SWODY1
SPC AC 221252
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0752 AM CDT WED APR 22 2009
VALID 221300Z - 231200Z
...NO SVR TSTM AREAS FORECAST...
...SYNOPSIS...
A DEEP TROUGH OVER THE OH VALLEY WILL MOVE EWD OVER NEW ENGLAND AS
AN EMBEDDED SHORTWAVE TROUGH OVER NC ROTATES NEWD OFF THE SE NEW
ENGLAND COAST. IN THE WAKE OF THE ERN TROUGH AND IN CONJUNCTION
WITH RISING HEIGHTS...A BAROCLINIC ZONE WILL LIFT SLOWLY NWD ACROSS
THE GULF COAST AND SRN PLAINS TO THE E/SE OF A LEE CYCLONE OVER THE
TX PANHANDLE. THE LEE CYCLONE WILL FORM AT LEAST PARTIALLY IN
RESPONSE TO A WEAK MID LEVEL TROUGH IN THE SRN STREAM WHICH WILL
PROGRESS EWD FROM CO TO KS...WHILE A SEPARATE UPSTREAM TROUGH MOVES
EWD FROM SRN CA TO SRN AZ. ANOTHER LARGER SCALE NRN STREAM TROUGH
WILL DIG ESEWD OVER THE PAC NW TOWARD THE NRN ROCKIES LATE IN THE
PERIOD.
LOW-LEVEL MOISTURE IS SOMEWHAT MARGINAL/LIMITED ACROSS THE MAJORITY
OF THE CONUS IN THE WAKE OF THE DEEP ERN TROUGH AND MULTIPLE FRONTAL
INTRUSIONS. HOWEVER...A SHALLOW MARINE LAYER IS RETURNING NWD/NEWD
ACROSS THE NW AND N CENTRAL GULF COAST...ALONG AND S OF THE
SLOW-MOVING FRONT. MEANWHILE...A VERY BROAD PLUME OF STEEP MID
LEVEL LAPSE RATES /ORIGINATING AS DEEP MIXED LAYERS OVER THE
INTERMOUNTAIN WEST/ WILL OVERSPREAD THE PLAINS TO THE MS VALLEY.
THE COMBINATION OF THE STEEP LAPSE RATES...MARGINAL MOISTURE..AND
STRONG SURFACE HEATING WILL SUPPORT A BROAD AREA OF THUNDERSTORM
POTENTIAL THIS AFTERNOON/EVENING FROM THE GREAT BASIN INTO THE
CENTRAL/NRN ROCKIES. THE STORMS SHOULD BE HIGH-BASED AND RATHER
ISOLATED IN COVERAGE...AND FOCUSED PRIMARILY BY LOCAL OROGRAPHY IN
PROXIMITY TO THE ZONES OF ASCENT NEAR AND DOWNSTREAM FROM THE
DIGGING TROUGH OVER THE PAC NW...AND THE WEAK WAVE OVER CO.
...PLAINS AREA LATE THIS AFTERNOON INTO TONIGHT...
STRONG SURFACE HEATING/MIXING AND BOUNDARY LAYER DEWPOINTS IN THE
40S MAY SUPPORT THE FORMATION OF ISOLATED HIGH-BASED STORMS INVOF
THE LEE TROUGH/CYCLONE AND RETURNING WARM FRONT ACROSS THE CENTRAL
HIGH PLAINS. THESE STORMS MAY PRODUCE STRONG OUTFLOW WINDS AND
MAINLY SMALL HAIL THIS AFTERNOON INTO EARLY TONIGHT ACROSS NE CO/SW
NEB/NRN KS. FARTHER SE... A GRADUAL INCREASE IN LOW-LEVEL MOISTURE
THROUGH ADVECTION AND EVAPOTRANSPIRATION WILL CONTRIBUTE TO MODERATE
INSTABILITY DEVELOPING ALONG THE WARM FRONT FROM OK INTO AR BY LATE
AFTERNOON. HOWEVER...SURFACE HEATING/MIXING ALONE APPEARS
INSUFFICIENT TO SUPPORT AFTERNOON THUNDERSTORM DEVELOPMENT ACROSS
THIS AREA. THE SOMEWHAT GREATER CHANCE FOR STORM FORMATION WILL BE
LATER TONIGHT IN A STRENGTHENING WAA REGIME ON THE NOSE OF THE
LLJ...WHICH WILL EXTEND AS FAR N AS KS OVERNIGHT. A SEPARATE LLJ
AND ZONE OF WAA MAY ALSO SUPPORT ISOLATED THUNDERSTORMS LATE TONIGHT
AS FAR NE AS SRN MN/NRN IA.
...OH AREA BY EARLY-MID AFTERNOON...
VERY COLD MID LEVEL TEMPERATURES /-30 C AT 500 MB/ WILL OVERSPREAD
THE OH AREA DURING THE DAY IN ASSOCIATION WITH PASSAGE OF THE LARGE
SCALE TROUGH AXIS. THE COLD TEMPERATURES AND SURFACE HEATING WILL
CONTRIBUTE TO SUFFICIENT INSTABILITY FOR CONVECTION AND A FEW
LIGHTNING STRIKES. HOWEVER...VERY LIMITED MOISTURE /BOUNDARY LAYER
DEWPOINTS IN THE LOW 30S/ AND WEAK INSTABILITY SUGGEST THAT THE
PROBABILITY OF SEVERE HAIL WILL BE TOO SMALL TO WARRANT AN OUTLOOK
AREA.
..THOMPSON/GRAMS.. 04/22/2009
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