SPC AC 181630
DAY 1 CONVECTIVE OUTLOOK
NWS STORM PREDICTION CENTER NORMAN OK
1130 AM CDT TUE AUG 18 2009
VALID 181630Z - 191200Z
...THERE IS A SLGT RISK OF SVR TSTMS THIS AFTERNOON INTO TONIGHT FOR
PARTS OF THE CENTRAL AND SRN HIGH PLAINS...
...SYNOPSIS...
THE PREVIOUSLY CLOSED LOW OVER ONTARIO WILL EJECT NEWD AS AN OPEN
WAVE AS AN UPSTREAM TROUGH OVER CENTRAL AB DIGS SEWD TOWARD THE NRN
HIGH PLAINS BY THE END OF THE PERIOD. A SEPARATE...SMALLER SCALE
TROUGH NOW OVER NE UT/NW CO WILL ALSO PROGRESS EWD OVER CO THROUGH
THIS EVENING...AND REACH WRN KS OVERNIGHT IN ADVANCE OF THE DIGGING
TROUGH FROM AB.
...E/SE CO AND NE NM THIS AFTERNOON TO NW OK OVERNIGHT...
THE REMNANTS OF OVERNIGHT CONVECTION PERSIST OVER OK TO THE N OF A
PRONOUNCED OUTFLOW BOUNDARY THAT HAS MOVED SEWD INTO SE OK AND N TX.
REGIONAL 12Z SOUNDINGS FROM DDC/AMA/OUN CONFIRM THE REDUCTION IN
LAPSE RATES EXPECTED FROM SUCH WIDESPREAD CONVECTIVE OVERTURNING...
AND THE RESIDUAL CLOUDS/RAIN-COOLED AIR MASS WILL LIMIT POTENTIAL
INSTABILITY THIS AFTERNOON ACROSS MUCH OF SRN KS AND OK. FARTHER
W...STEEPER MID LEVEL LAPSE RATES PERSIST IN PROXIMITY TO THE HIGHER
TERRAIN...AND BOUNDARY LAYER DEWPOINTS IN THE LOW-MID 50S HAVE
SPREAD WWD/NWWD TO THE FRONT RANGE IN CO/NE NM. AS THE MORNING
STRATUS CLOUDS ERODE AND TEMPERATURES WARM INTO THE 70S...AFTERNOON
MLCAPE VALUES WILL INCREASE TO 1000-2000 J/KG.
THUNDERSTORM DEVELOPMENT IS EXPECTED BY MID-LATE AFTERNOON
IMMEDIATELY E OF THE MOUNTAINS FROM NE NM INTO SE CO AS CONVECTIVE
INHIBITION IS REMOVED BY SURFACE HEATING/MIXING...AND AS THE BELT OF
ASCENT DOWNSTREAM FROM THE UT/WRN CO MID LEVEL TROUGH REACHES ERN
CO. THE MODERATE INSTABILITY WILL COMBINE WITH EFFECTIVE BULK SHEAR
OF 40-50 KT TO SUPPORT SUPERCELLS WITH THE POTENTIAL TO PRODUCE
ISOLATED VERY LARGE HAIL AND DAMAGING WINDS. LOW-LEVEL SHEAR AND
BOUNDARY LAYER MOISTURE MAY ALSO BE SUFFICIENT FOR A TORNADO OR TWO
THIS EVENING IN THE SE CO AREA. OVERNIGHT... CONVECTION IS EXPECTED
TO EVOLVE INTO ANOTHER MCS THAT WILL MOVE EWD/ESEWD ACROSS SW
KS...THE OK AND NRN TX PANHANDLES...AND NW OK LATE IN THE PERIOD.
THE THREAT FOR AT LEAST ISOLATED DAMAGING WINDS AND LARGE HAIL WILL
REACH AS FAR E AS NW OK.
...NEB/SD AREA THIS AFTERNOON INTO TONIGHT...
A WEAK LEE TROUGH WILL DEVELOP EWD INTO WRN NEB AND THE WRN DAKOTAS
THIS AFTERNOON /DOWNSTREAM FROM THE AMPLIFYING NRN STREAM TROUGH/.
A GRADUAL INCREASE IN LOW-LEVEL MOISTURE IS EXPECTED E OF THE TROUGH
ACROSS NEB/SD...WHERE SURFACE HEATING WILL RESULT IN THE DEVELOPMENT
OF WEAK-MODERATE INSTABILITY BY MID-LATE AFTERNOON. A FEW
THUNDERSTORMS COULD FORM ALONG THE BOUNDARY BY LATE AFTERNOON...AND
THE STRONGEST STORMS MAY PRODUCE MARGINALLY SEVERE HAIL AND ISOLATED
DAMAGING WINDS. SOME OF THIS CONVECTION MAY ALSO PERSIST INTO
TONIGHT AND SPREAD EWD INTO ERN SD AND ADJACENT AREAS IN CONJUNCTION
WITH LOW-LEVEL WAA AND WEAK ASCENT IN ADVANCE OF THE NRN PLAINS MID
LEVEL TROUGH.
...WRN PA/NY THIS AFTERNOON...
A BROKEN BAND OF PRE-FRONTAL CONVECTION IS EXPECTED THIS AFTERNOON
FROM ERN OH/WRN PA NEWD INTO WRN NY. ROUGHLY 30 KT LOW-LEVEL FLOW
WILL SUPPORT A MARGINAL DAMAGING WIND THREAT WITH THESE STORMS...BUT
WEAK INSTABILITY/POOR LAPSE RATES WILL LIMIT THE OVERALL THREAT.
..THOMPSON/GARNER.. 08/18/2009
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NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 2000Z
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