MESOSCALE PRECIPITATION DISCUSSION 0941
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
348 PM EST TUE NOV 21 2017

AREAS AFFECTED...FAR NORTHWESTERN OR INTO WESTERN WA

CONCERNING...HEAVY RAINFALL

VALID 212047Z - 220445Z

SUMMARY...RAINFALL RATES CURRENTLY PEAKING IN INTENSITY ACROSS
WESTERN WA WILL BEGIN TO WANE PAST 00Z. AN ADDITIONAL 2-4 INCHES
OF RAIN IS EXPECTED BY 04Z ACROSS THE OLYMPICS...SELECTED COASTAL
RANGES INTO THE WASHINGTON CASCADES.

DISCUSSION...BLENDED TPW IMAGERY THROUGH 18Z SHOWED A LARGE AREA
OF ANOMALOUS PRECIPITABLE WATER VALUES RANGING BETWEEN 200-300
PERCENT OF NORMAL...EXTENDING FROM THE PACIFIC NORTHWEST COAST TO
AND BEYOND 130W AND EXTENDING SWW TO ABOUT 25N. PRECIPITABLE WATER
VALUES WITHIN THE CORE OF THE ANOMALOUS MOISTURE AXIS...WHICH
SHOWED A DIRECT CONNECTION TO THE ITCZ...WERE WIDESPREAD OF 1.5+
INCHES PEAKING NEAR 2 INCHES AS FAR NORTH AS 34N 132W. GPS DATA
SHOWED MAXIMUM PRECIPITABLE WATER VALUES OF 1.3 TO 1.4 INCHES
ALONG MOST OF THE WA COASTLINE. 20Z VWP DATA FROM KLGX SHOWED 850
MB WINDS OF 75 KT FROM THE SSW...ARRIVING SLIGHTLY QUICKER THAN
EARLIER FORECASTS. THESE WINDS REPRESENT A 20 KT INCREASE SINCE
16Z WITH OBSERVED RAINFALL RATES OF NEAR 0.5 TO 0.6 IN/HR OBSERVED
IN THE OLYMPICS.

AS A SHORTWAVE NEAR 49N 132W CONTINUES TO TRACK NORTHEASTWARD INTO
BRITISH COLUMBIA OVER THE NEXT 3-6 HOURS...850 MB WIND SPEEDS
ALONG THE CENTRAL WASHINGTON COASTLINE WILL BEGIN TO
WEAKEN...REACHING 40 KT BY 03Z PER THE LATEST CONSENSUS OF HI-RES
MODEL FORECASTS...AND AS THE MOISTURE AXIS SLIDES SOUTH. MAXIMUM
WIND SPEEDS WITHIN THE MOISTURE AXIS ARE EXPECTED TO RANGE BETWEEN
50-60 KT BY 03Z ALONG THE CENTRAL TO SOUTHERN OR COAST. WEAKENING
TRENDS IN A 150 KT UPPER LEVEL SPEED MAX LOCATED EAST OF A LARGE
SCALE CLOSED LOW LOCATED NEAR 147W ARE ALSO EXPECTED TO CONTINUE
THROUGH 06Z.

THE RESULT WILL BE CONTINUING RAINFALL RATES PEAKING BETWEEN 0.5
AND 0.6 IN/HR FROM THE OLYMPICS INTO THE WASHINGTON CASCADES AND
THE COASTAL RANGES STRADDLING THE OR/WA BORDER OVER THE NEXT
SEVERAL HOURS. LOCALLY HIGHER RATES MAY BE OBSERVED...BUT
WEAKENING WILL OCCUR FROM NORTH TO SOUTH AS THE MOISTURE AXIS
EDGES DOWN THE COAST TONIGHT. AN ADDITIONAL 2-3 INCHES...LOCALLY
NEAR 4 INCHES OF RAIN IS EXPECTED IN THESE LOCATIONS THROUGH 04Z.

OTTO

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

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

LAT...LON   49102062 49032010 48572006 47932030 47512080
            46942120 46532134 45992169 45742199 45912293
            45472346 45632396 46442412 46992412 47542438
            47902427 48122376 47872299 48002252 48122225
            48642223 48942196 49072145