MESOSCALE PRECIPITATION DISCUSSION 0890
NWS WEATHER PREDICTION CENTER COLLEGE PARK MD
438 AM EDT WED OCT 18 2017

AREAS AFFECTED...NORTHWEST WA

CONCERNING...HEAVY RAINFALL

VALID 180838Z - 182038Z

SUMMARY...AN ATMOSPHERIC RIVER IS EXPECTED TO MOVE ASHORE WITH
HEAVY RAINFALL AFTER 15Z.  HOURLY RAIN TOTALS AT OR ABOVE 0.5" AND
LOCAL AMOUNTS IN THE 4-6" RANGE ARE EXPECTED.

DISCUSSION...THE SECOND IN A SERIES OF ATMOSPHERIC RIVER EVENTS IS
POISED TO MOVE INTO NORTHWEST WA.  THE DISTURBANCE RESPONSIBLE FOR
THE NEWEST INCURSION IS NEAR 49N 141W AT THE PRESENT TIME.  THE
MOISTURE CHANNEL/PLUME EXTENDS BACK TO TAIWAN.  PRECIPITABLE WATER
VALUES WITHIN THE PLUME EXCEED 1.5" PER THE BLENDED TPW PRODUCT. 
INFLOW AT 850/700 HPA HPA IS CURRENTLY OUT OF THE SOUTH TO WEST AT
~20 KTS.

AS THE DISTURBANCE APPROACHES, DIVERGENCE ALOFT AND BOUNDARY LAYER
MOISTURE CONVERGENCE ARE EXPECTED TO INCREASE.  THIS OCCURS ACROSS
THE OLYMPIC PENINSULA STARTING AT 15Z AND THE NORTHERN CASCADES
BEGINNING AT 18Z NEAR AND AFTER WARM FRONTAL PASSAGE AND REMAINS
QUASI- STATIONARY ALONG WITH THE MOISTURE PLUME THEREAFTER.  THE
INFLOW AT 850 HPA IS EXPECTED TO REACH 60-65 KTS WHILE MU CAPE
VALUES MIGHT REACH 100 J/KG.  INGREDIENT- WISE, THIS SUPPORTS
HOURLY RAIN TOTALS OF ~0.5".  THE MESOSCALE GUIDANCE CONCURS, WITH
VERSION 2 OF THE 00Z HREF PROBABILITIES SHOWING A 50-90% CHANCE OF
EXCEEDING 0.5"/HOUR TOTALS DURING THE PERIOD OF MAXIMUM BOUNDARY
LAYER MOISTURE CONVERGENCE.  SNOW LEVELS WILL BE CLIMBING WITHIN
THE WARM ADVECTION REGIME, ASCENDING TOWARDS 10,000 FEET WITH
TIME, WHICH WOULD AID WARM RAIN PROCESSES AND LEAD TO GREATER
PRECIPITATION EFFICIENCY DURING THE PERIOD OF MAXIMUM BOUNDARY
LAYER MOISTURE CONVERGENCE.  THE MESOSCALE GUIDANCE SHOWS A STRONG
SIGNAL FOR LOCAL AMOUNTS IN THE 4-6" RANGE ACROSS THE OLYMPICS AND
2-3" FOR THE NORTHERN CASCADES BY 21Z. 

ROTH

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

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

LAT...LON   49172297 49112199 49152075 47472096 47382106
            46692173 46732215 47122234 47012338 47022414
            48372479 48242320 48902314