SPC 2000Z Day 1 Outlook
DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 0252 PM CDT TUE AUG 14 2012 VALID 142000Z - 151200Z ...THERE IS A SLGT RISK OF SVR TSTMS FROM PORTIONS OF THE NORTHEAST U.S. SWD TO GA... ...THERE IS A SLGT RISK OF SVR TSTMS OVER PORTIONS OF THE SRN PLAINS... THE SRN PLAINS SLIGHT RISK AREA WAS ADJUSTED NORTH AND WEST ACROSS PORTIONS OF SWRN OK/SERN TX PANHANDLE. TSTMS DEVELOPING IN THE VICINITY OF FRONTAL BOUNDARY WILL LIKELY MOVE E/SE...POSSIBLY CONGEALING INTO SMALL CLUSTERS. LARGE TEMP-DEW POINT SPREADS S OF THE FRONT WILL RESULT IN PRIMARILY A DAMAGING WIND THREAT. NORTH OF THE FRONT A RISK OF DAMAGING WINDS AND LARGE HAIL WILL EXIST GIVEN 30-35 KT EFFECTIVE BULK SHEAR. EXPECT TSTM COVERAGE TO CONTINUE TO INCREASE ACROSS ERN U.S. SLIGHT RISK AREA OVER THE NEXT FEW HOURS AS MODEST ASCENT WITH UPPER TROUGH MOVES EWD. HAVE ALSO MAINTAINED THE SMALL 5 PERCENT TORNADO PROBABILITY AREA WHERE THE LOW-LEVEL FLOW...ALBEIT FAIRLY WEAK...IS SOMEWHAT BACKED. LOWER LCL HEIGHTS ALSO EXIST IN THIS AREA AND AN ISOLATED TORNADO CANNOT BE RULED OUT...ESPECIALLY WHERE STORM-SCALE INTERACTIONS OCCUR. ..BUNTING.. 08/14/2012 .PREV DISCUSSION... /ISSUED 1129 AM CDT TUE AUG 14 2012/ ...SYNOPSIS... LITTLE REMAINS OF THE SUMMERTIME SUBTROPICAL ANTICYCLONE THAT WAS THE DOMINANT FEATURE ACROSS THE SCNTRL AND SWRN U.S. DURING THE FIRST HALF OF AUGUST. A BROAD ZONE OF WEAK TO MODEST W/NWLY FLOW ALOFT NOW EXISTS FROM THE PACIFIC NORTHWEST TO THE MS VALLEY. THIS STREAM OF MID/UPPER LEVEL FLOW ROUNDS THE BASE OF A PERSISTENT LARGE SCALE TROUGH AXIS WITH NEUTRAL TILT FROM THE GREAT LAKES TO DEEP SOUTH BEFORE BECOMING W/SWLY DEEP-LAYER FLOW ALIGNED WITH THE ERN SEABOARD. THE LATEST IN SMALLER SCALE SHORTWAVE DISTURBANCES ENTERING THE LARGER MEAN ERN U.S. TROUGH WERE SITUATED FROM LOWER MI TO THE TN VALLEY THIS MORNING. THE MI SHORTWAVE WAS LIKELY MAINTAINING A WEAK SFC LOW OVER OHIO WITH A DIFFUSE COLD FRONT EXTENDING SWWD FROM THE LOW TO THE LOWER MS VALLEY. THIS BOUNDARY THEN STRETCHES NWWD AND FORMS A WEAK WARM FRONT OVER NRN TX. OTHER MID LEVEL DISTURBANCES LEAD THE MIDWEST/TN VALLEY SHORTWAVES INCLUDING ONE DRIVING A LOOSELY ORGANIZED MCS OVER NJ ATTM...AND ANOTHER NEARER THE BASE OF THE LARGE SCALE TROUGH THAT APPEARS TO BE AIDING IN THE DEVELOPMENT OF WIDELY SCATTERED STORMS FROM THE FL PNHDL NEWD TO GA. FARTHER WEST...A PAIR OF LOW AMPLITUDE SHORTWAVE TROUGHS ARE CROSSING THE GREAT PLAINS /ONE IN NEB THE OTHER IN KS/. THESE ARE EMBEDDED WITHIN THE LARGER SCALE W/NWLY FLOW REGIME. AND...A STRONGER SHORTWAVE TROUGH WAS DIGGING SEWD TOWARD SRN BC. THIS LATER IMPULSE WILL LIKELY PRODUCE THE GREATEST HEIGHT FALLS ACROSS THE CONUS OVER THE NEXT 24H WITH 12H 500MB HEIGHT FALLS NEARING 90M BY EARLY WED ACROSS NERN MT AND ND. ...EAST... IN THE WAKE OF MORNING MCS ACTIVITY OVER PA/NJ...CLEARING SKIES ACROSS NY AND PA AND RELATIVELY MOIST ANTECEDENT AIRMASS WILL CONTRIBUTE TO GRADUAL BUT GENERALLY WEAK DESTABILIZATION THROUGH THE LATE AFTERNOON. THIS PROCESS WILL OCCUR COINCIDENT WITH THE EWD ADVANCE OF A WEAK SFC LOW AND FRONT FROM THE OH VALLEY. THIS SHOULD PROMOTE BOTH AN INCREASE IN DEEP-LAYER ASCENT AND SCATTERED TSTMS