MESOSCALE PRECIPITATION DISCUSSION 0895
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
409 AM EDT SAT OCT 21 2017

AREAS AFFECTED...WESTERN WA...WESTERN OR

CONCERNING...HEAVY RAINFALL

VALID 210805Z - 211800Z

SUMMARY...HEAVY RAINS WILL BEGIN TO OVERSPREAD WESTERN WA/OR THIS
MORNING AS THE NEXT ATMOSPHERIC RIVER ARRIVES ACROSS THE PACIFIC
NORTHWEST.

DISCUSSION...THE LATEST GOES-15 WV IMAGERY SHOWS A STRENGTHENING
MID TO UPPER LEVEL TROUGH MOVING EAST TOWARD THE GULF OF AK WHICH
IS DRIVING A STRONG FRONTAL ZONE GRADUALLY TOWARD THE PACIFIC
NORTHWEST ALONG WITH A DEEP LONG-FETCH AXIS OF HIGHER PWATS AND
ENHANCED MOISTURE TRANSPORT. THERE IS ALREADY A WARM FRONT
APPROACHING THE COASTAL RANGES OF WESTERN WA/OR...AND A PAIR OF
AMSU PASSES AT 0454Z AND 0550Z RESPECTIVELY ARE INDICATING AN
UPTICK IN OFFSHORE RAINFALL RATES WHICH ARE SEEN AS HIGH AS 0.35
INCHES/HR. SOME COOLING CLOUD TOPS ASSOCIATED WITH A STRENGTHENING
OF THE WARM AIR ADVECTION PATTERN ARE SEEN JUST OFFSHORE WESTERN
OR IN PARTICULAR AND COINCIDES WITH THE HEAVIEST RATES.

OVER THE NEXT SEVERAL HOURS...THE WARM FRONT WILL BE APPROACHING
AND SOME MODERATE TO HEAVY RAINS OUT AHEAD OF IT WILL OVERSPREAD
ESPECIALLY NORTHWEST OR AND SOUTHWEST WA BASED ON THE LATEST AMSU
TRENDS. UPSTREAM THERE IS THE NOSE OF A 50 KT SOUTHWESTERLY LOW
LEVEL JET WHICH WILL BE ARRIVING ACROSS THE COASTAL RANGES BY
AROUND 12Z ALONG WITH ARRIVAL AND GRADUAL PASSAGE OF THE WARM
FRONT. SOUTHERLY FLOW AHEAD OF THE WARM FRONT WILL VEER AROUND TO
WEST-SOUTHWEST AND ALLOW UPSTREAM 850/700 MB MOISTURE FLUX
ANOMALIES OF 2 TO 3 SIGMAS ABOVE NORMAL TO BECOME AIMED ORTHOGONAL
TO THE COASTAL RANGES FROM WEST-CENTRAL AND NORTHWEST OR UP INTO
SOUTHWEST WA. THE LATEST GUIDANCE SUPPORTS SOME STRENGTHENING OF
THE WIND FIELD AND A FURTHER INCREASE OF THESE ANOMALIES TO AS
MUCH AS 3 TO 4 SIGMAS ABOVE NORMAL BY 18Z.

EXPECT ROBUST ATMOSPHERIC RIVER CONDITIONS AS A RESULT TO ENSUE BY
MID TO LATE MORNING WITH HEAVY RAIN AIDED BY PWATS OF OVER 1.25
INCHES INTO THE COASTLINE AND HIGHLY FAVORABLE OROGRAPHIC ASCENT
FOR ENHANCED RAINFALL RATES THAT ARE EXPECTED TO APPROACH AS MUCH
AS 0.50 TO 0.70 INCHES/HR.

THE HIRES MODEL CONSENSUS INCLUDING THE 00Z ARW/NMMB AND 06Z HRRR
SUPPORT RAINFALL AMOUNTS OF AS MUCH AS 2 TO 4 INCHES THROUGH
18Z...WITH THE HEAVIEST TOTALS ACROSS THE COASTAL RANGES OF
WESTERN OR JUST TO THE WEST OF THE WILLAMETTE VALLEY AND ALSO TO A
SLIGHTLY LESSER EXTENT OVER THE WEST FACING SLOPES OF THE CENTRAL
AND NORTHERN OR CASCADES. LESSER AMOUNTS OF AS MUCH AS 1 TO 3
INCHES CAN BE EXPECTED ELSEWHERE INCLUDING SOUTHWEST WA. ALL OF
THESE RAINS WILL BE FALLING OVER AREAS THAT JUST RECENTLY SAW
HEAVY RAINFALL...AND SO THERE WILL TEND TO BE AN INCREASED THREAT
OF RUNOFF PROBLEMS WITH TIME. ADDITIONAL HEAVY RAINS ARE EXPECTED
BEYOND THIS PERIOD AS WELL WHICH WILL FURTHER ENHANCE THE CONCERNS.

ORRISON

ATTN...WFO...EKA...MFR...PDT...PQR...SEW...

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

LAT...LON   47962402 47882344 47432309 47102301 46792274
            46832216 46892170 46552149 46132151 45582165
            44382181 43612191 42752195 42272217 42262256
            42502275 42752298 42872318 42922348 42592366
            42102376 41952414 42162449 42862458 44022425
            44742414 45362414 46342417 47232441 47762440