Flash Flood Guidance
Issued by NWS

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000
AWUS01 KWNH 240411
FFGMPD
NYZ000-NJZ000-PAZ000-DEZ000-MDZ000-VAZ000-DCZ000-241005-

MESOSCALE PRECIPITATION DISCUSSION 0548
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1210 AM EDT MON JUL 24 2017

AREAS AFFECTED...CNTRL/ERN PA INTO NJ AND THE NRN DELMARVA
PENINSULA

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

VALID 240409Z - 241005Z

SUMMARY...INTENSE RAINFALL RATES OF 2-3 IN/HR WILL BE POSSIBLE AS
CONVECTIVE COVERAGE INCREASES ACROSS PORTIONS OF THE NRN
MID-ATLANTIC REGION. 4-6 INCH TOTALS WILL BE EXPECTED WHERE
TRAINING MAXIMIZES THROUGH 10Z.

DISCUSSION...TRENDS IN REGIONAL RADAR OVER THE NRN MID-ATLANTIC
REGION SHOWED SCATTERED THUNDERSTORMS WITH REPORTED VERY
IMPRESSIVE RAINFALL RATES OF 2-3 IN/HR...AND 1 INCH IN AS LITTLE
AS 15 MINUTES IN NRN DE. GOES INFRARED CLOUD TOPS ARE RELATIVELY
WARM WITH MINIMAL LIGHTNING NOTED IN A FEW OF THE RAIN CORES OVER
THE REGION WHICH IS INDICATIVE OF WARM RAIN PROCESSES. GPS SITES
SHOWED A PLUME OF 2+ INCH PRECIPITABLE WATER VALUES EXTENDING FROM
ERN MD INTO ERN PA AND NJ AS OF 03Z. FLOW ALOFT WAS SWLY WITH
SMALL SCALE EMBEDDED VORTICITY MAXIMA HELPING TO CONTRIBUTE TO
CLUSTERS OF SHOWER/THUNDERSTORM ACTIVITY REGION-WIDE.

AS A NEUTRALLY ORIENTED MID-UPPER LEVEL TROUGH AXIS
APPROACHES...CURRENTLY LOCATED OVER THE OHIO VALLEY...RAP
FORECASTS INDICATE THE TROUGH BECOMING SLIGHTLY NEGATIVELY TILTED
THROUGH 10Z WITH MID-UPPER LEVEL FLOW BACKING OVER THE NRN
MID-ATLANTIC. AS THIS OCCURS...850 MB FLOW IS FORECAST TO INCREASE
INTO THE 20-30 KT RANGE IN THE VICINITY OF A SHARPENING TROUGH
AXIS...FORECAST TO EXTEND ESE ACROSS PA AND CNTRL NJ BY 06Z.
RELATIVELY WEAK STORM MOTIONS TOWARD THE NORTHEAST AND INCREASING
LOW LEVEL FLOW WILL ALLOW FOR TRAINING OF STORMS AND ADVECTION OF
HIGHER INSTABILITY FROM THE SOUTH.

RECENT ESRL HRRR FORECASTS SEEM TO BE HANDLING ONGOING CONVECTIVE
ACTIVITY THE BEST THOUGH IT IS FAR FROM PERFECT. HOWEVER...IT
CONSISTENTLY HAS BEEN ADVERTISING 4-6 INCH RAINFALL TOTALS OVER
PORTIONS OF NJ THROUGH 10Z...SOME OF WHICH MAY EXTEND BACK INTO
NERN MD AND ERN PA BASED ON CURRENT TRENDS IN RADAR. GIVEN HEAVY
RAINFALL WHICH HAS ALREADY IMPACTED THIS REGION EARLIER
TODAY...FLASH FLOODING APPEARS LIKELY.

OTTO

...PLEASE SEE WWW.WPC.NCEP.NOAA.GOV FOR GRAPHIC PRODUCT...

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

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

LAT...LON   42077642 42037553 41737491 41257418 40647348
            39757347 39127410 38977478 38837552 38677656
            38997722 39647772 40317803 41207788 41737736



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