MESOSCALE PRECIPITATION DISCUSSION 0142
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
207 PM EDT MON MAY 14 2018

AREAS AFFECTED...SERN OH...WV...SWRN PA...NRN VA...WRN MD...DC

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

VALID 141805Z - 152330Z

SUMMARY...ISOLATED FLASH FLOODING WILL BE POSSIBLE AS A SQUALL
LINE PUSHES THROUGH THE MID ATLANTIC REGION. RAIN RATES MAY REACH
1.0 TO 1.5 IN/HR IN LOCALIZED AREAS WHERE THUNDERSTORMS TRAIN OR
THE FORWARD MOTION OF THE LINE IS SLOWER.

DISCUSSION...A FORWARD PROPAGATING SQUALL LINE WAS DEVELOPING
ACROSS PORTIONS OF ERN OHIO AND WRN PENNSYLVANIA ON THE NORTHERN
CUSP OF A PLUME OF INCREASING SURFACE-BASED INSTABILITY. AS SBCAPE
CONTINUES TO INCREASE IN THE MID-ATLANTIC REGION AHEAD OF THE
SQUALL LINE...IT APPEARS LIKELY THAT THE LINE WILL PERSIST FOR AT
LEAST SEVERAL MORE HOURS AS IT PUSHES TO THE EAST. THE ENVIRONMENT
ACROSS THE REGION IS ALSO CHARACTERIZED BY PRECIPITABLE WATER
VALUES AROUND 1.3 TO 1.5 INCHES...ABOVE THE 90TH PERCENTILE FOR
THIS TIME OF YEAR. CIRA BLENDED TPW AND RAP ANALYSIS APPEAR TO BE
OVERESTIMATING PWATS SLIGHTLY...CLOSER TO 1.5 TO 1.6 INCHES
COMPARED WITH THE VALUES AROUND 1.3 INCHES ON THE 16Z SOUNDINGS
FROM KPIT AND KILN. HOWEVER...1.3 INCH PWATS WOULD BE SUFFICIENT
TO SUPPORT A FLASH FLOOD THREAT PROVIDED THAT CELLS TRAIN TO
INCREASE THE RESIDENCE TIME OF HEAVY RAIN (DUE TO DECREASED
PRECIPITATION EFFICIENCY).

CELL TRAINING SEEMS LESS LIKELY IN THE NORTHERN PORTION OF THE
MPD...PARTICULARLY IN PORTIONS OF PA...WHERE CELL MOTIONS APPEAR
TO BE FASTER DUE TO STRONGER MEAN FLOW FURTHER TO THE NORTH. AS
THE CONVECTIVE COLD POOL BECOMES BETTER ESTABLISHED...THIS PORTION
OF THE LINE MAY BEGIN TO ACCELERATE AS WELL. HOWEVER THIS AREA
FROM SW PA INTO FAR NRN WV AND THE PANHANDLE OF MD HAS THE MOST
HYDROLOGICALLY FAVORABLE CONDITIONS FOR FLASH FLOODING...WITH THE
LOWEST FFG...MUCH ABOVE NORMAL STREAMFLOW (PER THE NATIONAL WATER
MODEL ANALYSIS)...AND ELEVATED SOIL MOISTURE LEVELS.
THEREFORE...ALTHOUGH MESOSCALE CONDITIONS AND RADAR TRENDS WOULD
TEND TO INDICATE A LOWER FLASH FLOOD THREAT...IT CANNOT BE RULED
OUT GIVEN THE ANTECEDENT SOIL CONDITIONS.

FURTHER SOUTH...IT IS POSSIBLE THAT A TRAILING PORTION OF THE LINE
COULD SET UP IN A MORE WEST-EAST FASHION (ALIGNED WITH THE MEAN
FLOW VECTORS) IF LINE ACCELERATION OCCURS FURTHER TO THE NORTH. IF
THAT HAPPENED...IT WOULD INCREASE THE RESIDENCE TIME OF HEAVY RAIN
AT A GIVEN LOCATION AND COMPENSATE FOR THE SOMEWHAT MARGINAL
MOISTURE PROFILE ACROSS THE REGION...AND LESS FAVORABLE ANTECEDENT
CONDITIONS AS COMPARED TO FURTHER NORTH. LOW-LEVEL INFLOW WOULD BE
FROM THE WEST...WHICH COULD FAVOR SOME SLIGHT BACKBUILDING INTO
THE BACK OF THE LINE OR JUST ON THE UPSHEAR FLANK OF THE
CONVECTIVE COLD POOL AS WELL. THIS PARTICULAR SCENARIO OF
CONVECTIVE EVOLUTION IS NOT CERTAIN AND THE OVERALL CONVECTIVE
COMPLEX SHOULD BE FAIRLY PROGRESSIVE...THEREFORE ONLY ISOLATED
FLASH FLOODING SEEMS POSSIBLE AT THIS TIME.

LAMERS

ATTN...WFO...AKQ...CTP...ILN...LWX...PBZ...RLX...RNK...

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

LAT...LON   40707970 39867774 38927701 38117750 37727848
            37897954 38378031 38788115 39248229 39728262
            40158226 40698093