SPC 1200Z Day 1 Outlook
DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 0100 AM CDT FRI SEP 07 2012 VALID 071200Z - 081200Z ...THERE IS A SLGT RISK OF SVR TSTMS PORTIONS SERN LOWER MI/OH TO OZARKS AND MID-SOUTH...... ...SYNOPSIS... DOMINANT UPPER-AIR FEATURE FOR THIS FCST WILL BE STG SHORTWAVE TROUGH -- NOW VERY POSITIVELY TILTED FROM NWRN ONT WSWWD ACROSS SRN MB...MT AND CENTRAL ID. AS HEIGHTS RISE ACROSS MOST OF INTERMOUNTAIN W...500-MB TROUGH WILL TRANSLATE SEWD WITH ITS ERN PORTION INTENSIFYING. BY 8/00Z...TROUGH SHOULD EXTEND FROM NRN ONT SWWD ACROSS LS...THROUGH PRIMARY VORTICITY FIELD OVER WI...ACROSS IA...TO SWRN NEB/NWRN KS REGION. PERTURBATION WILL DEEPEN FURTHER AND BECOME LESS POSITIVELY TILTED WITH TIME...SUCH THAT BY 8/12Z...VORTICITY CENTER SHOULD ARRIVE AT NRN INDIANA...WITH TROUGH ARCHING SWD/SWWD OVER SERN INDIANA...SERN MO AND NRN OK. RELATED SFC COLD FRONT AT START OF PERIOD SHOULD EXTEND SWWD FROM FRONTAL-WAVE LOW OVER SRN IA THROUGH CENTRAL/SWRN KS TO NERN NM. AS LOW MOVES EWD TO NRN INDIANA BY 8/00Z...COLD FRONT SHOULD REACH SERN PORTIONS MO/OK/NM...WHILE WARM FRONT EXTENDS E OF LOW INVOF MI/OH BORDER AND LE. OVERNIGHT...COLD FRONT WILL SWEEP SEWD TO S TX...WRN LA...MID TN...AND OH...CONNECTING WITH DEEPENING SFC CYCLONE OVER SRN ONT BY 12Z. ...DIURNAL...LOWER MI AND INDIANA TO OZARKS AND OK... STRENGTHENING COLD FRONT IS EXPECTED TO MOVE SEWD INTO PROGRESSIVELY HIGHER-THETAE AIR MASS DURING DAY...WHILE DIABATIC SFC HEATING BOOSTS BUOYANCY AND REDUCES CINH. NET RESULT SHOULD BE DEVELOPMENT OF SCATTERED TSTMS INVOF FRONT DURING AFTERNOON...BUILDING NEWD AND SWWD WITH TIME INTO LENGTHY BAND OF SCATTERED-NUMEROUS STG-SVR TSTMS. MAIN THREAT WILL BE DAMAGING GUSTS...ALTHOUGH LOW-LEVEL AND DEEP SHEAR WILL SUPPORT SUPERCELL POTENTIAL GENERALLY N OF OH RIVER AND E OF MS RIVER FOR LOCALLY ENHANCED HAIL THREAT AND CONDITIONAL TORNADO POTENTIAL. LARGEST HODOGRAPHS SHOULD BE INVOF WARM FRONT DURING AFTERNOON...THEN AS LLJ STRENGTHENS FROM ABOUT 00Z ONWARD...IN SW-NE CORRIDOR ACROSS PORTIONS KY...INDIANA AND OH. SFC DEW POINTS UPPER 60S TO LOW 70S F ALREADY ARE IN PLACE ACROSS BROAD AREA OF MID-SOUTH AND OH VALLEY REGIONS. RELATED LOW-LEVEL MOISTURE WILL BE ADVECTED/TRANSPORTED POLEWARD AND AGAINST SFC WARM FRONT BY MID-LATE AFTERNOON. ACCORDINGLY MODIFIED MODEL SOUNDINGS YIELD 2500-3500 J/KG MLCAPE WITH LOCALLY HIGHER VALUES AHEAD OF COLD