SPC AC 191631
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1131 AM CDT WED AUG 19 2009
VALID 191630Z - 201200Z
...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTERNOON/TONIGHT FOR
OK/KS NEWD TO THE MID MS VALLEY...
...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTERNOON FROM CENTRAL PA
NEWD TO ERN NY/SRN VT...
...SYNOPSIS...
A COMPLICATED SCENARIO IS EXPECTED TODAY AS MULTIPLE EMBEDDED
SHORTWAVE TROUGHS EJECT NEWD OVER IA/MO IN ADVANCE OF AN AMPLIFYING
NRN STREAM TROUGH OVER THE NRN HIGH PLAINS. THE ASSOCIATED SURFACE
PATTERN IS EQUALLY COMPLEX AS AN INITIALLY WEAK SURFACE CYCLONE IN
NE SD DEVELOPS ENEWD ACROSS CENTRAL MN THIS EVENING TO THE DLH AREA
OVERNIGHT. A COUPLE OF COLD FRONTAL SURGES ARE NOTED IN SURFACE
ANALYSES ACROSS THE CENTRAL DAKOTAS/WRN NEB...AND A LEE TROUGH
EXTENDS FROM CENTRAL NEB TO THE TX PANHANDLE AS OF MID MORNING.
FARTHER E...AN APPARENT DUAL WARM FRONT STRUCTURE EXISTS WITH THE
NRN BAROCLINIC ZONE FROM SRN MN TO NRN IL...AND THE SECOND BOUNDARY
IS LIFTING NWD ACROSS THE LOWER MO VALLEY /ERN NEB TO CENTRAL MO/.
ADDITIONALLY...MULTIPLE OUTFLOW BOUNDARIES FROM OVERNIGHT CONVECTION
ARE STILL IDENTIFIABLE ACROSS CENTRAL/NE OK AND CENTRAL MO.
...NE MO/ERN IA/NW IL AREA THIS AFTERNOON...
A REMNANT MCV FROM OVERNIGHT CONVECTION WILL PROGRESS NEWD FROM NW
MO THIS MORNING TO ERN IA BY LATER THIS AFTERNOON. WIDESPREAD
CLOUDS PERSIST OVER MO/IA IN ADVANCE OF THIS WAVE AND ANOTHER
SHORTWAVE TROUGH MOVING NEWD FROM ERN NEB TOWARD WRN/NWRN IA...AND
THE CLOUDS WILL TEND TO LIMIT SURFACE HEATING IN THE DEVELOPING
SURFACE WARM SECTOR. HOWEVER...BOUNDARY LAYER DEWPOINTS OF 68-72 F
WILL SPREAD NWD FROM MO INTO ERN IA AS THE SRN WARM FRONT LIFTS
NWD...AND THIS WILL CONTRIBUTE TO LOW-LEVEL DESTABILIZATION.
LOW-LEVEL SHEAR WILL ALSO BE SOMEWHAT STRONGER IN ADVANCE OF THE MCV
WITH A LLJ SEGMENT. THE NET RESULT WILL BE AN ENVIRONMENT FAVORABLE
FOR A FEW SUPERCELLS IMMEDIATELY E OF THE MCV...WHERE THE MOIST
BOUNDARY LAYER AND EFFECTIVE SRH AOA 200 M2/S2 MAY SUPPORT A COUPLE
OF TORNADOES THIS AFTERNOON.
...OK/ERN KS/WRN MO THIS AFTERNOON INTO TONIGHT...
THE REMNANT OF THE MID LEVEL TROUGH CROSSING CO YESTERDAY APPEARS TO
BE LOCATED OVER CENTRAL KS AS OF MID MORNING...THOUGH PRIOR
CONVECTION HAS SUBSTANTIALLY COMPLICATED THE TRACKING AND
IDENTIFICATION OF INDIVIDUAL EMBEDDED SHORTWAVE TROUGHS. THE
ELEVATED CONVECTION ONGOING OVER NRN OK AND SRN/SERN KS IS EXPECTED
TO SPREAD EWD AND GRADUALLY DIMINISH IN COVERAGE THIS
AFTERNOON...WITH BOUNDARY LAYER WARMING/MOISTENING IN ITS WAKE. A
WEAK OUTFLOW BOUNDARY /NOW JUST N OF I-40 IN OK/ COULD PERSIST INTO
THIS AFTERNOON...WHILE THE ORIGINAL OUTFLOW TRAILING FROM THE MCV
REMNANTS IN MO WILL LIFT NWD FROM THE RED RIVER AND DISSIPATE IN
RESPONSE TO STRONG SURFACE HEATING ACROSS SRN OK.
