MESOSCALE PRECIPITATION DISCUSSION 0774
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
906 AM EST THU NOV 24 2016
 
AREAS AFFECTED...WESTERN WA...WESTERN OR 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 241400Z - 242130Z
 
SUMMARY...HEAVY RAINS WITH STRONG ONSHORE FLOW WILL CONTINUE TO
IMPACT WESTERN WA...AND GRADUALLY SETTLE SOUTH DOWN OVER THE
COASTAL RANGES OF WESTERN OR THROUGH MIDDAY.

DISCUSSION...A DEEP LAYER TROUGH AND ASSOCIATED FRONTAL OCCLUSION
IS HELPING TO FOCUS A DEEP PLUME OF PACIFIC MOISTURE INTO THE
PACIFIC NORTHWEST THIS MORNING WHICH IS RESULTING IN A SW/NE
ORIENTED AXIS OF HEAVY RAINFALL IMPACTING WESTERN WA. THE PW
ANOMALIES AHEAD OF THE FRONT ARE ONLY NORMAL TO SLIGHTLY ABOVE
NORMAL FOR THIS TIME OF YEAR...WITH VALUES OF 0.70 TO 0.80 INCHES.
HOWEVER...STRONG SOUTHWEST ONSHORE FLOW OF 50 TO 60 KTS AT 850 MB
AIMING INTO THE COASTAL RANGES ALONG WITH NOTABLE LEFT-EXIT REGION
UPPER JET DYNAMICS IS HELPING TO COMPENSATE AND IS RESULTING IN
HEAVIER RAINFALL RATES. 

RAINFALL RATES ACROSS SOUTHWEST WA THIS MORNING HAVE ALREADY BEEN
AS HIGH AS 0.40 TO 0.70 INCHES/HR WITHIN THE CORE OF STRONGEST
FORCING...AND THESE RATES WILL TEND TO CONTINUE ON OCCASION WITH
OROGRAPHICS AND ALSO THE ROBUST UPPER LEVEL JET DYNAMICS PROMOTING
STRONGER DEEP LAYER ASCENT. GOES-WEST IR SATELLITE IMAGERY SHOWS A
CORE OF IMPRESSIVELY COLD CLOUD TOPS EXTENDING FROM WESTERN WA TO
OFFSHORE THE OR COASTAL RANGES IN ASSOCIATION WITH THE UPPER JET
SUPPORT.

THE LATEST HIRES GUIDANCE LED BY THE ARW/NMMB AND RECENT RUNS OF
THE HRRR SUGGESTS THE MAIN FRONTAL ZONE OFFSHORE THE PACIFIC
NORTHWEST GRADUALLY SETTLING SOUTHEAST OVER THE NEXT FEW HOURS
WHICH WILL ALLOW THE AXIS OF HEAVY RAINFALL TO SETTLE SOUTH DOWN
ACROSS THE COASTAL RANGES OF WESTERN OR THROUGH MIDDAY. THE HIRES
GUIDANCE SUGGESTS RAINFALL RATES OCCASIONALLY REACHING AS HIGH AS
0.50 TO 0.75 INCHES FOR ESPECIALLY NORTHWEST OR...WHERE RAINFALL
TOTALS THROUGH 21Z OF 2 TO 4 INCHES WILL BE EXPECTED...ALONG WITH
SOME ISOLATED HEAVIER TOTALS FOR THE FAVORED SOUTHWEST FACING
SLOPES. SOME ENHANCED THREAT OF RUNOFF PROBLEMS WITH MUDSLIDES AND
DEBRIS FLOWS CANNOT BE RULED OUT HERE. ADDITIONAL AMOUNTS FOR
WESTERN WA WILL GENERALLY BE 1 TO 2 INCHES WITH SOME LOCALLY
HEAVIER TOTALS. WILL CONTINUE TO MONITOR.

ORRISON

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

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

LAT...LON   48092290 46972233 44572263 43262329 43182418 
            43802457 45192461 45422441 47742426