Flash Flood Guidance
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000
AWUS01 KWNH 161348
FFGMPD
MEZ000-MAZ000-NHZ000-RIZ000-CTZ000-NYZ000-162000-

MESOSCALE PRECIPITATION DISCUSSION 0111
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
947 AM EDT MON APR 16 2018

AREAS AFFECTED...SOUTHEAST NY...LONG
ISLAND...CT...RI..MA...SOUTHEAST NH

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

VALID 161400Z - 162000Z

SUMMARY...STORM TRANSITIONING FROM HIGH RAIN RATE CONVECTION TO
BROADER MODERATE BUT PERSISTENT RAINFALL WITH SMALL EMBEDDED
CONVECTION SHIFTING THREAT FROM FLASH FLOODING TO INUNDATION
FLOODING.

DISCUSSION...CURRENT SYNOPTIC SETUP PER OBSERVATIONS AND GOES-16
VIS/RGB DEPICT AN EXPOSED OCCLUDED LOW OVER THE CENTRAL PIEDMONT
OF VA WITH THE OCCLUDED FRONT EXTENDING NNE TOWARD THE TRIPLE
POINT LOW/CLOSED WIND CIRCULATION NEAR TRENTON NJ.  THE WARM FRONT
THEN EXTENDS NEAR DUE EAST JUST SOUTH OF LONG ISLAND TO NEAR BUOY
44008 SE OF NANTUCKET ISLAND.  THE COLD FRONT /BROKEN SQUALL LINE
IS DENOTED WELL SOUTH OF BEST CYCLONIC INFLECTION EAST OF THE
VIRGINIA CAPES OF THE DELMARVA.  IT IS NORTH OF THIS INFLECTION
THAT CONVECTION BECOMES A BIT LESS CONCENTRATED AND
BROADER/FRACTURED BOTH DEPICTED IN GOES-VIS/10.3 UM IR AS WELL AS
COMPOSITE FROM OKX/DIX RADARS.

IN THE WARM SECTOR THE SURFACE WINDS ARE BACKING QUICKLY FURTHER
EAST SOUTH OF RI/SE MA AS THE WARM FRONT LIFTS NORTH PROVIDING
STRONG MOISTURE CONVERGENCE AT THE BOUNDARIES.  AIDED BY DEEPER
ISENTROPIC ASCENT ACROSS THE BOUNDARY WITH SFC WINDS TO 50KTS AND
850MB LLJ OF 75KTS NEAR THE COLD FRONT BUT STILL OVER 50KTS AS FAR
EAST AS 70W.  MUCAPES ARE MODEST NEAR AND SOUTH OF LONG ISLAND IN
THE 100-200 J/KG RANGE HELPING TO MAINTAIN THIS BROKEN CONVECTION
THOUGH EXPECTED TO CONTINUE TO SLOWLY WANE WITH TIME AND EASTWARD
PROGRESSION INTO EARLY AFTERNOON.  GIVEN THE BREADTH OF THE
SOUTHERLY FLOW ISENTROPIC ASCENT...HIGHLY FRACTURED
SHALLOWER/SMALLER CONVECTIVE CORES HAVE DEVELOPED ALONG THE WARM
FRONT AIDED BY FRICTIONAL CONVERGENCE AT LONG ISLAND TO BROADEN
MODERATE SHOWERS WITH HOURLY RAIN RATES OF .25"/HR OCCASIONALLY
REACHING .4 TO .5" IN AN HOUR FOR THE NEXT 3-4 HRS AS THE MAIN
COLD FRONT APPROACHES. PRECURSORY .75-1.25" TOTALS WILL FULLY
SATURATE THE ALREADY WET GROUND CONDITIONS ACROSS SOUTHEAST NEW
ENGLAND (AS NOTED BY 90-100% SATURATED SOILS IN THE HRRR/HRRRV3
TOP 1 CM FIELDS).

THE COLD FRONT WILL STILL FOCUS THE GREATEST MAGNITUDE OF
RAINFALL/RATES GIVEN STRENGTH OF MOISTURE FLUX/IVT VALUES (WELL
OVER 3 STDEV FROM NORMAL) WITH TPWS OF 1.5-1.7" DETECTED BY
BLENDED TPW IMAGERY.  AS IT SWEEPS SLOWLY EASTWARD...RATES UP TO
.75"/HR ARE EXPECTED EARLY/FURTHER EAST BUT SHOULD SLOWLY REDUCE
TOWARD 21Z...EASTERN MA TOWARD .5".  GIVEN THE SEVERING/BLOCKING
OF BEST DEEP SOUTHERLY FLOW/MOISTURE FEED /INSTABILITY NOTED BY
THE COLD FRONT BULGE FURTHER SOUTH...AN ADDITIONAL 1-1.5" IN
SHORTER DURATION (1-2 HOURS) OVER THE SATURATED SOILS MAY LEAD TO
HIGHLY LOCALIZED FLASH FLOODING.  HOWEVER...IT IS MORE LIKELY THE
COMBINATION/DURATION OF THE OVERALL RAINFALL WILL LEAD TO FURTHER
INUNDATION STYLE/SLOWER RISES ESPECIALLY ACROSS URBAN/NEAR SWOLLEN
STREAMS MAINTAINING A POTENTIALLY SIGNIFICANT HAZARD. THIS IS IN
LINE WITH CURRENT HI-RES CAM GUIDANCE PARTICULARLY THE ARW/ARW2
AND HRRRV3.

GALLINA

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

ATTN...WFO...ALY...BOX...GYX...OKX...

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

LAT...LON   43427118 43267090 42887066 42357065 42167053
            41847069 41377094 40997133 40937202 40537295
            40497397 40977389 41447384 42017323 42367268
            42697180 43027154


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