FRONT. EXAMINING AREAS SWWD FROM CATEGORICAL OUTLOOK AREA ACROSS OK...PREFRONTAL AIR MASS WILL BE CHARACTERIZED BY HOTTER AND MORE DEEPLY MIXED BOUNDARY LAYER. COMPARED TO FARTHER NE...EXPECT LESS MOISTURE...LOWER MLCAPE...AND MORE VEERED SFC FLOW CONTRIBUTING TO WEAKER CONVERGENCE. STILL...COMBINATION OF INTENSE PREFRONTAL/DIABATIC HEATING AND FRONTAL LIFT WILL LEAD TO AT LEAST WIDELY SCATTERED TSTM DEVELOPMENT WITH SPORADIC SVR GUSTS/HAIL PSBL. THIS THREAT WILL BE MORE LIMITED IN SPACE AND TIME THAN OVER CATEGORICAL OUTLOOK AREA...AND SHOULD DIMINISH AFTER DARK. ...OH TO MID-SOUTH REGION OVERNIGHT... LATE IN PERIOD...FROM ABOUT 8/06Z ONWARD...STRENGTHENING WIND FIELDS ALOFT WILL OVERSPREAD LARGER PROPORTION OF WARM SECTOR AS NRN PART OF MID-UPPER LEVEL SHORTWAVE PERTURBATION DEEPENS AND APCHS NEUTRAL TILT. THIS CAUSES SOME CONCERN FOR SVR THREAT EXTENDING INTO OVERNIGHT HOURS ACROSS PORTIONS OH...CENTRAL/ERN KY AND MIDDLE TN...EVEN THOUGH FOREGOING AIR MASS WILL BE CHARACTERIZED BY SOMEWHAT WEAKER AMBIENT THETAE...AND INSTABILITY WILL DECREASE DUE TO DIABATIC SFC COOLING. FCST SOUNDINGS STILL SHOW EFFECTIVE INFLOW PARCELS SFC-BASED OR VERY NEARLY SO EVEN AS LATE AS AROUND 8/09Z...IN NARROW CORRIDOR AHEAD OF FRONT/CONVERGENCE LINE. LAPSE RATES AND BUOYANCY EACH WILL BE WEAK...WITH MLCAPE GENERALLY AOB 300 J/KG. SOME CAPPING IN 750-850 MB LAYER IS EVIDENT IN PROGS...WHICH MAY BE OVERCOME BY FRONTAL FORCING. GIVEN THESE UNCERTAINTIES...SVR PROBABILITIES AT CATEGORICAL THRESHOLDS ARE EXPANDED FARTHER E...BUT LOWER THAN PRIOR AFTERNOON/EVENING DUE TO CONDITIONAL NATURE OF INSTABILITY. ...INTERIOR MID-ATLC AND SRN NEW ENGLAND REGIONS... WIDELY SCATTERED TO SCATTERED TSTMS -- MOSTLY MULTICELLULAR IN MODE -- ARE FCST TO DEVELOP DURING AFTERNOON. ALTHOUGH SPECIFIC FOCI FOR INITIATION APPEAR NEBULOUS...MOIST BOUNDARY-LAYER AIR MASS SHOULD EXPERIENCE STG SFC HEATING TO WEAKEN CINH. MLCAPE SHOULD RISE INTO 1000-2000 J/KG RANGE OVER MUCH OF THIS AREA...LOCALLY APCHG 3000 J/KG. ALTHOUGH LOW-MIDDLE LEVEL FLOW WILL BE MODEST THROUGH MOST OF DAYLIGHT HOURS...LIMITING VERTICAL SHEAR...STRENGTHENING UPPER/ANVIL-LEVEL FLOW WILL AID IN STORM ORGANIZATION. ISOLATED SVR DOWNDRAFTS AND SPORADIC LARGE HAIL EACH ARE POSSIBLE. ..EDWARDS.. 09/07/2012
via SPC Forecast Products http://www.spc.noaa.gov/products/outlook/day1otlk_1200.html
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