Aug 21, 2009 2000 UTC Day 1 Convective Outlook
Updated: Fri Aug 21 22:12:32 UTC 2009
Probabilistic to Categorical Outlook Conversion Table (Effective Feb 14, 2006).
Categorical Graphic
20090821 2000 UTC Day 1 Outlook Graphic
Probabilistic Tornado Graphic
20090821 2000 UTC Day 1 Tornado Probabilities Graphic
Torndao Legend Image
Probabilistic Damaging Wind Graphic
20090821 2000 UTC Day 1 Damaging Wind Probabilities Graphic
Wind Legend Image
Probabilistic Large Hail Graphic
20090821 2000 UTC Day 1 Large Hail Probabilities Graphic
Hail Legend Image
   SPC AC 212209
   
   DAY 1 CONVECTIVE OUTLOOK AMEND 2
   NWS STORM PREDICTION CENTER NORMAN OK
   0509 PM CDT FRI AUG 21 2009
   
   VALID 212150Z - 221200Z
   
   ...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE NORTHEAST STATES/NEW
   ENGLAND AND DELMARVA...
   
   ...THERE IS A SLGT RISK OF SVR TSTMS ACROSS SOUTHERN AZ...
   
   AMENDED FOR SEVERE RISKS IN MAINE/WW 730 AND AZ
   
   ...NORTHEAST STATES/NEW ENGLAND AND DELMARVA...
   A NUMBER OF SEVERE TSTMS WILL CONTINUE TO DEVELOP/SPREAD
   EAST-NORTHEASTWARD INTO THE EVENING HOURS ACROSS WESTERN/SOUTHERN
   NEW ENGLAND AND NJ/NYC VICINITY AND DELMARVA WITHIN LOW LEVEL
   INSTABILITY/CONFLUENCE AXIS. WITH A VERY MOIST AIRMASS IN
   PLACE...AND AMPLE HEATING CONTRIBUTING TO MODERATE INSTABILITY
   ESPECIALLY NJ TO SOUTHERN NEW ENGLAND...SCENARIO IS SUPPORTED ON THE
   LARGE SCALE BY A LEAD SHORTWAVE TROUGH/SPEED MAXIMA PIVOTING
   NORTHEASTWARD OVER THE LOWER GREAT LAKES/NY VICINITY. A FEW
   QUASI-DISCRETE/LINE-EMBEDDED SUPERCELLS WITH BRIEF TORNADO POTENTIAL
   WILL REMAIN POSSIBLE OVER THE NEXT FEW HOURS MAINLY ACROSS
   SOUTHERN/WESTERN NEW ENGLAND...BUT LARGELY UNIDIRECTIONAL LOW/MID
   TROPOSPHERIC WINDS /PER 18Z OBSERVED RAOBS AND WSR-88D VWPS/ SHOULD
   MAINTAIN A DECIDEDLY LINEAR CONVECTIVE MODE WITH SMALL BOWS AND
   DAMAGING WINDS AS THE PRIMARY HAZARD.
   
   ...AZ...
   HAVE INTRODUCED A SLIGHT RISK FOR PORTIONS OF SOUTHERN AZ. FOR
   ADDITIONAL DETAILS...SEE MESOSCALE DISCUSSION 1949.
   
   AS AN INCREASINGLY MOIST AIRMASS CONTINUES TO DEVELOP INTO AZ PER
   GPS DERIVED PW DATA AND 12Z/18Z OBSERVED RAOBS...AMPLE HEATING OVER
   THE HIGHER TERRAIN SHOULD RESULT IN INCREASING TSTMS THROUGH LATE
   AFTERNOON/EARLY EVENING ACROSS SOUTHEAST AZ. PER THE 18Z TUCSON
   OBSERVED RAOB...MODESTLY STRONG EAST-SOUTHEASTERLY STEERING FLOW
   BETWEEN 2-6 KM SHOULD ALLOW SOME STORMS TO PROPAGATE ONTO THE
   SOUTHERN AZ DESERT FLOOR...WITH A FEW MICROBURSTS POSSIBLE.
   
   ...ELSEWHERE...
   EARLIER FORECAST/REASONING GENERALLY HOLDS...WITH MAINLY ISOLATED
   SEVERE TSTMS POSSIBLE ACROSS PORTIONS OF TX TO THE SOUTHEAST
   STATES/FL...AND PERHAPS PORTIONS OF WESTERN MT AS WELL.
   