ACROSS THE REGION. POCKETS OF EFFECTIVE SHEAR AROUND 30KT SUGGEST THAT A FEW STORMS OR CLUSTERS OF STORMS MAY BECOME BETTER ORGANIZED OR PERSIST FOR A SUFFICIENT LENGTH OF TIME THAT A FEW WIDELY SCATTERED DAMAGING WIND EVENTS MAY ENSUE. MUCH OF THE ACTIVITY IN THE EAST WILL LIKELY BE DRIVEN BY STORM SCALE DYNAMICS WHICH COULD BE TIED TO THE WEAK SURFACE LOW DEVELOPING EAST TOWARD SRN PA AND THE DELMARVA AREA DURING LATE EVENING PER LATEST NAM-WRF. STORM SCALE GUIDANCE FROM THE 00Z WRF-ARW-NSSL AND THE NAM-KF CONTROL MEMBER OF THE 09Z SREF DO SUGGEST LOW LEVEL WIND PROFILES RESULTING IN ENLARGED HODOGRAPHS SUPPORTIVE OF LOW LEVEL STORM ROTATION DURING THE EVENING. THE CHANCE FOR A TORNADO EXISTS IF DISCRETE CONVECTION CAN EVOLVE IN THIS FORECAST KINEMATIC REGIME. HOWEVER...LATEST NAM-WRF GUIDANCE SHOWS MUCH WEAKER LOW LEVEL SHEAR WITH LITTLE OR NO TORNADO POTENTIAL INDICATED. GIVEN RELATIVELY HIGH CONVECTIVE QPF SIGNAL NEAR A FAVORABLE CYCLOGENESIS REGION IT SEEMS PRUDENT TO MAINTAIN A SMALL RELATIVELY HIGHER 5 PERCENT PROBABILITY TORNADO FORECAST FOR THE TIME BEING FROM NRN DELMARVA INTO SRN PA/CAPE MAY AREA. FARTHER SOUTH...FROM VA TO THE CAROLINAS/ERN GA...WIDELY SCATTERED CONVECTION IS EXPECTED THIS AFTERNOON WITH AT LEAST WEAK LARGE SCALE ASCENT AND AREAS OF STRONGER HEATING CONTRIBUTING TO A UP-TICK IN STORM INTENSITY. AS A BAND OF SLIGHTLY STRONGER LOW THROUGH MID LEVEL FLOW /15-20KT AT 850MB TO ABOUT 25KT AT 500MB/ SPREADS EAST WITHIN THE BASE OF THE LARGER SCALE TROUGH...SLIGHTLY FASTER STORM MOTIONS COUPLED WITH STEEP LOW LEVEL LAPSE RATES SHOULD RESULT IN A FEW DAMAGING WIND/TREE DOWN EVENTS. ACTIVITY SHOULD SPREAD EWD INTO EARLY EVENING AND WEAKEN AFTER SUNSET. ...NRN TX/SRN PLAINS... SREF TSTM GUIDANCE IS IN MODEST AGREEMENT WITH SPC STORM SCALE ENSEMBLE AS TO WHERE THE GREATEST LIKELIHOOD FOR TSTM DEVELOPMENT WILL BE FROM SOUTH OF THE RED RIVER NWWD TO THE TX PNHDL LATER TODAY. LIFT ACROSS THE DIFFUSE FRONTAL SEGMENT SHOULD BE ENHANCED AS SHORTWAVE TROUGH CROSSES KS/OK. WHILE STRONGER SHEAR/FLOW WILL RESIDE NORTH OF THE SFC BOUNDARY...MODERATE DESTABILIZATION SEEMS LIKELY IN A CORRIDOR FROM AMA TO DFW. DEVELOPING CONVECTION COULD TAP THE GREATER INSTABILITY WHERE A HOT AND WELL MIXED BOUNDARY LAYER WILL SUPPORT A CHANCE FOR A FEW STRONG TO SEVERE TSTM WIND EVENTS. ...ELSEWHERE... VERY ISOLATED STORMS ARE FORECAST WITHIN WARM ADVECTION REGIME AHEAD OF A LEADING SHORTWAVE TROUGH MOVING FROM NEB TOWARD SD/MN LATER TODAY AND TONIGHT. WHILE HAIL IS POSSIBLE WITH THIS CONVECTION...OVERALL POTENTIAL APPEARS TOO LIMITED FOR A PROBABILITY AREA. ALSO...STRONG FORCING FOR ASCENT WILL ACCOMPANY THE UPPER TROUGH AND COLD FRONT SPREADING SOUTH FROM CANADA INTO MT LATE TONIGHT. THIS FORCING SEEMS SUFFICIENT TO SPUR TSTM DEVELOPMENT DESPITE A WEAK INSTABILITY ENVIRONMENT. IF GREATER CAPE THAN CURRENTLY FORECAST IS REALIZED AHEAD OF THE FRONT THEN ISOLATED HAIL/WIND DAMAGE COULD OCCUR. LATEST INDICATIONS ARE THAT THE PROBABILITY OF SEVERE WITH THIS FROPA REMAINS TOO LOW FOR AN OUTLOOK AREA.
via SPC Forecast Products http://www.spc.noaa.gov/products/outlook/day1otlk_2000.html
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