MESOSCALE PRECIPITATION DISCUSSION 0503
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
855 PM EDT SAT JUL 21 2018

AREAS AFFECTED...NORTHERN MID-ATLANTIC STATES

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

VALID 220054Z - 220654Z

SUMMARY...VERY HEAVY RAINFALL CURRENTLY NEAR THE POTOMAC RIVER IS
EXPECTED TO LESSEN IN MAGNITUDE AS IT LIFTS NORTHWARD INTO PA. 
HOURLY RAIN TOTALS TO 2" WITH LOCAL AMOUNTS TO 4" ARE EXPECTED
THROUGH 07Z, PARTICULARLY ACROSS MD & PA.

DISCUSSION...THE 00Z SURFACE ANALYSIS REVEALS AN OCCLUDED CYCLONE
IN SOUTHERN CHESAPEAKE BAY WITH AN INCREASINGLY PRONOUNCED DRY
SLOT NEAR THE VA CAPES AND EASTERN SHORE.  VERY HEAVY RAINFALL
NEAR THE POTOMAC RIVER IS BEGINNING TO LESSEN IN MAGNITUDE AND
SHIFT SOMEWHAT EASTWARD WITHIN THE TROWAL OF THIS CYCLONE, SHOWING
UP AS A SURFACE TROUGH ACTING AS A BOUNDARY BETWEEN NORTHERLY AND
EASTERLY SURFACE WINDS.  PRECIPITABLE WATER VALUES ARE 1.6-2.5"
PER GPS VALUES AND THE WALLOPS VA SOUNDING.  INFLOW AT 850 HPA IS
EAST-SOUTHEAST AT 25-40 KTS PER VAD WIND PROFILES.  EFFECTIVE BULK
SHEAR IS 30-40 KTS IN THIS REGION, WHICH HAS ORGANIZED CONVECTION
INTO A BAND NEAR THE NATION'S CAPITAL.

THE EXPECTATION BASED ON RAP MASS FIELDS IS FOR THE TROWAL TO
MORPH INTO A PRONOUNCED COMMA HEAD IN THE NEXT SEVERAL HOURS.  THE
GUIDANCE SHOWS DECREASING INSTABILITY WITH TIME, SO BELIEVE THAT
THE ONGOING FLASH FLOOD EVENT SHOULD EVOLVE INTO A LONGER DURATION
HEAVY RAIN EVENT BY 07Z AS HOURLY RAIN TOTALS SLOWLY FADE. 
HOWEVER, EVEN BY 07Z, AVAILABLE INSTABILITY SHOULD CONTINUE TO
SUPPORT HOURLY RAIN TOTALS OVER 1", WHICH WOULD BE PROBLEMATIC IN
URBAN AREAS.  THE MESOSCALE GUIDANCE ADVERTISES LOCAL AMOUNTS UP
TO 4" IN MD/PA AND UP TO 2" IN NJ/NEW YORK CITY THROUGH 07Z. 
COMBINED WITH PREVIOUS RAINS, STORM TOTALS OF 6-7" ARE POSSIBLE
ACROSS MD BEFORE THE HEAVY RAIN SHIFTS NORTHWARD/ENDS.

ROTH

ATTN...WFO...AKQ...BGM...CTP...LWX...OKX...PHI...

ATTN...RFC...MARFC...NERFC...

LAT...LON   41427629 41317504 41007389 40747363 40357382
            39607409 38987454 38687496 38507551 38197611
            37977654 38557738 39637828 40917798