Flash Flood Guidance
Issued by NWS

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000
AWUS01 KWNH 281409
FFGMPD
PAZ000-MDZ000-VAZ000-WVZ000-282015-

MESOSCALE PRECIPITATION DISCUSSION 0280
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1009 AM EDT SUN MAY 28 2017

AREAS AFFECTED...NORTHWEST VA...NORTHEAST WV...EXT WESTERN
MD...SOUTHWEST PA

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

VALID 281415Z - 282015Z

SUMMARY...EFFICIENT RAIN PRODUCING CONVECTION EXPECTED TO DEVELOP
OVER SATURATED SOILS/LOW FFG VALUES POSING FLASH FLOODING THREAT
THROUGH EARLY AFTERNOON HOURS.

DISCUSSION...GOES-16 CONUS VIS IMAGERY ALONG WITH 13Z SURFACE
ANALYSIS DENOTES A SHALLOW WAVE ALONG VA/NC LINE NEAR DAN THAT IS
EXPECTED TO LIFT NORTHWARD WITH EASTERLY FLOW BANKING UP AGAINST
BLUE RIDGE...WHILE ON THE OTHER SIDE OF THE APPALACHIANS HEIGHT
FALLS AND EXPANDING SWLY FLOW IS OVERSPREADING S OH/W WV SPARKING
A FEW CONVECTIVE SHOWERS ON MAXIMIZED WAA CONVERGENCE.  BETWEEN
THESE SYSTEMS LOW LEVEL FLOW WILL CONTINUE TO TIGHTEN IN
CONFLUENCE ACROSS E WV/NW VA.  LOW LEVEL ENVIRONMENT IS QUITE
SATURATED WITH LARGE AREA OF LOW 60S TDS.  INSTABILITY IS
GENERALLY LACKING DUE TO POOR LAPSE RATES PER IAD/PIT 12Z
RAOBS...THOUGH BREAKS IN LOW LEVEL CLOUD DECK EAST OF CONTINENTAL
DIVIDE ARE LIKELY TO AID SURFACE HEATING FOR SOME MILD INSTABILITY
IN THE 250-500 J/KG.

TOWARD 15-16Z...LOW LEVEL INFLOW WILL VEER FROM E TO SLY AND
INCREASE THROUGH 850MB FURTHER SUPPORTING MST CONVERGENCE ACROSS
THE EASTERN WV PANHANDLE SUPPORTING CONVECTIVE DEVELOPMENT.  DUE
TO THE LACK OF INSTABILITY...CONVECTION IS LIKELY TO BE SHALLOW
WITH RELATIVELY SMALL CORES.  THOUGH WITH THE STRONG MST
CONVERGENCE/CONFLUENCE...TPWS ARE EXPECTED TO INCREASE STEADILY
ALLOWING FOR GOOD RAINFALL EFFICIENCY. LARGER SCALE HEIGHT FALLS
AND SWLY FLOW ALOFT WEST OF THE DIVIDE SHOULD SUPPORT WEAK
STEERING/CLOUD ENVIRONMENT WITH SLOW CELLS MOTIONS BUT ALSO
REDEVELOPMENT ALONG TERRAIN WEST TO ALLOW FOR SOME REPEAT/CROSS
TRACKS OF RAIN SWATHS.   MUCH OF THE HI-RES GUIDANCE SUITE IS
SUPPORTIVE OF THIS SCENARIO UNFOLDING WITH 1-1.5" TOTALS ANCHORED
BY CONSISTENCY IN THE LAST FEW RUNS OF THE HRRR AND ESRL
EXPERIMENTAL HRRR.  GIVEN SATURATED SOILS/LOW FFG (PARTICULALRY
FURTHER S OVER E WV/NW VA) COULD POSE LOCALIZED FF CONCERNS.

TOWARD MIDDAY (18Z-20Z) MAIN FORCING FROM WAA FROM SW ALONG WITH
INCREASED INSTABILITY OVER 500 J/KG PER RAP/HRRR...SHOULD EXPAND
CONVECTIVE COVERAGE FURTHER N INTO SW PA...DEEPER CORES WITH
FURTHER SATURATED ENVIRONMENT/SFC MST CONVERGENCES LIFTING
N...SHOULD ALSO ALLOW FOR INCREASED RAINFALL EFFICIENCY LIKELY
OVER 1"/HR. GIVEN CELLS ARE LIKELY TO BE ANCHORED TO TERRAIN FOR
SOME DURATION.  WILL CONTINUE TO MONITOR THIS EVOLUTION AND UPDATE
MPD AS NEEDED.

GALLINA

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

ATTN...WFO...CTP...LWX...PBZ...RLX...RNK...

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

LAT...LON   40817952 40487914 39947875 39137853 38327850
            37767913 37698000 38578004 39278023 40138037
            40758023



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