Posted by JAC on 5/1/2009, 12:49 pm

DAY 1 CONVECTIVE OUTLOOK NWS STORM PREDICTION CENTER NORMAN OK 1131 AM CDT FRI MAY 01 2009 VALID 011630Z - 021200Z ...THERE IS A MDT RISK OF SVR TSTMS ACROSS PARTS OF SOUTHERN OKLAHOMA AND PARTS OF NORTHERN TEXAS... ...THERE IS A SLGT RISK OF SVR TSTMS AROUND THE MODERATE FROM NWRN TX TO THE LOWER OH VALLEY... ...THERE IS A SLGT RISK OF SVR TSTMS IN THE MID ATLANTIC REGION... ...SWRN OK INTO NRN TX... MODERATE TO STRONG INSTABILITY AND /SBCAPE TO 4000 J PER KG/ AND DEEP LAYER SHEAR 30-40 KT /SUFFICIENT FOR SUPERCELLS/ SHOULD CONTINUE TO EVOLVE AHEAD OF THE ADVANCING COLD FRONT AND DRYLINE FROM SWRN OK ACROSS THE RED RIVER VALLEY THIS AFTERNOON. THERE IS NEARLY UNANIMOUS AGREEMENT ACROSS A BROAD RANGE OF AVAILABLE MODEL GUIDANCE THAT SURFACE-BASED CONVECTION WILL INITIATE BY EARLY EVENING NEAR FRONT/OUTFLOW/DRYLINE TRIPLE POINT AROUND SPS. WHILE SYNOPTIC SCALE FORCING FOR ASCENT ACROSS THIS AREA OF INTEREST IS EXPECTED TO BE WEAK IT WILL NOT NECESSARY BE HOSTILE TO STORM DEVELOPMENT AS EVIDENCED IN THE NAM/GFS GUIDANCE AND RELATIVELY FLAT MID-LEVEL HEIGHT PATTERN AND WLY 500MB WINDS AOB 30KT. THE EXISTENCE OF STRONG LOW LEVEL FORCING ON THE MESOSCALE APPEARS TO AID CONVECTIVE INITIATION IN A STRONGLY UNSTABLE AND SUFFICIENT SHEARED ENVIRONMENT FOR STORMS TO QUICKLY ACQUIRE SUPERCELLULAR STRUCTURE. MAGNITUDE OF CAPE WILL SUPPORT ROBUST UPDRAFTS AND MAY COMPENSATE SOMEWHAT FOR LACK OF STRONG SYNOPTIC ASCENT WITH STROM MERGERS AND COLD POOL DEVELOPMENT AIDING UPSCALE ORGANIZATION ACROSS NRN TX THROUGH THE EVENING. GIVEN THE STRONG SIGNAL FOR STORM DEVELOPMENT...AND DEGREE OF DESTABILIZATION EXPECTED...A SMALL HIGHER PROBABILITY HAIL AREA HAS BEEN ADDED TO THIS FORECAST. BASEBALL OR LARGER HAIL APPEARS QUITE POSSIBLE WITH THESE STORMS...ESPECIALLY WITHIN A FEW HOURS OF INITIATION. LACK OF STRONGER DEEP SHEAR AND WEAK LARGER SCALE FORCING MAY LIMIT STORM COVERAGE. THIS FACTOR...AND GENERALLY MARGINAL FORECAST HODOGRAPH STRUCTURE...INDICATE TORNADO POTENTIAL MAY BE LIMITED IN TIME/SPACE. HOWEVER...MOIST BOUNDARY LAYER AND LIKELIHOOD FOR SOME STORM-SCALE AND MESOSCALE ENHANCEMENT TO THE LOW LEVEL WIND FIELD...MAY PLAY A ROLE IN FAVORING A TORNADIC SUPERCELL OR TWO IN THIS REGIME. EVENTUALLY...STRONG INSTABILITY AND RESULTANT COLD POOLS SHOULD RESULT IN A SMALL MCS ACROSS THE RED RIVER/NWRN TX. THE POTENTIAL FOR BOWING LINE SEGMENTS/SUPERCELLS WITH SEVERE HAIL AND WIND COULD SHIFT ESEWD ACROSS FAR NRN TX/DFW THROUGH EVENING/LATE EVENING. ...NERN OK TO AR AND ACROSS LWR/MID MS VALLEY... LEADING PORTION OF SYNOPTIC FRONTAL ZONE ACROSS NW AND NCNTRL OK IS BEING REINFORCED BY ONGOING MCS NOW SPREADING ESEWD ACROSS NERN OK AND THE OZARKS. MODEST CONTINUING INFLUX OF LOW LEVEL MOISTURE AND MASS CONVERGENCE INTO THE MCS OUTFLOW/COLD POOL WILL MAINTAIN SOME POTENTIAL FOR SEVERE HAIL AND WIND FROM NERN OK ACROSS AR THIS AFTERNOON. POSSIBLE MCV EVOLVING FROM THIS ONGOING SYSTEM MAY SUPPORT THE DEVELOPMENT AND PERSISTENCE OF ADDITIONAL STORMS ALONG THE STALLING COLD FRONT...ALONG A LIT-MEM-BNA LINE...THROUGH LATE TONIGHT. GENERALLY WEAKER DEEP LAYER FLOW ACROSS THIS ZONE SUGGESTS THAT PRIMARY THREAT MAY BECOME EXCESSIVE RAIN. HOWEVER...A FEW SEVERE STORMS ALSO APPEAR POSSIBLE IN THIS CORRIDOR THROUGH EARLY SAT. ...LOWER/MID OH VALLEY... A MARGINALLY UNSTABLE AIR MASS AHEAD OF THE SLOWLY ADVANCING COLD FRONT ACROSS THIS AREA SHOULD LEAD TO AN INCREASE IN SHOWERS AND THUNDERSTORMS THROUGH THIS AFTERNOON. DESPITE WEAK LAPSE RATES...DEEP LAYER WLY FLOW MAY SUPPORT SOME ORGANIZATION TO THIS ACTIVITY INTO THE EVENING HOURS WITH WIND AND MARGINALLY SEVERE HAIL BEING THE PRIMARY HAZARDS. ...MID-ATLANTIC... LOW AMPLITUDE SHORT WAVE TROUGH AND DIURNAL DESTABILIZATION WILL CONTRIBUTE TO AN INCREASING CHANCE FOR THUNDERSTORM DEVELOPMENT ALONG ADVANCING LEE/SYNOPTIC TROUGH THIS AFTERNOON.WARMING INTO THE 70S SHOULD YIELD SBCAPE AROUND 500-1000 J/KG. THIS INSTABILITY ALONG WITH UNIDIRECTIONAL WLY WINDS AT 30-40 KT BETWEEN 850-300 MB SHOULD BE SUFFICIENT FOR SPORADIC WIND DAMAGE WITH THE SQUALL LINE...BEFORE IT PASSES OFF THE ATLANTIC SEABOARD THIS EVENING. ..CARBIN/GUYER.. 05/01/2009