MESOSCALE PRECIPITATION DISCUSSION 0931
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
1251 AM EST WED NOV 15 2017

AREAS AFFECTED...COASTAL RANGES OF WASHINGTON AND OREGON

CONCERNING...HEAVY RAINFALL

VALID 150551Z - 151751Z

SUMMARY...AN ENHANCED JET OF SOUTHWESTERLY LOW-MID LEVEL FLOW WILL
BEGIN TO SLOWLY DROP SOUTH ALONG THE WASHINGTON AND OREGON
COASTLINES THIS MORNING. THIS SHOULD MAINTAIN A RIBBON OF MODERATE
TO OCCASIONALLY HEAVY RAINFALL OVER THE NEXT 6-12 HOURS WITH RAIN
RATES LIKELY TO REACH 0.5 IN/HR ALONG THE IMMEDIATE COAST AND
FAVORED UPSLOPE REGIONS...AND LOCALIZED MAX RATES AS HIGH AS 1
IN/HR POSSIBLE.

DISCUSSION...LANGLEY HILL (KLGX) RADAR ON THE WASHINGTON COAST
SHOWED A SUBSTANTIAL RIBBON OF PRECIPITATION PUSHING INTO THE
WASHINGTON COAST AS OF 05Z...WITH A PROMINENT CONVECTIVE BANDING
FEATURE FROM NEAR QUILLAYUTE WA...TO ABOUT 50MI WEST OF ASTORIA
OR. THE 00Z SOUNDING FROM QUILLAYUTE SHOWED A STRONG LOW-LEVEL JET
(61 KNOT WINDS OBSERVED AT 1KM AGL) WITH A SMALL AMOUNT OF
INSTABILITY (AROUND 100 J/KG MUCAPE). PRECIPITABLE WATER VALUES
WERE ABOVE NORMAL (0.73 IN)...AROUND THE 80TH PERCENTILE RELATIVE
TO THE STATION SOUNDING CLIMATOLOGY. THE CIRA BLENDED TPW PRODUCT
SHOWED HIGHER PWATS LURKING JUST OFFSHORE ON THE ORDER OF 0.8 TO
0.9 INCHES AND MODEL FORECASTS DO SHOW AN INCREASE IN PWATS ALONG
THE COAST OVER THE NEXT SEVERAL HOURS...PARTICULARLY IN SW
WASHINGTON AND NW OREGON. AN AMSU 89 GHZ PASS AROUND 23Z SHOWED A
LIKELY REGION OF HIGHER ATMOSPHERIC MOISTURE CONTENT WITH A
LEADING EDGE AROUND 38N/130W. ALTHOUGH THIS WILL MOSTLY BE THE
FOCUS FOR PRECIPITATION DOWN THE PACIFIC COAST LATER TODAY...THERE
ARE SOME INDICATIONS ON THE RAP MODEL THAT ACCELERATING FLOW
AROUND THE SOUTH SIDE OF THE TROUGH MAY BE ABLE TO TAP THIS DEEPER
MOISTURE AND ENHANCE PWATS TOWARD THE END OF THE DISCUSSION WINDOW
(14-18Z) ALONG THE OREGON COAST.

ALTHOUGH PWATS MAY INCREASE LATE IN THE PERIOD...LOW-LEVEL
MOISTURE TRANSPORT MAGNITUDES ARE EXPECTED TO WEAKEN SLIGHTLY
(PARTICULARLY AFTER 12Z). THIS MAY MITIGATE THE INCREASE IN
MOISTURE SOMEWHAT...AND KEEP THE HIGHEST RAIN RATES ALONG THE
WA/OR COASTLINES CONCENTRATED GENERALLY PRIOR TO 15Z. UNTIL THAT
TIME...THE SYNOPTIC SETUP FAVORS MAINTENANCE OF BANDS OF HEAVY
RAIN WITH STRONG LOW-LEVEL FLOW CONTINUING TO IMPINGE ON THE COAST
AND COASTAL RANGES...AND BROAD UPPER LEVEL DIFFLUENCE (EVIDENT IN
THE WEDGE-SHAPED APPEARANCE OF THE COLDER CLOUD TOPS ON GOES-15 IR
SATELLITE).

RAIN RATES OFFSHORE WERE GENERALLY PEAKING IN THE 0.2 TO 0.5 IN/HR
RANGE PER GLOBAL HYDRO-ESTIMATOR SATELLITE ESTIMATES AND
DISCUSSION WITH NESDIS SAB. THESE RATES COULD POTENTIALLY BE
DOUBLED IN THE MORE SUBSTANTIAL RAIN BANDS IMMEDIATELY ALONG THE
COAST AND IN FAVORED UPSLOPE REGIONS (W/SW FACING SLOPES IN THE
COASTAL RANGES). QUILLAYUTE WA OBSERVED AN HOURLY TOTAL AS HIGH AS
0.55 INCHES SO FAR...AND EVEN REPORTED 0.18 INCHES IN JUST 4
MINUTES (0223-0227Z). THEREFORE...VERY HEAVY RAIN RATES DO EXIST
IN SOME OF THE BANDS APPROACHING THE COAST...BUT GIVEN HOW NARROW
THEY ARE THOSE SORT OF RAIN RATES MAY NOT BE SUSTAINED FOR A
SIGNIFICANT LENGTH OF TIME. THE 00Z HREF PROBABILITY MATCHED MEAN
RAINFALL HAS PROVIDED A REASONABLE UPPER BOUND FOR OBSERVED RAIN
RATES THUS FAR...AND IT SHOWS RATES PEAKING AS HIGH AS 0.8 TO 1.0
IN/HR NEAR THE COAST AS FAR SOUTH AS NEWPORT OR THROUGH 13Z. RAIN
RATES THIS HIGH MAY PRODUCE SOME FLOODING...PARTICULARLY GIVEN
ANTECEDENT CONDITIONS SUGGESTED BY RAINFALL AROUND 150 PERCENT OF
NORMAL OVER THE PAST MONTH IN THE REGION...AND ELEVATED
NEAR-SURFACE SOIL MOISTURE LEVELS PER THE NATIONAL WATER MODEL.

LAMERS

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

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

LAT...LON   48472480 48372443 48272405 47832401 47502378
            47162316 46282295 45612302 44682328 43922353
            43542404 43782436 44642422 45582413 46582422
            47312444 47952477