MESOSCALE PRECIPITATION DISCUSSION 0290
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1247 AM EDT MON JUN 11 2018

AREAS AFFECTED...PHILADELPHIA METRO AREA

CONCERNING...HEAVY RAINFALL...FLASH FLOODING LIKELY

VALID 110447Z - 110817Z

SUMMARY...A HEAVY RAIN BAND WAS NEARLY STATIONARY OVER THE
PHILADELPHIA METRO AREA AS OF 12:30 AM AND WAS PRODUCING RAIN
RATES OF 1 TO 2 IN/HR. THIS WILL LIKELY LEAD TO FLASH FLOODING
ACROSS THE PHILADELPHIA METRO AREA...WITH SIGNIFICANT FLASH
FLOODING POSSIBLE IN SOME AREAS.

DISCUSSION...A BAND OF HEAVY RAIN WAS BEING FORCED BY A
STRENGTHENING FRONTOGENETIC CIRCULATION. IT ROUGHLY COINCIDED WITH
THE MAXIMUM IN 925-850MB FRONTOGENESIS AND THIS IS PROJECTED TO
MOVE VERY LITTLE THROUGH 06Z (ANOTHER HOUR). AFTER THAT...IT MAY
BEGIN TO DRIFT SLOWLY SOUTH AS A LOW-LEVEL FRONTAL WAVE BEGINS TO
RELOCATE FURTHER SOUTHEAST. THE RAIN BAND SHOULD FOLLOW
SUIT...WHICH LIKELY MEANS ANOTHER COUPLE HOURS OF VERY HEAVY
RAINFALL FOCUSED OVER THE PHILADELPHIA METRO AREA.

SIGNIFICANT RAIN RATES OF 1-2 IN/HR HAVE BEEN ESTIMATED BY MRMS
AND KDIX DUAL POL RADAR...AND THESE SEEM REASONABLE GIVEN SEVERAL
MESONET REPORTS OF AROUND 3 INCHES OF RAIN IN 2 HOURS ON THE NORTH
SIDE OF PHILADELPHIA. THESE KIND OF RATES IN AN URBANIZED AREA ARE
VERY LIKELY TO LEAD TO FLASH FLOODING...AND COULD LEAD TO SOME
POCKETS OF SIGNIFICANT FLASH FLOODING.

HYDROLOGIC RESPONSE IS BEING NOTED ON CREST MODEL MAXIMUM UNIT
STREAMFLOW...WITH VALUES OVER 10 (CAN BE ASSOCIATED WITH MORE
SIGNIFICANT FLOODING) IN THE PHILADELPHIA AREA...AS WELL AS SOME
STREAM GAGES. THE FRANKFORD CREEK AT PHILADELPHIA HAS APPROACHED
MAJOR FLOOD STAGE FROM BASE FLOW IN ROUGHLY TWO HOURS. THIS SORT
OF HYDROLOGIC RESPONSE SHOULD CONTINUE OVER THE NEXT SEVERAL HOURS
AND SIGNIFICANT FLASH FLOODING WILL BE POSSIBLE.

LAMERS

ATTN...WFO...PHI...

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

LAT...LON   40217500 40157463 39997433 39787435 39757465
            39797511 39847539 40017563 40187544