MESOSCALE PRECIPITATION DISCUSSION 0181
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
103 PM EDT TUE MAY 22 2018

AREAS AFFECTED...SRN/ERN WV...FAR ERN
KY...VIRGINIA...MARYLAND...DC

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

VALID 221700Z - 222300Z

SUMMARY...SLOW-MOVING SHOWERS AND THUNDERSTORMS WILL GRADUALLY
PUSH EAST ACROSS PORTIONS OF THE MID-ATLANTIC AND WILL POSE A
THREAT FOR FLASH FLOODING. HOURLY RAIN RATES MAY REACH 1-2 IN/HR
WITH LOCALIZED TOTALS AS HIGH AS 3-5 INCHES.

DISCUSSION...REGIONAL RADARS AND GOES-16 VISIBLE SATELLITE SHOW A
CONSIDERABLE UPWARD TREND IN CONVECTIVE ACTIVITY (BOTH INTENSITY
AND COVERAGE) BETWEEN 1530Z AND 17Z FROM PORTIONS OF KENTUCKY AND
TENNESSEE EAST-NORTHEAST INTO SRN WEST VIRGINIA AND WRN VIRGINIA.
THESE SHOWERS AND STORMS WERE DEVELOPING IN A BROAD AREA OF
LOW-LEVEL CONFLUENCE AS INSOLATION LEADS TO INCREASING INSTABILITY
AND SOME AREAS APPROACHING THEIR CONVECTIVE TEMPERATURES. HI-RES
MODELS SHOW SOME VARIABILITY IN THE EVOLUTION AND PROGRESSION OF
THIS CONVECTION...BUT IN GENERAL SHOW IT MOVING FROM AREAS OF
HIGHER TERRAIN EARLY THIS AFTERNOON INTO THE MORE UNSTABLE AIR
MASS SITUATED OVER VIRGINIA AND ADJACENT PORTIONS OF MARYLAND. RAP
ANALYZED CAPE VALUES WERE MORE LIMITED (AROUND 500-1000 J/KG) OVER
THE MOUNTAINS OF WEST VIRGINIA...BUT INCREASED TO OVER 2000 J/KG
OVER CENTRAL VIRGINIA. AS CONVECTION PROGRESSES TO THE EAST...THE
COMBINATION OF MODERATE-STRONG INSTABILITY AND EFFECTIVE BULK
SHEAR AROUND 20 KNOTS SHOULD BE SUFFICIENT FOR INCREASINGLY
ORGANIZED MULTICELL CLUSTERS AND THE POTENTIAL FOR TRAINING AND/OR
INCREASED RESIDENCE TIME OF HEAVY RAINFALL.

THE NATIONAL WATER MODEL ANALYSIS AND USGS HISTORICAL STREAMFLOW
ANALYSIS SHOWS VERY ELEVATED STREAMFLOW ACROSS MUCH OF THE
OUTLINED AREA...PARTICULARLY ERN WEST VIRGINIA AND WRN/CTRL
VIRGINIA. MANY STREAMS ARE EXPERIENCING FLOWS ABOVE THE 95TH
PERCENTILE...AND MUCH OF THE SAME AREA HAS SEEN OVER 200 PERCENT
OF NORMAL RAINFALL OVER THE PAST TWO WEEKS. THEREFORE...ANTECEDENT
CONDITIONS ARE MORE FAVORABLE THAN NORMAL FOR EXCESSIVE RUNOFF
FROM HEAVY RAIN AND SHOULD PROMOTE A FLASH FLOOD THREAT WITH ANY
SLOW-MOVING STORMS TODAY.

HI-RES MODELS SUPPORT A RISK OF RAIN RATES AS HIGH AS 2
IN/HR...AND THIS WOULD BE SUPPORTED BY AN ENVIRONMENT WITH
PRECIPITABLE WATER VALUES OVER 1.5 INCHES (ABOVE THE 90TH
PERCENTILE FOR MAY) AND EXPECTED STORM MOTIONS SLOWER THAN 20
KNOTS.

LAMERS

ATTN...WFO...AKQ...JKL...LWX...MRX...PBZ...PHI...RLX...RNK...

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

LAT...LON   39637814 39587713 39087635 38267642 37597699
            37257772 37107835 36867954 36718068 36698281
            37078345 37568307 37968227 38458080 38927954
            39317876