SWODY1
SPC AC 191230
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0730 AM CDT THU APR 19 2007
VALID 191300Z - 201200Z
..NO SVR TSTM AREAS FORECAST...
..SYNOPSIS...
A COMPLEX FLOW REGIME WILL PERSIST OVER THE CONUS WITH A BROAD
TROUGH OVER THE WRN STATES...A RIDGE FROM THE SRN PLAINS TO THE
GREAT LAKES...AND ANOTHER TROUGH OVER THE WRN ATLANTIC AND ERN
SEABOARD. WITHIN THIS BROADER FLOW REGIME...ONE SHORTWAVE TROUGH
WILL EJECT NNEWD FROM CO/WY THIS MORNING TO ERN MT/WRN ND BY TONIGHT
AS ANOTHER STRONG TROUGH APPROACHES CA FROM THE ERN PAC. FARTHER
E...A COMPACT MID LEVEL LOW OVER THE LOWER OH VALLEY WILL MOVE SEWD
OVER THE SAVANNAH RIVER VALLEY BY THIS EVENING. THE PRIMARY
THUNDERSTORM THREAT AREAS WILL BE FOCUSED ALONG THE PATHS OF THE NRN
PLAINS AND SERN STATES SHORTWAVE TROUGHS AND ASSOCIATED SURFACE
REFLECTIONS.
..CENTRAL AND NRN PLAINS TODAY INTO TONIGHT...
A STRONG MID LEVEL SHORTWAVE TROUGH IS EJECTING NNEWD AROUND THE WRN
PERIPHERY OF THE BLOCKING RIDGE FROM ONTARIO SSWWD INTO THE SRN
PLAINS. A DEEP SURFACE CYCLONE WILL DEVELOP GENERALLY NWD ALONG A
BAROCLINIC ZONE ACROSS ERN MT...WHILE A WEAKENING PACIFIC COLD FRONT
SWEEPS EWD TODAY ACROSS NEB/SD.
A WNW-ESE ORIENTED BAND OF CLOUDS/RAIN WILL SPREAD NEWD ACROSS THE
DAKOTAS/MT TODAY IN THE REGION OF STRONG ASCENT DOWNSTREAM FROM THE
MID LEVEL TROUGH...THOUGH FORCING FOR ASCENT SHOULD WEAKEN GRADUALLY
AS THE ENTIRE SYSTEM BEGINS TO LOSE AMPLITUDE. INSTABILITY WILL BE
QUITE LIMITED WITHIN THE INITIAL PRECIPITATION BAND GIVEN THE RATHER
POOR LAPSE RATES AND LIMITED LOW-LEVEL MOISTURE ACROSS THE NRN
PLAINS. HOWEVER...THERE WILL BE SOME THREAT FOR ADDITIONAL STORM
DEVELOPMENT THIS AFTERNOON IN SD WITH THE MID LEVEL THERMAL TROUGH
AND STEEPER LOW-MID LEVEL LAPSE RATES.
SOMEWHAT GREATER MOISTURE /BOUNDARY LAYER DEWPOINTS IN THE LOW-MID
50S/ WILL SPREAD NWD ACROSS KS TO SRN NEB BY LATE AFTERNOON IN
ADVANCE OF THE SURFACE BOUNDARY SAGGING SEWD. THIS BOUNDARY SHOULD
STALL BY EARLY TONIGHT...AND PERHAPS BEGIN TO LIFT NWWD AS A WARM
FRONT ACROSS WRN KS AND CENTRAL NEB TOWARD MORNING. THE INCREASING
MOISTURE AND AFTERNOON TEMPERATURES NEAR 70 F WILL CONTRIBUTE TO
MLCAPE VALUES NEAR 500 J/KG ALONG THE BOUNDARY IN KS...WHERE COOLING
IN THE 850-700 MB LAYER AND REDUCED CONVECTIVE INHIBITION COULD
ALLOW ONE OR TWO THUNDERSTORMS TO DEVELOP BY LATE AFTERNOON. IF
STORMS FORM...THE WEAK INSTABILITY AND MODEST DEEP LAYER SHEAR MAY
SUPPORT SPLITTING/SUPERCELL STORMS WITH A MARGINAL HAIL/WIND THREAT.
BY TONIGHT...THE POSSIBILITY WILL ALSO EXIST FOR ELEVATED STORMS IN
A WEAK WAA REGIME ALONG AND JUST NW OF THE RETREATING SURFACE
BOUNDARY IN KS...WHERE THE THREAT FOR SOME HAIL COULD PERSIST
OVERNIGHT.
..SRN APPALACHIANS/SC AREA FROM MIDDAY INTO EARLY TONIGHT...
A CONCENTRATED REGION OF ASCENT WILL ACCOMPANY THE COMPACT MID LEVEL
LOW ACROSS THE SRN APPALACHIANS AND SC TODAY INTO THIS EVENING. A
WEAK SURFACE LOW IS EXPECTED TO DEVELOP SEWD ALONG A PRE-EXISTING
BOUNDARY /ROUGHLY ALONG THE SAVANNAH RIVER/...AND THE AREA NEAR AND
IMMEDIATELY SE OF THE SURFACE LOW SHOULD BE THE FOCUS FOR
THUNDERSTORM DEVELOPMENT TODAY. DESPITE THE PRIOR PASSAGE OF A
WEAKENING SRN STREAM TROUGH AND ASSOCIATED COLD FRONTAL PASSAGE...A
NARROW CORRIDOR OF 48-52 F DEWPOINTS SHOULD PERSIST ALONG AND JUST E
OF THE SECONDARY BOUNDARY ALONG THE SC/GA BORDER. THIS RESIDUAL
MOISTURE...ALONG WITH SOME DAYTIME HEATING AND STEEPER LAPSE RATES
WITH THE SMALL COLD CORE LOW...WILL RESULT IN AFTERNOON MLCAPE
VALUES OF 200-400 J/KG. RELATIVELY COOL MID LEVEL TEMPERATURES AND
EFFECTIVE BULK SHEAR OF 30-35 KT COULD SUPPORT A FEW
ORGANIZED/MARGINAL SUPERCELL STORMS CAPABLE OF PRODUCING SOME HAIL
AND STRONG OUTFLOW GUSTS THIS AFTERNOON INTO EARLY TONIGHT.
.THOMPSON/GRAMS.. 04/19/2007
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