Flash Flood Guidance
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984
AWUS01 KWNH 210853
FFGMPD
WAZ000-212052-

MESOSCALE PRECIPITATION DISCUSSION 0940
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
352 AM EST TUE NOV 21 2017

AREAS AFFECTED...PORTIONS OF WESTERN WA

CONCERNING...HEAVY RAINFALL

VALID 210852Z - 212052Z

SUMMARY...AN APPROACHING WARM FRONT ALONG WITH AN IMPINGING
ATMOSPHERIC RIVER ARE EXPECTED TO INCREASE HOURLY RAIN TOTALS
ABOVE 0.5" BEYOND 17Z WITH LOCAL AMOUNTS UP TO 4" WITHIN FAVORED
TOPOGRAPHY BY 21Z.

DISCUSSION...GOES-15 INFRARED SATELLITE IMAGERY AND SATELLITE
DERIVED WINDS REVEAL A CURVED 150+ KT UPPER LEVEL JET STREAM
ARCING OUT OF THE NORTHEAST PACIFIC INTO SOUTHWEST BRITISH
COLUMBIA AND THEN INTO WESTERN MT.  WATER VAPOR IMAGERY AND
GFS-BASED INTEGRATED VAPOR TRANSPORT SHOW OFF AN ATMOSPHERIC RIVER
RISING OUT OF THE TROPICS NEAR THE 140TH MERIDIAN AND EXTENDING TO
THE WEST OF OR. SURFACE OBSERVATIONS AT 08Z REVEAL A WARM FRONT
MOVING INTO SOUTHWEST OR AT THE LEADING EDGE OF THE MOISTURE
PLUME.  GPS-BASED PRECIPITABLE WATER VALUES NEAR AND SOUTH OF THE
WARM FRONT ARE 1.4-1.6" FROM THE CENTRAL OR COAST DOWN INTO FAR
NORTHWEST CA.  PER A RECENT BRIEFING FROM SAB/NESDIS, PRECIPITABLE
WATER VALUES OFFSHORE WITHIN THE PLUME ARE CLOSE TO 2".  INFLOW AT
700/850 HPA IS SOUTHWEST AT ~30 KTS PER VAD WIND PROFILES.

THE GFS ADVERTISES LOW- TO MID-LEVEL FRONTOGENESIS TO SPREAD
ACROSS WESTERN WA BEYOND 09Z WHILE RAP-BASED FIELDS SHOW
INCREASING 850/700 HPA INFLOW OUT OF THE SOUTH-SOUTHWEST WITH
TIME, RISING BEYOND 50 KTS BY 17Z.  THE SURFACE WARM FRONT IS
EXPECTED TO SHOW SLOW PROGRESS INTO WESTERN WA BY 21Z.
PRECIPITABLE WATER VALUES SHOULD RISE TO ~1.5" AT SEA LEVEL AND
~1" IN THE MOUNTAINS AFTER 12Z.  FREEZING LEVELS ARE FORECAST TO
RISE TO NEAR 700 HPA/10,000 FEET ABOVE SEA LEVEL WITH TIME WHICH
SHOULD AID WARM RAIN PROCESSES.  INTEGRATED VAPOR TRANSPORT IS
FORECAST BY THE GFS TO MAXIMIZE NEAR WESTERN WA FROM ~18Z ONWARD.

HREF FORECASTS INDICATE MU CAPE VALUES COULD LOCALLY BREACH 100
J/KG, WHICH COMBINED WITH THE EXPECTED MOISTURE AND LOW-LEVEL
INFLOW SHOULD LEAD TO 0.5"+/HOUR TOTALS.  BOTH THE 00Z SSEO
NEIGHBORHOOD PROBABILITIES AND 00Z HREF PROBABILITIES AGREE,
INDICATING A RAMP UP OF RAIN RATES ACROSS WESTERN WA 12-17Z BEFORE
0.5"+/HOUR TOTALS ARE REACHED THEREAFTER.  THE MESOSCALE GUIDANCE
SHOWS A GOOD SIGNAL FOR LOCAL AMOUNTS IN THE 3-4" RANGE THROUGH
21Z.  PER INFORMATION FROM SEW/THE SEATTLE WA FORECAST OFFICE,
THIS RAINFALL COULD LEAD TO LANDSLIDES.

ROTH

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

ATTN...WFO...PDT...PQR...SEW...

ATTN...RFC...NWRFC...

LAT...LON   48392443 48262379 48052297 47512210 47102137
            46732108 46332114 45782168 45762230 46002267
            46732292 47012346 47202388 47672453 48322476




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