   ..GUYER.. 08/21/2009
   
   .PREV DISCUSSION... /ISSUED 1108 AM CDT FRI AUG 21 2009/
   
   ...NORTHEAST STATES...
   LARGE UPPER LOW/TROUGH OVER THE GREAT LAKES/OH VALLEY REGION WILL
   ROTATE NEWD ACROSS THE NERN STATES THIS AFTERNOON....WITH STRONG
   SHORTWAVE TROUGH/SPEED MAX SHIFTING NEWD THROUGH WRN PA/NY INTO THE
   EVENING HOURS. STORMS HAVE ALREADY DEVELOPED AHEAD OF THE UPPER
   TROUGH ACROSS PORTIONS OF ERN PA/SRN NY...REFERENCE WW/S 726 AND
   727...AND AS FORCING SPREADS ACROSS THE REGION...WIDESPREAD STRONG
   TO SEVERE THUNDERSTORMS ARE ANTICIPATED.
   
   THE VERY MOIST AIR MASS IN PLACE...DEWPOINTS 68 TO 75...COMBINED
   WITH PW VALUES AROUND 2 INCHES AND ADDITIONAL HEATING SHOULD RESULT
   IN MLCAPES TO RANGE FROM 3000 J/KG IN AREAS THAT RECEIVE STRONG
   WARMING...TO 1500 J/KG WHERE CLOUDS ARE MORE PERSISTENT. A 50-60 KT
   MID LEVEL SPEED MAX WILL RESULT IN SUFFICIENT SHEAR FOR SUPERCELLS
   AND LINE SEGMENTS/BOW ECHOES THROUGHOUT THE AFTERNOON/EARLY EVENING
   HOURS. UNIDIRECTIONAL SWLY WINDS THAT INCREASE WITH HEIGHT ABOVE THE
   BOUNDARY LAYER WILL BE MOSTLY FAVORABLE FOR DAMAGING WINDS.
   HOWEVER...THE PRESENCE OF A VERY MOIST BOUNDARY LAYER AND VEERING
   WINDS WITHIN THE BOUNDARY LAYER MAY SUPPORT A TORNADO OR TWO.
   
   ...CENTRAL/ERN TX ENEWD INTO THE CAROLINAS...
   STRONG TO OCCASIONALLY SEVERE ELEVATED STORMS WERE ONGOING ACROSS
   PORTIONS OF NRN TX THIS MORNING WITHIN AREA OF MODERATE INSTABILITY
   AND EFFECTIVE SHEAR AROUND 30 KT. THESE STORMS SHOULD DIMINISH AS
   WEAK MID LEVEL WARM ADVECTION SUBSIDES. HOWEVER...DESPITE WEAK
   DYNAMICAL FORCING...CONVERGENCE ALONG OUTFLOW BOUNDARIES/EDGE OF
   THICKER CLOUDS AND WHERE STRONGER HEATING/DESTABILIZATION
   OCCURS...WILL FAVOR SCATTERED STRONG STORMS DURING THE
   AFTERNOON/EVENING HOURS. SINCE THE SHEAR WILL NOT BE PARTICULARLY
   STRONG AND THE THREAT FOR SEVERE STORMS APPEARS LOCALIZED...ONLY LOW
   SEVERE PROBABILITIES ARE FORECAST.
   
   ...NRN CONTINENTAL DIVIDE AREA...
   SOUTHERN TAIL OF UPPER TROUGH IN BC WILL MOVE EWD ACROSS THE NRN
   ROCKIES LATER TODAY. CONVECTION HAS OCCURRED IN FAR NWRN MT THIS
   MORNING IN ASSOCIATION WITH A SHORTWAVE TROUGH LIFTING NEWD INTO
   ALBERTA. MORE STORMS SHOULD DEVELOP LATER TODAY AS LARGE SCALE
   FORCING SPREADS IN FROM THE WEST. DEEP LAYER SHEAR AT 30 TO 40
   KT...MUCAPES NEAR 2000 J/KG AND STEEP LOW-MID LEVEL LAPSE RATES
   SHOULD SUPPORT A FEW STORMS WITH STRONG AND GUSTY WINDS.
   
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   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z