Mesoscale Precipitation Discussion 1008
NWS Weather Prediction Center College Park MD
1108 PM EDT Sat Nov 03 2018

Areas affected...Western and Northern Washington

Concerning...Heavy rainfall

Valid 040240Z - 041400Z

Summary...An atmospheric river event will develop across portions
of northern and western Washington into early Sunday morning and
likely bring heavy rainfall.

Discussion...Latest GOES satellite IR imagery shows a well defined
plume of moisture pushing onshore the far southern British
Columbia and northwest Washington coastlines. The latest blended
TPW product shows the plume focused on the coastline, with values
around 1 to 1.2 inches. This is confirmed by the UIL 00z sounding
which had a PWAT of 1.19 inches. These values are around 2
standard deviations above normal.

Recent radar returns show widespread rain invading the area and
recent estimates and surface observations indicate hourly totals
of 0.10-0.20" are already occurring.

The latest hi-res guidance shows rainfall intensity will increase
steadily through the overnight period, with the best potential for
reaching or exceeding hourly totals of 0.50" between 08-12z across
the Olympic and portions of the Cascades. This should coincide
with the arrival of 50+ kt 850mb jet helping to drive the moisture
flux anomalies as well as coupled with the 100-150 kt jet streak
at 300 mb. All of this should lead to enhanced orographic forcing
allowing rainfall rates to reach 0.50-0.75" per hour. Recent
rainfall events over the last week have primed soils and the area
is running 150-300 percent above normal for rainfall.

Total rainfall amounts of 2-4" (locally higher amounts up to 6")
will be possible through 12z over the Olympic Peninsula and the
northern Cascades of WA. Lesser amounts are expected farther south
across the coastal ranges of southwest WA and southern WA
Cascades.

Taylor

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

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

LAT...LON   48902182 48452139 48002131 47582140 47002151
            46502166 46142181 46002219 45892277 46092329
            46392395 46792417 48222491 48452461 48152372
            47812342 47302345 46902328 46752283 47252215
            47762206 48462225