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MESOSCALE DISCUSSION 0537
NWS STORM PREDICTION CENTER NORMAN OK
0109 PM CDT MON APR 20 2009
AREAS AFFECTED...VA/NORTHERN NC
CONCERNING...SEVERE POTENTIAL...WATCH LIKELY
VALID 201809Z - 202015Z
SEVERE THREAT IS EXPECTED TO GRADUALLY INCREASE ACROSS PORTIONS OF
VA/NORTHERN NC BY MID/LATE AFTERNOON. A TORNADO WATCH WILL LIKELY BE
NEEDED WITHIN THE NEXT 1-2 HOURS ACROSS PORTIONS OF VA/NORTHERN NC.
LATEST SURFACE ANALYSIS FEATURES A SECONDARY SUB-SYNOPTIC LOW ACROSS
FAR WESTERN VA EARLY THIS AFTERNOON...WITH A GENERAL WEST-EAST
ORIENTED WAVY WARM FRONT CONTINUING TO SLOWLY PROGRESS NORTHWARD
ACROSS SOUTHERN VA. IN THE WAKE OF EARLY DAY PRECIPITATION AND CLOUD
COVER...70+ F SURFACE TEMPERATURES AND UPPER 50S/LOWER 60S F
DEWPOINTS ARE RESULTING IN A WEAKLY CAPPED/INCREASINGLY UNSTABLE
BOUNDARY LAYER THIS AFTERNOON...WITH AROUND 500-900 J/KG MLCAPE.
WITH INCREASING/DEEPENING CU FIELD NOTED PER VISIBLE IMAGERY...TSTMS
SHOULD STEADILY INCREASE THROUGH MID/LATE AFTERNOON...AIDED BY A MID
LEVEL IMPULSE CURRENTLY CROSS KY/TN PER WATER VAPOR IMAGERY. WITH
30-35 KT SOUTHWESTERLY FLOW AROUND 1 KM...SOMEWHAT BACKED LOW LEVEL
FLOW AHEAD OF THE SECONDARY SURFACE LOW WILL CONTRIBUTE TO
SUFFICIENT LOW LEVEL SRH IN THE WARM SECTOR ACROSS SOUTHERN
VA/NORTHERN NC FOR SUPERCELLS/POSSIBLE TORNADOES...ALTHOUGH THE
PRIMARY TORNADO THREAT MAY BE TIED TO ANY STORM INTERACTING
WITH/TRACKING PARALLEL TO THE WARM FRONT ACROSS VA.
OTHERWISE...STEEP LAPSE RATES/AMPLE VERTICAL SHEAR WILL SUPPORT A
RISK FOR LARGE HAIL/DAMAGING WINDS.
..GUYER.. 04/20/2009
ATTN...WFO...AKQ...MHX...RAH...RNK...
LAT...LON 37447902 38087736 37527650 35917703 35387805 36057973
37447902
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