EJECTING SHORTWAVE TROUGH CURRENTLY LOCATED OVER LA.
ISOLATED TORNADOES WILL BE THE MAIN THREAT...THOUGH OVERALL THREAT
COULD REMAIN SOMEWHAT LIMITED BY ONGOING CONVECTION AND WIDESPREAD
CLOUDS IN THE WARM SECTOR.

..DIAL.. 09/17/2012

.PREV DISCUSSION... /ISSUED 1119 AM CDT MON SEP 17 2012/

...SYNOPSIS...

CONSIDERABLE AMPLIFICATION OF THE LARGE-SCALE PATTERN WILL OCCUR
DURING THE D1 PERIOD OWING TO THE FORMATION OF A FULL-LATITUDE
TROUGH FROM HUDSON BAY INTO THE LOWER MS VALLEY. WITHIN THIS
BROADER-SCALE PATTERN...A POLAR BRANCH SHORTWAVE TROUGH AND
ASSOCIATED MIDLEVEL JETSTREAK WILL PROGRESS FROM THE NRN PLAINS INTO
MIDWEST. MEANWHILE...A SUBTROPICAL BRANCH VORTICITY MAXIMUM OVER
SERN TX WILL TRANSLATE EWD INTO THE LOWER MS VALLEY.

IN THE LOW LEVELS...A COLD FRONT ASSOCIATED WITH THE POLAR BRANCH
UPPER SYSTEM CURRENTLY FROM LOW PRESSURE OVER THE U.P. OF MI SWWD
THROUGH THE MID MO VALLEY INTO SRN HIGH PLAINS WILL PROGRESS SEWD.
BY 18/12Z...THIS FRONT IS FORECAST TO STRETCH FROM A DEVELOPING
CYCLONE OVER SWRN ONTARIO/UPPER OH VALLEY THROUGH THE MID SOUTH INTO
CNTRL TX. ELSEWHERE...A SURFACE LOW ASSOCIATED WITH THE SUBTROPICAL
BRANCH VORTICITY MAXIMUM WILL DEVELOP FROM S-CNTRL LA TO W-CNTRL AL
BY THE END OF THE D1 PERIOD WITH A WEAK WARM FRONT LIFTING NWD THE
TN VALLEY AND CAROLINAS.

...CNTRL/ERN GULF COAST THIS AFTERNOON INTO TONIGHT...

THE 12Z NEW ORLEANS SOUNDING APPEARS REPRESENTATIVE OF WARM SECTOR
ENVIRONMENT...FEATURING PW VALUES AOA TWO INCHES AND MLCAPE VALUES
OF 1500-2000 J/KG...DESPITE WEAK LAPSE RATES AND WIDESPREAD
CLOUDS/PRECIPITATION. THE PRIMARY FACTOR LIMITING A MORE IMMEDIATE
SEVERE THREAT IS A RATHER WEAKLY SHEARED ENVIRONMENT PER CURRENT VAD
DATA. LATEST MESOSCALE MODEL GUIDANCE REMAINS CONSISTENT IN
SUGGESTING AN INCREASE IN THE DEEP-LAYER WIND FIELD LATER TODAY INTO
TONIGHT IN CONJUNCTION WITH PHASING OF THE SRN STREAM VORTICITY
MAXIMUM WITH THE MORE INTENSE POLAR BRANCH SYSTEM. AS SUCH...EXPECT
A CORRESPONDING INCREASE IN SUPERCELL POTENTIAL MAINLY DURING THE
LATTER HALF OF THE FORECAST PERIOD WITH AN ASSOCIATED RISK FOR A
TORNADO OR TWO AND LOCALLY DAMAGING WIND GUSTS.

...PIEDMONT OF THE CNTRL/SRN APPALACHIANS TONIGHT INTO TUESDAY
MORNING...

FORCING FOR ASCENT AND VERTICAL SHEAR WILL INCREASE MARKEDLY IN
RESPONSE TO THE PHASING OF MULTI-STREAM TROUGHS AND OVERALL
DEEPENING OF SYNOPTIC SYSTEM. THIS WILL PROMOTE AN INCREASE IN TSTM
DEVELOPMENT ACROSS THE REGION...ALONG AND S OF RETREATING WARM/WEDGE
FRONT. THOUGH INSTABILITY WILL REMAIN WEAK...THE POTENTIAL WILL
EXIST FOR EMBEDDED ROTATING STORMS WITH AN INCREASING RISK FOR A
TORNADO OR TWO TOWARD 18/12Z.

...UPPER GREAT LAKES INTO MID MS VALLEY THIS AFTERNOON AND
EVENING...

CONSIDERABLE HEIGHT FALLS/FORCING FOR ASCENT WILL OVERSPREAD THE
REGION IN TANDEM WITH THE SURFACE COLD FRONT...CONTRIBUTING TO AN
INCREASE IN TSTM DEVELOPMENT THIS AFTERNOON INTO TONIGHT. SOME HAIL
AND PERHAPS LOCALLY STRONG WIND GUSTS WILL BE POSSIBLE WITH THE
STRONGEST STORMS. HOWEVER...A MORE ROBUST SEVERE THREAT WILL BE
PRECLUDED BY MARGINAL MOISTURE CONTENT AND THE PRESENCE OF AN
INVERSION/WARM LAYER AROUND 700 MB WHICH WILL LIMIT THE DEGREE OF
AIR MASS DESTABILIZATION.

...WRN PARTS OF TX/OK LATE THIS AFTERNOON AND EVENING...

WIDELY SCATTERED TSTMS WILL BE POSSIBLE ALONG THE COLD FRONT LATER
THIS AFTERNOON INTO TONIGHT AS DEEP-LAYER FORCING FOR ASCENT IS
AUGMENTED BY A VORTICITY MAXIMUM PROGRESSING SEWD INTO THE SRN HIGH
PLAINS. STEEP MIDLEVEL LAPSE RATES AND MODEST DEEP-LAYER SHEAR MAY
PROMOTE A FEW STRONGER STORMS CAPABLE OF SOME SMALL HAIL AND LOCALLY
STRONG WIND GUSTS. CURRENTLY...IT APPEARS THAT THE ENVIRONMENT WILL
BECOME ONLY WEAKLY UNSTABLE...LIMITING SEVERE WEATHER POTENTIAL AND
PRECLUDING THE NECESSITY FOR LOW WIND AND HAIL PROBABILITIES.

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