MESOSCALE PRECIPITATION DISCUSSION 0048
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
106 AM EST THU FEB 09 2017
 
AREAS AFFECTED...WESTERN WA AND OR...NORTHERN CA 
 
CONCERNING...HEAVY RAINFALL 
 
VALID 090605Z - 091800Z
 
SUMMARY...AN IMPRESSIVE SURGE IN LOW-LEVEL MOISTURE FLUX IS
EXPECTED TO REACH THE WEST COAST WITHIN THE NEXT FEW HOURS. RAIN
RATES SHOULD INCREASE ACROSS THE COASTAL RANGES OF NORTHERN CA,
THE NORTHERN SIERRA NEVADA, AND THE COASTAL RANGES OF OREGON AND
WASHINGTON, AS WELL AS PORTIONS OF THE CASCADES. RAINFALL RATES IN
THE 0.50-0.75 IN/HR RANGE ARE POSSIBLE OF MUCH OF THE OUTLINED
AREA, WITH RATES NEAR 0.50 IN/HR POSSIBLE ACROSS THE OLYMPICS.

DISCUSSION...A PLUME OF DEEP TROPICAL MOISTURE IS STREAMING
NORTHWARD AHEAD OF A DEEPENING LOW PRESSURE SYSTEM ACROSS THE
NORTH PACIFIC. LATEST AMSU 89GHZ PASS INDICATES A T-BONE LIKE
FRONTAL STRUCTURE WITH A WAVE OF LOW PRESSURE ALONG THE FRONT NEAR
39N/131W. THE PRIMARY MOISTURE PLUME/ATMOSPHERIC RIVER WAS
OBSERVED AHEAD OF THIS SURFACE LOW, WITH THE LATEST BLENDED TPW
DATA INDICATING PWATS IN THE 1.25 TO 1.6 INCH RANGE IN THIS
REGION. ACCORDING TO LATEST RUNS OF THE GLOBAL MODELS, THE
ATMOSPHERIC RIVER WAS POSITIONED WITHIN THE FAVORABLE DIVERGENT
RIGHT-REAR QUADRANT OF A >150 KT JET STREAM CENTERED NEAR
42N/130W. AT THE LOWER LEVELS, LATEST GLOBAL MODELS SUGGEST SSW
FLOW OF 40-60 KT WITHIN THE ATMOSPHERIC RIVER. LATEST MICROWAVE
ESTIMATED RAINFALL RATES OFFSHORE WITHIN THE PLUME WERE FROM 0.10
TO 0.3 IN/HR.

RAINFALL RATES ARE EXPECTED TO INCREASE FIRST AT THE COAST
FOLLOWED BY FARTHER INLAND AS THE MOISTURE PLUME MOVES ONSHORE
OVER THE NEXT FEW HOURS. ADDITIONALLY, UPPER DIVERGENCE WILL
GRADUALLY INCREASE AS THE NOTED 250 HPA JET STREAM MOVES
NORTHEASTWARD. THE LATEST WRF-ARW AND NMM DATA SUGGEST THAT RAIN
RATES NEAR 0.50 IN/HR ARE POSSIBLE ACROSS SWRN OR AND NWRN CA AS
SOON AS 08-09Z, WITH THESE RATES EXPANDING AFTER THAT TIME.
WRF-ARW AND NMM OUTPUT ALSO SUGGESTS THAT RATES COULD APPROACH
0.75 IN/HR IN PORTIONS OF THE OR AND NRN CA COASTAL RANGES AFTER
12Z. DUE TO THE PROLONGED PERIOD OF ONSHORE FLOW, SNOW LEVELS HAVE
INCREASED SUBSTANTIALLY AND PRECIPITATION IS EXPECTED TO FALL AS
RAIN EVEN AT MANY HIGHER ELEVATION LOCATIONS IN THE NORTHERN
SIERRA NEVADA AND PORTIONS OF THE CASCADES. THROUGH 18Z,
ADDITIONAL RAINFALL AMOUNTS OF 2 TO 4 INCHES ARE POSSIBLE, WITH
LOCALLY HIGHER AMOUNTS IN THE FAVORED TERRAIN. ADDITIONAL RAIN
ALONG WITH RISING SNOW LEVELS AND SNOW MELT WILL EXACERBATE
FLOODING AND POTENTIAL ROCK/MUD SLIDE CONCERNS ACROSS THE OUTLINED
AREA.

RYAN

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

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

LAT...LON   48452362 47662300 47172288 47022228 47102155 
            46482124 45962145 45202196 44542213 44242248 
            43892312 43522325 42382317 41782265 41682215 
            41322169 40832149 40102120 39292147 38752205 
            37962271 38192327 39112391 40142460 41722497 
            43232485 45302416 46902426 48122451 
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