SPC AC 211934
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0234 PM CDT TUE APR 21 2009
VALID 212000Z - 221200Z
...NO SVR TSTM AREAS FORECAST...
...ERN U.S...
LEADING EDGE OF LARGE SCALE FORCING IS NOW SPREADING EAST OF THE
HIGHER TERRAIN FROM CNTRL PA...SWD INTO WRN NC. ALONG THE LEADING
EDGE OF THIS FORCING...MULTIPLE BANDS OF DEEP CONVECTION WITH
EMBEDDED THUNDERSTORMS ARE SPREADING INTO AN AXIS OF STEEP LAPSE
RATES WHERE STRONG BOUNDARY LAYER HEATING HAS ALLOWED SFC
TEMPERATURES TO WARM INTO THE 60S TO LOWER 70S. CU FIELD IS
SPREADING/DEVELOPING NWD ACROSS THE DELMARVA INTO A REGION THAT IS A
BIT MORE CONVERGENT THAN AREAS ACROSS NC/VA. AS A
RESULT...ORGANIZATIONAL POTENTIAL MAY BE ENHANCED ALONG WRN FRINGE
OF RECOVERING AIRMASS OVER CNTRL PA WHERE BUOYANCY IS EXPECTED TO
INCREASE OVER THE NEXT FEW HOURS. OTHERWISE...MARGINALLY SEVERE
HAIL AND GUSTY WINDS MAY ACCOMPANY ACTIVITY AS IT SPREADS ACROSS THE
MIDDLE ATLANTIC REGION.
...NERN GULF COAST...
NARROW BAND OF SHALLOW ELEVATED CONVECTION IS SAGGING SEWD ACROSS
CNTRL MS/AL. THIS ACTIVITY IS ASSOCIATED WITH A SECONDARY BAND OF
FORCING THAT MAY TRIGGER THUNDERSTORMS ALONG ERN PLUME OF
INSTABILITY WITHIN THE NEXT FEW HOURS.
...ELSEWHERE...
CU FIELD IS JUST BEGINNING TO DEVELOP ACROSS THE HIGHER TERRAIN FROM
NRN CA INTO SWRN ID. A FEW THUNDERSTORMS MAY YET DEVELOP ACROSS
THIS REGION AS PARCELS REACH THEIR CONVECTIVE TEMPERATURES.
..DARROW.. 04/21/2009
.PREV DISCUSSION... /ISSUED 1127 AM CDT TUE APR 21 2009/
...MID-ATLANTIC/NORTH CAROLINA...
AN UPPER-LEVEL LOW OVER THE GREAT LAKES WILL GRADUALLY OPEN UP TODAY
AS A SHORTWAVE TROUGH EVIDENT ON WATER VAPOR IMAGERY OVER THE OH
VALLEY ROTATES EWD AROUND THE UPPER-LEVEL TROUGH. LARGE-SCALE ASCENT
ASSOCIATED WITH THE SHORTWAVE AND SFC HEATING SHOULD RESULT IN AN
INCREASE IN THUNDERSTORM DEVELOPMENT ACROSS THE CNTRL
APPALACHIANS...MID-ATLANTIC AND NORTH CAROLINA THIS AFTERNOON.
REGIONAL FORECAST SOUNDINGS AT 21Z SHOW WEAK INSTABILITY WITH
MODERATE TO STRONG VERTICAL SHEAR ASSOCIATED WITH A BAND OF
MID-LEVEL FLOW EXTENDING NNEWD ACROSS THE ERN SEABOARD. THIS ALONG
WITH COLD TEMPS ALOFT /500 MB TEMPS OF -23 TO -25C/ ON THE ERN SIDE
OF THE UPPER-LEVEL TROUGH SHOULD BE SUFFICIENT FOR HAIL WITH THE
STRONGER UPDRAFTS. THE LATEST RUC ANALYZES THE STEEPEST MID-LEVEL
LAPSE RATES OVER THE SRN APPALACHIAN MTNS EXTENDING NEWD INTO WRN
NORTH CAROLINA WHERE 850 TO 700 MB LAPSE RATES EXCEED 7.0 C/KM. DUE
TO THE PROXIMITY TO THE STEEPEST MID-LEVEL LAPSE RATES...NRN NC AND
VIRGINIA COULD HAVE THE GREATEST POTENTIAL FOR HAIL LATE THIS
AFTERNOON.
...ERN GULF COAST STATES...
THE SRN EXTENT OF AN ERN U.S. UPPER-LEVEL TROUGH WILL BE IN PLACE
ACROSS THE ERN GULF COAST STATES TODAY. A SHORTWAVE TROUGH IS
EVIDENT ON WATER VAPOR IMAGERY OVER THE LOWER MS VALLEY. AS THIS
SHORTWAVE TROUGH DRIFTS EWD AND SFC TEMPS WARM ACROSS SRN AL AND SW
GA...ISOLATED THUNDERSTORMS MAY DEVELOP THIS AFTERNOON.
HOWEVER...MODEL FORECASTS ARE IN DISAGREEMENT CONCERNING CONVECTIVE
INITIATION PROBABLY DUE TO WEAK LOW-LEVEL CONVERGENCE. IF
THUNDERSTORMS CAN INITIATE...STRONG VERTICAL SHEAR AND COLD TEMPS
ALOFT EVIDENT ON FORECAST SOUNDINGS IN SRN AL COULD BE SUFFICIENT
FOR HAIL. WILL NOT ADD A 5 PERCENT HAIL AREA DUE TO CONVECTIVE
UNCERTAINTY.
CLICK TO GET WUUS01 PTSDY1 PRODUCT
NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 0100Z
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