MESOSCALE PRECIPITATION DISCUSSION 0683
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1126 AM EST THU DEC 17 2015
 
AREAS AFFECTED...SOUTHWEST WA...WESTERN OR 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 171625Z - 180425Z
 
SUMMARY...LOCALLY HEAVY RAINS WILL CONTINUE TO FOCUS IN ACROSS
PORTIONS OF SOUTHWEST WA...AND ESPECIALLY THE OR COASTAL RANGES
AND THE FOOTHILLS OF THE OR CASCADES FOR THE BALANCE OF THE DAY.

DISCUSSION...A DEEP LAYER SOUTHWESTERLY FETCH OF PACIFIC MOISTURE
ALONG AND JUST AHEAD OF A COLD FRONT WILL CONTINUE TO AIM INTO
SOUTHWEST WA AND WESTERN OR THROUGHOUT THE DAY. THIS ATMOSPHERIC
RIVER WILL FOCUS LOCALLY HEAVY RAINS IN ACROSS ESPECIALLY THE WEST
AND SOUTHWEST FACING SLOPES OF THE OR COASTAL RANGES AND THE
FOOTHILLS OF THE OR CASCADES GIVEN STRONG ONSHORE FLOW ASSOCIATED
WITH A SOUTHWEST LOW LEVEL JET OF 40 TO 50 KTS.

THIS IS HELPING TO TRANSPORT AN AXIS OF 1.25 INCH PWATS OFFSHORE
THE WEST COAST AHEAD OF THE FRONT IN TOWARD THE HIGHER TERRAIN OF
SOUTHWEST WA AND WESTERN OR AS PART OF A STRONG WARM AIR ADVECTION
PATTERN. MEANWHILE...THE LATEST WV IMAGERY SHOWS EVIDENCE OF AN
UPSTREAM SHORTWAVE IMPULSE RIDING NORTHEAST UP ALONG THE COLD
FRONT...THIS NEXT IMPULSE IS EXPECTED TO FOSTER AN INCREASE IN
SOUTHWEST ONSHORE FLOW THIS AFTERNOON AND THIS EVENING. THE 00Z
GFS INDICATES A MARKED INCREASE IN 850 MB MOISTURE FLUX ANOMALIES
BY 00Z...REACHING 2 TO 3 STD DEV ABOVE NORMAL. THESE ANOMALIES
SHOULD INCREASE FURTHER TOWARD 06Z AND REACH 3 TO 4 STD DEV ABOVE
NORMAL JUST AHEAD OF THE APPROACHING COLD FRONT.

THE RESULT OF STRONG ONSHORE FLOW AND ANOMALOUS MOISTURE TRANSPORT
WILL BE HEAVY RAINS FOR THE FAVORED COASTAL RANGES OF WESTERN OR
AND INCLUDING THE FOOTHILLS OF THE OR CASCADES. HEAVY RAINS WILL
ALSO BE FOCUSING FARTHER NORTH INTO SOUTHWEST WA. OFFSHORE
RAINFALL RATES AS PER LATEST AMSU DATA IS CLOSE TO 0.25
INCHES/HR...BUT THIS WILL INCREASE THIS AFTERNOON AND EVENING.
RAINFALL RATES SHOULD INCREASE TO LOCALLY OVER 0.50 INCHES/HR AS
THE NEXT SHORTWAVE IMPULSE ARRIVES...AND RATES MAY EVEN REACH OVER
0.75 INCHES/HR FOR SOME PARTS OF THE SOUTHWEST OR COASTAL RANGES
WHERE MOISTURE TRANSPORT/OROGRAPHICS WILL ENHANCE RATES.

THE LAST COUPLE RUNS OF THE HRRR SUGGEST ADDITIONAL RAINFALL
AMOUNTS OF 3 TO 5 INCHES FOR THE HIGHER TERRAIN...WITH ISOLATED
HEAVIER AMOUNTS POSSIBLY EXCEEDING 6 INCHES FOR SOUTHWEST OR.
EXPECT LESSER AMOUNTS OF 1 TO 3 INCHES FOR THE INTERIOR VALLEYS OF
WESTERN OR. IN ANY EVENT...THESE ADDITIONAL RAINS ARE LIKELY TO
EXACERBATE ONGOING FLOODING PROBLEMS ASSOCIATED WITH RECENT HEAVY
RAINFALL OVER THE LAST COUPLE OF WEEKS. WILL CONTINUE TO MONITOR.

ORRISON

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

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

LAT...LON   46652321 46422228 44772216 44262222 43352258 
            42392281 42092310 41982389 42112432 42522452 
            43332445 44382411 45742406 46382379 
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