548
AWUS01 KWNH 092112
FFGMPD
ORZ000-WAZ000-100910-

Mesoscale Precipitation Discussion 1259
NWS Weather Prediction Center College Park MD
411 PM EST Tue Dec 09 2025

Areas affected...Northern Oregon and Western Washington

Concerning...Heavy rainfall...Flash flooding possible

Valid 092110Z - 100910Z

Summary...The ongoing atmospheric river event over the Pacific
Northwest will continue into tonight over portions of western
Oregon and Washington. The axis of heaviest rainfall is expected
to shift back north into Washington and also increase in
intensity, which will bring an increasing flood and landslide risk
to these areas.

Discussion...The expectation is that the atmospheric river
impacting northern OR will shift northward back into western WA
this evening. After a temporary drop in Integrated Water Vapor
Transport (IVT) today, values are forecast to increase again
towards 03z resulting in an uptick in rainfall coverage and
intensity into the overnight hours. With no forecast instability,
rainfall rates will be driven mostly by the magnitude of moisture
transport into the terrain. This will put a cap on the upper bound
of rainfall rates, although this will be compensated by IVT values
above the climatological 99th percentile and layered precipitable
water (PW) generally above the 95th percentile at all atmospheric
levels. This deep moisture and strong moisture transport will
support hourly rainfall in the 0.25"-0.40" range, potentially
approaching 0.50" in the highest mountain peaks. These rates
should increase in coverage after 03z (per the 12z HREF
probabilities), moving from northern OR into western WA overnight.
Additional rainfall through 09z Wednesday is generally expected to
be in the 1-3" range.

Under typical conditions, rainfall rates and totals of this
magnitude would not pose a significant hazard to this region.
However, the past 24 hours have already brought 4-10" of rain to
these locations, leading to saturated ground and elevated streams
and rivers. Although parts of WA saw a brief lull today allowing
for some recovery, hydrologic conditions remain sensitive.
Consequently, when heavier rain returns this evening into tonight,
additional flood impacts are expected.

The primary impacts are expected to be flooding of rivers and
streams, other low-lying flood-prone areas, and poor drainage and
urban areas. Given the saturated ground, the risk of landslides
and debris flows in areas of steep terrain will also increase
tonight as rainfall intensity ramps up.

Chenard

...Please see www.wpc.ncep.noaa.gov for graphic product...

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

ATTN...RFC...PTR...NWC...

LAT...LON   47942123 47642074 46792112 46432130 45912145
            45412151 45322145 44952164 44572184 44412215
            44522255 44712277 45042250 45422224 45882270
            45822308 45252331 44842354 44972399 45782413
            46532407 47232417 47682412 47842384 47782333
            47392322 47042294 46852251 47122208 47482197
            47762182