MESOSCALE PRECIPITATION DISCUSSION 0582
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1040 AM EDT FRI JUL 28 2017

AREAS AFFECTED...FAR SOUTHERN PA...MD...D.C...NORTHERN VA

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

VALID 281440Z - 282040Z

SUMMARY...SHOWERS AND THUNDERSTORMS WITH VERY HEAVY RAINFALL RATES
ARE EXPECTED TO DEVELOP AND EXPAND IN COVERAGE OVER THE NEXT
SEVERAL HOURS. RAINFALL RATES OF UP TO 2.5 INCHES/HR ARE
EXPECTED...AND FLASH FLOODING WILL GRADUALLY BECOME LIKELY.

DISCUSSION...THE LATEST EXPERIMENTAL GOES-16 6.9 MICRON/WV AND
RGB/AIRMASS IMAGERY SHOWS A SIGNIFICANT SHORTWAVE TROUGH RAPIDLY
AMPLIFYING ACROSS THE LOWER GREAT LAKES REGION...WITH A CLOSED 500
MB CENTER ALREADY DIGGING SOUTHEAST ACROSS NORTHERN LOWER MI.
MEANWHILE...UPSTREAM OVER SOUTHERN WI...THERE IS A SIGNIFICANT
AMOUNT DRYING/DARKENING SEEN IN THE WV IMAGERY WHICH IS
CORRESPONDING TO A 90+KT 250 MB JET MAX THAT IS DIGGING SOUTHEAST
IN TOWARD THE BASE OF THE AMPLIFYING TROUGH.

ALL OF THIS ENERGY WILL BE ENCROACHING ON THE MID-ATLANTIC REGION
BY EARLY THIS AFTERNOON AND WILL BE INTERACTING WITH MULTIPLE
BOUNDARIES/SURFACE CONVERGENCE ZONES INCLUDING A FRONTAL ZONE
DRAPED OVER THE UPPER OH VALLEY AND EXTENDING EAST ACROSS THE
MASON-DIXON LINE. MEANWHILE...A LEE-SIDE TROUGH IS NOTED EAST OF
THE BLUE RIDGE FROM JUST WEST OF BALTIMORE SOUTH TO D.C AND DOWN
THROUGH CENTRAL/SOUTHERN VA. A WAVE OF LOW PRESSURE IS NOTED ALONG
IT SOUTH OF D.C. ALONG THE POTOMAC RIVER. CONSEQUENTLY THERE ARE
MULTIPLE FOCI AT THE SURFACE FOR CONVECTION TO INITIATE AS THE
UPSTREAM ENERGY APPROACHES THIS AFTERNOON...AND THIS WILL INCLUDE
OROGRAPHIC FORCING OVER THE BLUE RIDGE AND ADJACENT FOOTHILLS.

THE AIRMASS OVER THE MID-ATLANTIC ALONG AND SOUTH OF THE FRONT IS
VERY MOIST AND BECOMING INCREASINGLY UNSTABLE. PWATS OF GENERALLY
1.75 TO 2.0 INCHES AND MUCAPE VALUES APPROACHING OR LOCALLY
EXCEEDING 2000 J/KG ARE IN PLACE.

OVER THE NEXT FEW HOURS...STRENGTHENING DPVA AND INCREASINGLY
DIVERGENT FLOW ALOFT WILL BE INTERACTING WITH THE FRONT AND WITH
THE AIRMASS SOUTH OF IT FOR AN EXPANSION OF CONVECTIVE ACTIVITY.
THE AFOREMENTIONED SURFACE BOUNDARIES COUPLED WITH ADDITIONAL
DIURNAL HEATING AND OROGRAPHICS WILL ALSO FACILITATE THE INCREASED
COVERAGE OF CONVECTION.

EXPECT RAINFALL RATES TO BE QUITE INTENSE...WITH RATES AS HIGH AS
2.5 INCHES/HR WITHIN THE STRONGER CONVECTIVE CORES. THIS COUPLED
WITH RELATIVELY SLOW CELL MOTION SHOULD FOSTER SOME VERY HEAVY
RAINFALL TOTALS THROUGH MID TO LATE AFTERNOON. THE LATEST
HRRR/HRRRX GUIDANCE SUPPORT SOME LOCALIZED 3 TO 5 INCH RAINFALL
AMOUNTS THROUGH LATE AFTERNOON. THIS WILL LIKELY RESULT IN SOME
FLASH FLOODING...WITH A PARTICULAR CONCERN OVER THE HIGHER TERRAIN
AND ALSO THE URBANIZED I-95 CORRIDOR FROM BALTIMORE TO D.C. AND
NORTHERN VA...ALONG WITH THE ADJACENT SUBURBS.

ORRISON

ATTN...WFO...AKQ...CTP...LWX...PBZ...PHI...

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

LAT...LON   40077873 39927764 39927673 39667605 38817571
            38277622 38187695 38267755 38467832 38707892
            39257904 39777955 40007936