MESOSCALE PRECIPITATION DISCUSSION 0132
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1213 PM EDT MON MAY 07 2018

AREAS AFFECTED...EASTERN WV & WESTERN VA

CONCERNING...HEAVY RAINFALL...FLASH FLOODING POSSIBLE

VALID 071612Z - 072212Z

SUMMARY...SLOW-MOVING THUNDERSTORMS ARE EXPECTED TO DEVELOP OVER A
REGION WITH SATURATED SOILS/HIGH STREAM FLOW.  HOURLY RAIN TOTALS
TO 1.5" WITH LOCAL AMOUNTS TO 2" ARE EXPECTED.

DISCUSSION...THE APPROACH OF AN ELONGATED SHORTWAVE -- SHOWING UP
ON WATER VAPOR IMAGERY AS A NARROW ZONE OF MID-LEVEL DRYING --
INTO WV AND FAR SOUTHWEST VA IN COMBINATION WITH DAYTIME HEATING
HAS LED TO RECENT CUMULUS DEVELOPMENT OVER SOME OF THE TERRAIN,
WITH EARLY SHOWERS DEVELOPING ACROSS SOUTHWEST PA NEAR A FRONTAL
ZONE.  PRECIPITABLE WATER VALUES ARE IN THE 0.75-1" RANGE.  THE
MORNING SOUNDING FROM PITTSBURGH SHOWED SATURATION FROM THE
SURFACE UP TO 650 HPA.  ML CAPE VALUES HAVE RISEN INTO THE 100-250
J/KG RANGE NEAR AREAS OF ENHANCED CUMULUS DEVELOPMENT.  INFLOW AT
850 HPA IS OUT OF THE EAST AT 20 KTS PER THE STERLING VA VAD WIND
PROFILE.

THE GUIDANCE SHOWS A RAMP UP FOR SHOWER AND THUNDERSTORM
DEVELOPMENT FROM HERE ON OUT AS INSTABILITY RISES THIS AFTERNOON. 
WITH 850 HPA CONVERGENT FLOW BECOMING MORE EASTERLY REGIONALLY AND
THE MEAN 850-400 HPA WIND LIGHT WESTERLY, THERE IS CONCERN THAT
INDIVIDUAL CELL MOTION WOULD BE SLOW MOVING/NON-EXISTENT. 
EFFECTIVE BULK SHEAR IS ~15 KTS PER RAP FORECASTS, IMPLYING A
PULSE MODE TO CONVECTION.  OUTFLOW BOUNDARIES SHOULD DRIVE
THUNDERSTORM DEVELOPMENT ALONG A PREFERRED PATH SOUTH TO
SOUTH-SOUTHEAST DOWN THE INSTABILITY GRADIENT/INVERTED TROUGH,
WITH THE MEAN WIND FEEDING THE STORMS WITH INSTABILITY FROM THE
WEST. 

DESPITE A PULSE MODE, THERE IS THE CHANCE FOR SOME REPEAT/SHORT
PERIOD OF TRAINING AS THE INSTABILITY POOL TO WEST ISN'T EXPECTED
TO ERODE/SHIFT SLOWLY EASTWARD WITH THE INVERTED TROUGH.  THE
AVAILABLE MOISTURE IN THE TERRAIN SUPPORTS HOURLY RAIN TOTALS TO
1.5", WITH AVAILABLE MESOSCALE GUIDANCE SHOWING LOCAL AMOUNTS OF
2" POSSIBLE.  CONSIDERING SATURATED SOILS AND ELEVATED STREAM
FLOW, HEAVY RAIN WOULD BE PROBLEMATIC.

ROTH

ATTN...WFO...LWX...PBZ...RLX...RNK...

ATTN...RFC...MARFC...OHRFC...SERFC...

LAT...LON   39898039 39867967 39327954 39177933 39257887
            38997857 38767851 38487871 38537895 38267931
            37787981 37228039 37018089 37288149 38568156
            39518103