Mesoscale Precipitation Discussion 1005
NWS Weather Prediction Center College Park MD
600 PM EDT Thu Nov 01 2018

Areas affected...Western and Northern WA

Concerning...Heavy rainfall

Valid 012200Z - 021000Z

Summary...A modest atmospheric river event will ensue this
afternoon and overnight for western and northern Washington with
heavy rainfall likely.

Discussion...The latest GOES-WEST IR satellite imagery shows a
well-defined frontal system offshore the Pacific Northwest
associated with an upper trough which will advance east over the
next 6 to 12 hours and move inland across the Pacific Northwest by
tomorrow morning.

Already there is plenty of warm advection rainfall overspreading
areas of the Olympic Peninsula and up across the Cascades of
northern WA as a warm front approaches Vancouver Island. Rainfall
rates over the last couple of hours have been locally approaching
a 0.25 inch/hr, and these rates will gradually increase through
the late afternoon time frame and especially after 00Z as
strengthening low to mid level westerly flow enhances isentropic
lift and couples with improving orographic ascent for notably
heavier rainfall rates.

The latest blended-TPW product shows a very well-defined axis of
subtropical moisture focused up along and ahead of the cold front
offshore, with PWATs of as much as 1.25 to 1.5 inches. Already
there are PWATs as high as 1.3 to 1.4 pooling up across Vancouver
Island as per the GPS-derived data, and these values are about 2.5
standard deviations above normal.

The latest hires model consensus suggests the bulk of this
offshore moisture arriving overnight in the 06-12Z time frame as
the cold front begins moving inland and coincides with a peak
westerly low level jet of 40 to 50 kts. This will help drive
moisture flux anomalies in the 850/700 mb layer of 3 to 4 standard
deviations above normal. This enhanced moisture transport coupled
with the nose of a 130-150+kt 250 mb jet streak and strong
orographic forcing will result in enhanced rainfall rates during
this time frame which are likely to peak up between 0.50 to 0.75
inches/hr.

The 12Z/18Z HREF mean supports storm totals of as much as 3 to 6
inches with isolated heavier amounts going through 12Z on Friday.
The dominant focus of this heavy rainfall will be over the Olympic
Peninsula and also up over the Cascades of northern WA. Lesser
amounts of 1 to 3 inches are expected farther south across the
coastal ranges of southwest WA and the southern WA Cascades.

Orrison

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

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

LAT...LON   49052183 49032135 48652107 48022097 47302094
            46582119 46112177 46082236 46262287 46392377
            46632415 47032432 47922471 48252451 48162378
            47692321 47222311 47022278 47132213 47572204
            48142214 48822227