DAY1SVR: Day 1 Convective Outlook

ACUS01 KWNS 201240
SWODY1
SPC AC 201238

DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
0738 AM CDT SUN MAY 20 2007

VALID 201300Z - 211200Z

..THERE IS A SLGT RISK OF SVR TSTMS THIS AFTERNOON INTO TONIGHT
ACROSS THE NRN HIGH PLAINS...

..NRN HIGH PLAINS THIS AFTERNOON INTO TONIGHT...
THE MID LEVEL LOW JUST W OF VANCOUVER WILL MOVE ESEWD TO WA BY
TONIGHT...WHILE A LEAD SHORTWAVE TROUGH NEAR THE ORE COAST
PROGRESSES ENEWD TO WRN MT BY THIS EVENING. CYCLOGENESIS IS
EXPECTED IN ADVANCE OF THE MID LEVEL TROUGH ALONG A LOW-LEVEL
BAROCLINIC ZONE NEAR THE WY/MT BORDER. THE RESULTANT SURFACE LOW
WILL THEN DEVELOP NEWD INTO WRN ND BY THE END OF THE PERIOD.
FARTHER E IN THE NRN PLAINS...A SURFACE FRONT ACROSS SRN SD AND SRN
MN WILL BEGIN TO RETREAT NWD AS A WARM FRONT IN RESPONSE TO THE
PRESSURE FALLS ACROSS THE NRN HIGH PLAINS BY LATER TODAY INTO
TONIGHT.

THE SURFACE WARM SECTOR ACROSS THE CENTRAL/NRN PLAINS CONSISTS OF A
MODIFIED CONTINENTAL POLAR AIR MASS WITH DEWPOINTS GENERALLY IN THE
MID-UPPER 50S...WHICH CONTINUES TO BE LESS THAN SHOWN IN SHORT TERM
MODEL FORECASTS. THIS MOISTURE...IN COMBINATION WITH AFTERNOON
TEMPERATURES WELL INTO THE 80S AND STEEP MID LEVEL LAPSE
RATES...WILL RESULT IN MLCAPE VALUES OF 1000-2000 J/KG S OF THE WARM
FRONT AND E OF THE LEE TROUGH ACROSS NEB/SD. VERTICAL SHEAR WILL BE
RATHER WEAK OVER MUCH OF THE WARM SECTOR DURING THE
AFTERNOON...WHICH SUGGESTS THAT THE AFTERNOON CONVECTION ALONG THE
LEE TROUGH AND WARM FRONT SHOULD BE LARGELY MULTICELL CLUSTERS WITH
ONLY SHORT-LIVED THREATS FOR DOWNBURST WINDS OR HAIL.

VERTICAL SHEAR WILL BECOME STRONGER TOWARD NERN WY AND MT IN CLOSER
PROXIMITY TO THE MID LEVEL JET. HERE...MORE ORGANIZED STORM TYPES
ARE EXPECTED...INCLUDING SUPERCELLS AND BOWING SEGMENTS.
THUNDERSTORM DEVELOPMENT IS EXPECTED BY MID AFTERNOON ACROSS S
CENTRAL MT IN ADVANCE OF THE APPROACHING MID LEVEL WAVE AND SURFACE
LOW...AND STORMS WILL SUBSEQUENTLY SPREAD EWD/NEWD TOWARD SERN MT
AND THE WRN DAKOTAS BY TONIGHT. SOMEWHAT LIMITED LOW-LEVEL MOISTURE
/BOUNDARY LAYER DEWPOINTS ONLY IN THE LOWER 50S/ AND STEEP LAPSE
RATES WILL GENERALLY FAVOR LARGE HAIL AND DAMAGING WINDS AS THE
PRIMARY SEVERE THREATS. THE DAMAGING WIND THREAT WILL BE PARTIALLY
DEPENDENT UPON UPSCALE GROWTH OF CONVECTION INTO A FORWARD
PROPAGATING MCS BY THIS EVENING/EARLY TONIGHT ACROSS SE MT AND SW
ND.

..SRN NM INTO FAR W TX THIS AFTERNOON...
A BROAD/WEAK SRN STREAM TROUGH WILL PERSIST OVER SRN AZ/SW NM AND NW
MEXICO. BOUNDARY LAYER DEWPOINTS INTO THE MID 50S HAVE SPREAD WWD
INTO SRN NM/W TX IN THE WAKE OF A PRIOR FRONTAL INTRUSION...AND A
PLUME OF STEEP LOW-MID LEVEL LAPSE RATES EXISTS OVER THIS AREA. THE
NRN FRINGE OF THE STRONGER MID-UPPER FLOW WITH THE SRN STREAM WILL
EXTEND AS FAR N AS THE BIG BEND...WHERE AN ISOLATED SUPERCELL OR TWO
MAY OCCUR. GIVEN THE SOMEWHAT MARGINAL NATURE OF THE THREAT...WILL
OUTLOOK ONLY LOW HAIL AND WIND PROBABILITIES.

.THOMPSON/TAYLOR.. 05/20/2007

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