Flash Flood Guidance
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000
AWUS01 KWNH 231729
FFGMPD
NYZ000-PAZ000-232325-

MESOSCALE PRECIPITATION DISCUSSION 0544...CORRECTED
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
129 PM EDT SUN JUL 23 2017

CORRECTED FOR SECOND PARAGRAPH

AREAS AFFECTED...WESTERN NY & NORTH-CENTRAL PA

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

VALID 231725Z - 232325Z

SUMMARY...SHOWERS AND THUNDERSTORMS ARE BEGINNING TO DEVELOP
ACROSS WESTERN NY.  HOURLY RAIN RATES UP TO 1.75" WITH LOCAL
AMOUNTS TO 3" ARE EXPECTED.

DISCUSSION...A FRONTAL WAVE IS LOCATED NEAR BUFFALO NY AT 17Z,
WITH AN ASSOCIATED WARM FRONT EXTENDING FROM WESTERN NY INTO
SOUTHEAST PA.  A FEW SHOWERS/THUNDERSTORMS HAVE FORMED RECENTLY
NEAR THE WESTERN PA/NY BORDER.  WITHIN ITS WARM SECTOR, ML CAPE
VALUES HAVE BEEN RISING DUE TO DAYTIME HEATING, WITH 500-1000 J/KG
NOTED ON RECENT SPC MESOANALYSES.  PRECIPITABLE WATER VALUES ARE
1.3-1.5" PER RECENT GPS DATA.  INFLOW AT 850 HPA IS 10-15 KTS OUT
OF THE SOUTHWEST, SOMEWHAT BELOW THE VALUE OF THE MEAN 850-400 HPA
WIND.  HOWEVER, EFFECTIVE BULK SHEAR OF 25-35 KTS EXISTS IN THE
REGION WHICH COULD ORGANIZE CONVECTION.  WATER VAPOR IMAGERY
IMPLIES A NEARBY UPPER LEVEL JET STREAK TO THE SOUTHEAST OF AN
UPPER LOW CENTERED ACROSS MI.

RECENT EXPERIMENTAL GOES-16 0.86 MICRON/"VEGGIE BAND" IMAGERY
SHOWS PARTLY CLOUDY SKIES ACROSS THE REGION, WHICH SHOULD ALLOW
FOR AN ADDITIONAL 1000 J/KG OF ML CAPE TO BUILD DURING THE DAY
TODAY.  THE EFFECTIVE BULK SHEAR AND UNIDIRECTIONAL FLOW WITH
HEIGHT SHOULD CONSPIRE TO CREATE WEST-EAST THUNDERSTORM BANDS
WHICH COULD TRAIN.  RAP GUIDANCE SUGGESTS A SECOND LOW-LEVEL
CYCLONIC CIRCULATION 70 MILES WEST OF THE CURRENT LOW ACROSS
SOUTHERNMOST ONTARIO MOVES IN FROM THE WEST AND MERGES IN WITH
TIME, WHICH KEEPS THE WESTERN NY CONVERGENCE ZONE IN PLACE.
THUNDERSTORM OUTFLOWS COULD NUDGE CONVECTION SOMEWHAT SOUTHEAST
WITH TIME INTO NORTH-CENTRAL PA.  INSTABILITY SHOULD CONTINUE TO
CREEP NORTHEAST, WITH THE WARM FRONT POSSIBLY JUMPING INTO THE
LOW-LEVEL CONVERGENCE ZONE NEAR 42.8 LATITUDE SO LONG AS OUTFLOW
BOUNDARIES DON`T HOLD UP ITS NORTHWARD PROGRESSION.  THUNDERSTORM
MOTION SHOULD BE TO THE EAST AT ~30 KTS.  HOURLY RAIN RATES UP TO
1.75" ARE EXPECTED.  THE MESOSCALE GUIDANCE SHOWS A GOOD SIGNAL
FOR LOCAL AMOUNTS OF 3".  OVER PARTIALLY SATURATED SOILS, THIS
DEGREE OF RAINFALL COULD BE PROBLEMATIC.

ROTH

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

ATTN...WFO...BGM...BUF...CTP...

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

LAT...LON   43167763 42947558 41177588 41567780 42367915
            42887912 43057883



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