SWODY1
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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