THE ERN EXTENT OF A STEEP MID LEVEL LAPSE RATE PLUME WILL OVERLAP
THE MOIST BOUNDARY LAYER ALONG AND E OF I-35 IN OK/KS...RESULTING IN
THE DEVELOPMENT OF MLCAPE IN EXCESS OF 2000 J/KG THIS AFTERNOON.
THE MORE PROBABLE SCENARIO WILL BE FOR STORMS TO DEVELOP LATE THIS
AFTERNOON ALONG A WEAK LEE TROUGH FROM CENTRAL KS INTO N/NW OK...AND
THEN STORMS WILL SPREAD EWD/SEWD TONIGHT ACROSS KS/OK/WRN MO.
MEANWHILE...VERTICAL SHEAR WILL BE SUFFICIENT FOR SUPERCELLS AS SLY
LOW-LEVEL FLOW IS MAINTAINED AND 30-40 KT MID LEVEL FLOW PERSISTS
OVER THE WARM SECTOR AS ANOTHER EMBEDDED SHORTWAVE TROUGH MOVES
ESEWD FROM CENTRAL NEB/WRN KS. MODIFIED 12Z SOUNDINGS AND MODEL
FORECAST SOUNDINGS SHOW PROFILES FAVORABLE FOR LARGE HAIL AND
DAMAGING WINDS...AND POTENTIALLY A COUPLE OF TORNADOES IF A ZONE OF
ENHANCED LOW-LEVEL SHEAR CAN BE MAINTAINED THROUGH THE AFTERNOON
ALONG THE REMNANT OUTFLOW BOUNDARY IN OK.
...ERN NEB/SE SD/WRN IA THIS AFTERNOON/EVENING...
THE PRIMARY MID LEVEL TROUGH IS DIGGING SEWD OVER THE WRN
DAKOTAS...AND OTHER SPEED MAXIMA ARE EJECTING GENERALLY EWD OVER
NEB/SD IN ADVANCE OF THE MAIN WAVE. AS THE SURFACE COLD FRONT
PROGRESSES SEWD THIS AFTERNOON AND SOME LOW-LEVEL WARMING/MOISTENING
OCCURS IN THE WAKE OF THE WRN IA CONVECTION... THE ENVIRONMENT WILL
FAVOR THUNDERSTORM DEVELOPMENT BY MID-LATE AFTERNOON ALONG THE FRONT
FROM E/SE SD SWD INTO ERN NEB. VERTICAL SHEAR WILL NOT BE
PARTICULARLY STRONG /ESPECIALLY FROM NE NEB NWD/...BUT A FEW STORMS
COULD PRODUCE AT LEAST ISOLATED LARGE HAIL AND DAMAGING GUSTS THIS
AFTERNOON/EVENING.
...CENTRAL PA NEWD TO ERN NY/SRN VT THIS AFTERNOON...
A MID LEVEL SPEED MAX IS MOVING EWD FROM WRN NY...WITH A BELT OF
ENHANCED MID LEVEL FLOW SWD INTO NRN PA. SURFACE HEATING WILL
CONTRIBUTE TO MODERATE DESTABILIZATION DOWNSTREAM FROM THIS EJECTING
WAVE...AND SCATTERED STORMS SHOULD FORM THIS AFTERNOON FROM CENTRAL
PA NEWD INTO NY. THE COMBINATION OF MODERATE INSTABILITY AND MODEST
VERTICAL SHEAR COULD SUPPORT A FEW STORMS WITH DAMAGING GUSTS THIS
AFTERNOON.
..THOMPSON/GARNER.. 08/19/2009
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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 2000Z
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