MESOSCALE PRECIPITATION DISCUSSION 0704
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1224 PM EDT FRI AUG 18 2017

AREAS AFFECTED...N VIRGINIA...C MARYLAND...E PENNSYLVANIA...N NEW
JERSEY...SE NEW YORK

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

VALID 181624Z - 182224Z

SUMMARY...NUMEROUS THUNDERSTORMS WITH VERY HEAVY RAIN ARE FORECAST
TO DEVELOP DURING THE MID TO LATE AFTERNOON HOURS EAST OF THE
APPALACHIANS AND EXTENDING EASTWARD TO THE INTERSTATE 95 CORRIDOR.
 THE POTENTIAL EXISTS FOR 2 TO 4 INCHES OF RAIN IN SOME LOCATIONS
BY 6PM, AND RAINFALL RATES COULD EXCEED TWO INCHES PER HOUR WITH
THE STRONGEST CONVECTION.

DISCUSSION...A FAVORABLE ENVIRONMENT FOR EXCESSIVE RAINFALL HAS
DEVELOPED ACROSS MUCH OF THE NORTHERN MID-ATLANTIC REGION AND
EXTENDING INTO PARTS OF UPSTATE NEW YORK.  AN ANOMALOUS SURGE OF
DEEP MOISTURE AHEAD OF A COLD FRONT IS LEADING TO PWS ON THE ORDER
OF 2 TO 2.3 INCHES EAST OF THE APPALACHIAN MOUNTAINS, ALONG WITH
SURFACE DEWPOINTS IN THE MID TO UPPER 70S.  AN 80KT JET STREAK
SITUATED OVER NORTHERN NEW YORK, COUPLED WITH AN APPROACHING 500MB
TROUGH AND VORTICITY ADVECTION, WILL AID DEEP LAYER ASCENT AND
SUSTAIN CONVECTION ONCE IT DEVELOPS.  LOCAL RAOBS ARE INDICATING
NEARLY UNIDIRECTIONAL SW FLOW ALOFT.  MLCAPE IS APPROACHING 2000
J/KG WITH EFFECTIVE BULK SHEAR ON THE ORDER OF 30KT BASED ON THE
LATEST SPC MESOANALYSIS.

THE LATEST SUITE OF HI-RES MODEL GUIDANCE IS ON BOARD WITH THE
IDEA OF WIDESPREAD 1 TO 3 INCH AMOUNTS FROM THE GREATER WASHINGTON
DC METRO AREA TO SOUTHEAST NEW YORK, WITH SOME ISOLATED SPOTS
GETTING POTENTIALLY 4 INCHES OR MORE OF RAINFALL.  THE THREAT OF
FLASH FLOODING WILL BE GREATER FOR THOSE LOCATIONS THAT HAVE
RECEIVED SIGNIFICANT RAINFALL OVER THE PAST 24 HOURS AND FOR URBAN
LOCATIONS.

HAMRICK

ATTN...WFO...ALY...BGM...BOX...CTP...LWX...OKX...PHI...

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

LAT...LON   43017306 42397283 41157330 40497379 40057461
            39507554 38797653 38577700 38497754 38577835
            38837884 39657845 40597767 41817652 42987510