Flash Flood Guidance
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420
AWUS01 KWNH 170524
FFGMPD
WAZ000-171723-

MESOSCALE PRECIPITATION DISCUSSION 0888
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
123 AM EDT TUE OCT 17 2017

AREAS AFFECTED...TERRAIN OF WESTERN WA

CONCERNING...HEAVY RAINFALL

VALID 170523Z - 171723Z

SUMMARY...AN ATMOSPHERIC RIVER WITH ITS ASSOCIATED HEAVY
PRECIPITATION IS POISED TO MAKE LANDFALL IN NORTHWEST WA AROUND
11Z, LEADING TO HOURLY RAIN TOTALS UP TO 0.7" AND LOCAL AMOUNTS TO
4" BY 18Z.

DISCUSSION...A COLD FRONT IS SHIFTING SOUTHEAST ACROSS BRITISH
COLUMBIA AND THE ADJACENT PACIFIC IN THE DIRECTION OF WASHINGTON.
A RECENT SAB/NESDIS BRIEFING NOTED THAT AN AREA OF PRONOUNCED
SURFACE MOISTURE CONVERGENCE WAS SEEN WITHIN GOES HIGH DENSITY
WIND FIELDS NEAR THE FRONT.  THE MOISTURE CHANNEL/ ATMOSPHERIC
RIVER IS LONG AND STRETCHES BACK TO JAPAN AND SOUTHERN CHINA.  THE
NEXT SIGNIFICANT DISTURBANCE ALOFT UPSTREAM WAS NOTED NEAR 46N
140W AS AN AREA OF DARKENING ON GOES-15 WATER VAPOR IMAGERY.
PRECIPITABLE WATER VALUES (PWS) OF 1.25-1.5" LIE WITHIN THE RIVER
BASED ON SATELLITE ESTIMATES OFFSHORE.  CURRENT PWS ARE 1-1.25"
ACROSS THE REGION PER GPS DATA.  INFLOW AT 850 HPA IS SOUTHWEST
20-25 KTS PER VAD WIND PROFILES AND THE WIND FLOW IS NEARLY
UNIDIRECTIONAL WITH HEIGHT.

RAP MASS FIELDS SUGGEST THAT THE FRONTAL ZONE AND ITS ASSOCIATED
BOUNDARY LAYER MOISTURE CONVERGENCE REACHES QUILLAYUTE ~11Z AS
DIVERGENCE ALOFT INCREASES AND MOVES THROUGH THE COASTAL RANGES
AND CASCADES WITH TIME.  LATE IN THE PERIOD, AFTER 15Z, SNOW
LEVELS START DROPPING IN EARNEST, BOTTOMING OUT NEAR 5500 FEET IN
THE NORTHERN CASCADES AND OLYMPIC PENINSULA BY 18Z, BUT THIS IS
EXPECTED TO OCCUR AS THE HEAVIER PRECIPITATION SHIFTS SOUTHWARD.
MUCAPE VALUES OF 100-200 J/KG (PER SREF MEAN OUTPUT) COMBINED WITH
INFLOW AT 850 HPA/THE MEAN 850-400 HPA WIND RISING TO ~55 KTS
SUGGEST THAT HOURLY RAIN RATES SHOULD MAXIMIZE IN 0.6-0.7" RANGE,
WHICH IS SUPPORTED BY 12Z ARW AND 00Z HRRR RUNS.  THE MESOSCALE
GUIDANCE IS IN GOOD AGREEMENT ON LOCAL AMOUNTS OF 3-4" BEING
POSSIBLE, MAINLY DURING THE 12-18Z PERIOD.

ROTH

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

ATTN...WFO...OTX...PDT...SEW...

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

LAT...LON   49042286 49012097 48092082 47372111 46822167
            46632261 47042404 47362453 47922475 48422475
            48212280 48392308 48652326 48762290



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