SWODY1
SPC AC 241949
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0249 PM CDT SAT OCT 24 2009
VALID 242000Z - 251200Z
...THERE IS A SLGT RISK OF SVR TSTMS ACROSS THE MID ATLANTIC
STATES...
...MID ATLANTIC STATES...
FEW CHANGES TO ONGOING SCENARIO/OUTLOOK...WITH AT LEAST ISOLATED
DAMAGING WINDS/PERHAPS A TORNADO STILL REMAINING POSSIBLE ACROSS THE
MID ATLANTIC SEABOARD/DELMARVA VICINITY THROUGH EARLY/PERHAPS MID
EVENING. A MOIST AIRMASS CHARACTERIZED BY UPPER 60S/LOWER 70S F
SURFACE DEWPOINTS CONTINUES TO STREAM NORTHWARD AHEAD OF A LEE SIDE
LOW ACROSS CENTRAL PORTIONS OF VA/MD AND AN IMMEDIATE PREFRONTAL
CONVECTIVE BAND. MID LEVEL LAPSE RATES/TOTAL BUOYANCY WILL REMAIN
WEAK AS PREVIOUSLY MENTIONED...AND HAS LIKELY BEEN LIMITING UPDRAFT
VIGOR/TSTM COVERAGE THUS FAR THIS AFTERNOON. BUT WITH A MOIST
BOUNDARY LAYER AND AN APPROACHING SHORTWAVE TROUGH/SPEED MAX OVER
THE UPPER OH RIVER VALLEY PER AFTERNOON WATER VAPOR IMAGERY...AMPLE
VERTICAL SHEAR/LOW LEVEL SRH IS IN PLACE TO SUPPORT POTENTIAL FOR
SUPERCELLULAR STRUCTURES AND/OR EMBEDDED SMALL BOWS WITH ATTENDANT
THREAT FOR DAMAGING WINDS AND/OR A TORNADO MAINLY ALONG/EAST OF THE
I-95 CORRIDOR.
..GUYER.. 10/24/2009
.PREV DISCUSSION... /ISSUED 1118 AM CDT SAT OCT 24 2009/
...SYNOPSIS...
A MID LEVEL TROUGH OVER THE GREAT LAKES AND MS VALLEY WILL PROGRESS
NEWD OVER THE OH VALLEY/MID ATLANTIC/NEW ENGLAND THROUGH TONIGHT AS
GRADUAL AMPLIFICATION OF AN UPSTREAM TROUGH OCCURS OVER THE ROCKIES
AND HIGH PLAINS. WITHIN THE LARGER SCALE TROUGH...AN EMBEDDED SPEED
MAX NOW OVER AL/ERN TN/KY WILL EJECT NEWD TO THE CENTRAL
APPALACHIANS THIS EVENING...AND SRN NEW ENGLAND OVERNIGHT. THE ERN
US TROUGH IS ACCOMPANIED BY AN OCCLUDED CYCLONE JUST E OF LAKE
SUPERIOR...A TRAILING COLD FRONT IN THE IMMEDIATE LEE OF THE
CENTRAL/SRN APPALACHIANS...AND A WARM FRONT MOVING NWD FROM THE NRN
MID ATLANTIC INTO SRN NEW ENGLAND. A WEAK SURFACE WAVE WILL FORM
JUST E OF THE BLUE RIDGE THIS AFTERNOON /AT THE OCCLUSION POINT/ AND
PROGRESS NEWD INTO SRN NEW ENGLAND TONIGHT. THIS WILL ALLOW A
MODIFIED WARM SECTOR TO SPREAD NWD INTO ERN PA/NJ/SE NY BY THIS
EVENING...FOLLOWED BY COLD FRONT PASSAGE EARLY TONIGHT.
...MID ATLANTIC REGION THROUGH THIS EVENING...
THE SURFACE COLD FRONT IS CROSSING THE BLUE RIDGE IN VA AND MOVING
EWD INTO THE PIEDMONT OF NC/SC AS OF LATE MORNING. THE WARM SECTOR
AHEAD OF THE FRONT IS CHARACTERIZED BY SURFACE TEMPERATURES IN THE
MID-UPPER 70S ACROSS MUCH OF VA AND NEAR 80 IN ERN NC...AND
DEWPOINTS HAVE BEEN GRADUALLY INCREASING TO THE MID-UPPER 60S.
MEANWHILE...REGIONAL 12Z SOUNDINGS REVEALED RATHER POOR MID LEVEL
LAPSE RATES /AOB 6 C PER KM IN THE 850-500 MB LAYER/. THE
COMBINATION OF THESE FACTORS...ALONG WITH SOME ADDITIONAL SURFACE
HEATING...WILL SUPPORT AFTERNOON MLCAPE VALUES OF 500-1000 J/KG AND
MINIMAL CONVECTIVE INHIBITION.
THUNDERSTORMS ARE EXPECTED TO INCREASE IN COVERAGE/INTENSITY DURING
THE AFTERNOON IN ONE OR MORE BROKEN BANDS ALONG AND JUST E OF THE
SURFACE COLD FRONT FROM CENTRAL NC NWD INTO CENTRAL VA...AND
CONVECTION WILL SPREAD NNEWD INTO ERN PA/NJ AS THE WARM SECTOR
DEVELOPS NWD. DEEP LAYER AND LOW-LEVEL VERTICAL SHEAR WILL REMAIN
WELL WITHIN THE RANGE FAVORING SUPERCELLS...THOUGH THE MAGNITUDE OF
THE DEEP LAYER SHEAR MAY BE A LITTLE STRONG COMPARED TO THE
RELATIVELY WEAK BUOYANCY. THUS...EXPECT A GRADUAL EVOLUTION OF
CONVECTION INTO EMBEDDED SUPERCELL STRUCTURES AND SHORT LINE
SEGMENTS THAT WILL BECOME CAPABLE OF PRODUCING DAMAGING GUSTS AND
POTENTIALLY A COUPLE OF TORNADOES THIS AFTERNOON INTO THIS EVENING.
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