MESOSCALE PRECIPITATION DISCUSSION 0004
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1007 AM EST THU JAN 11 2018

AREAS AFFECTED...WRN WASHINGTON...NWRN OREGON

CONCERNING...HEAVY RAINFALL

VALID 111506Z - 120106Z

SUMMARY...QUICK INCREASE OF ATMOSPHERIC MOISTURE AND WEST TO
SOUTHWEST LOW-MID LEVEL FLOW TODAY SHOULD PRODUCE PERIODS OF HEAVY
RAINFALL. RAIN RATES MAY EXCEED 0.5 IN/HR FOR SEVERAL HOURS IN
FAVORED UPSLOPE REGIONS OF THE COASTAL RANGES...OLYMPICS...AND
CASCADES.

DISCUSSION...LATEST GOES-15 WATER VAPOR SATELLITE LOOPS SHOW A
COMPACT LOW PRESSURE SYSTEM JUST OFFSHORE OF THE PACIFIC NORTHWEST
WITH AN UPPER LEVEL +PV MAXIMUM CENTERED NEAR 45N/134W. AN
OCCLUDED SURFACE LOW WAS CENTERED NEARBY...AND BOTH RAP ANALYSIS
AND CIRA BLENDED TPW PRODUCTS SHOW A PLUME OF ENHANCED PWATS OVER
1.0 INCHES IN THE WARM SECTOR JUST OFFSHORE OF WA/OR. AS THE
OCCLUDED CYCLONE MAKES STEADY PROGRESS EAST...THE PLUME OF DEEPER
MOISTURE SHOULD ARRIVE ALONG AND WEST OF THE CASCADES SHORTLY. THE
12Z SLM SOUNDING SHOWED A SATURATED PROFILE ESSENTIALLY THROUGH
THE DEPTH OF THE TROPOSPHERE WITH A PWAT APPROACHING 1 INCH...WELL
ABOVE THE 90TH PERCENTILE ON THE STATION CLIMATOLOGY. RADAR VWP
FROM KRTX ALSO REVEALED STEADILY INCREASING WINDS IN THE 3-7KFT
LAYER OVER THE PAST SEVERAL HOURS WITH A SLIGHT VEERING AS WELL.
THIS SHOULD CONTINUE AS OBJECTIVE ANALYSIS INDICATES WSW LLJ
OFFSHORE INCREASING TO 50-60 KNOTS...AND THIS IS FORECAST TO
ARRIVE IN THE HIGHLIGHTED PORTION OF WA/OR BY 18Z.

THE COMBINATION OF INCREASING LOW-MID LEVEL FLOW...A DEEP
SATURATED PROFILE...AND RISING SNOW LEVELS SHOULD LEAD TO
INCREASING RAIN RATES AND MORE EFFICIENT RAIN PROCESSES INTO THE
LATE MORNING AND AFTERNOON...PARTICULARLY IN FAVORED UPSLOPE
REGIONS OF THE NORTH-SOUTH ORIENTED RANGES. BETWEEN 15Z AND
21Z...THERE IS A STRONG SIGNAL ON THE 06Z HREF FOR HEAVY
RAINFALL...WITH OVER 70 PERCENT PROBABILITIES OF AT LEAST 0.5
IN/HR RAIN RATES IN PORTIONS OF THE NRN OREGON COASTAL
RANGES...AND THE WA/OR CASCADES. SLIGHTLY LOWER PROBABILITIES
EXIST IN OTHER RANGES ACROSS THE REGION...AND OVERALL ON THE
HRRR-TLE (ALTHOUGH THERE IS STILL SOME SIGNAL ON THAT ENSEMBLE AS
WELL). THE NATIONAL WATER MODEL DOES INDICATE NEAR SURFACE SOIL
MOISTURE IS AT ELEVATED LEVELS IN MOST OF THE AREA WEST OF THE
CASCADES...AND STREAMFLOW ANOMALIES ARE WELL ABOVE NORMAL FLOW IN
PORTIONS OF NW OREGON. THIS SUGGESTS THAT ANY MORE FOCUSED PERIODS
OF HEAVY RAINFALL COULD LEAD TO SOME EXCESS RUNOFF CONCERNS.

LAMERS

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

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

LAT...LON   48932182 48762111 48292078 47402084 46462118
            45592143 45032159 44492161 43932169 44002232
            44472279 45112273 45242310 44702335 44452385
            44892410 45822414 46442414 47082428 47462445
            47932474 48392478 48452437 48252370 48082321
            47662281 47732236 48372256 48892244