524
AWUS01 KWNH 081653
FFGMPD
ORZ000-WAZ000-090400-

Mesoscale Precipitation Discussion 1258
NWS Weather Prediction Center College Park MD
1152 AM EST Mon Dec 08 2025

Areas affected...Western Washington...Far Northwest Oregon...

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

Valid 081700Z - 090400Z

SUMMARY...Very strong atmospheric river with persistent .25-.5"/hr
rates falling on increasingly saturated soils will result in high
run-off, swelling rivers and poor drainage/urban flooding but at
minimum further set the stage for expected additional rainfall
into the mid-week.

DISCUSSION...Regional RADAR mosaic is showing the leading
warm/moisture advection surge from the atmospheric river is
expanding across the Pacific Northwest and into the Washington
Cascades at this time.  CIRA LPW animation depicts a long fetch
core to the atmospheric river with a long tap back to the tropics
near the Dateline; though the strength of the polar stream with
150-170kt fairly zonal upper-level jet has resulted in very strong
deep layer flow  which has resulted in a fairly long/gradual
isentropic incline to deplete any potential of vertical
instability, the strength of moisture flux anomalies are pushing
the maximum climatological percentiles for this time of year. This
is generally confirmed by CIRA LPW percentiles in the 95-99th
range through all layers but most noted in the 700-500mb at this
time.

The total PWat values are at or above 1.25" and have come ashore
with the subtle but fast moving warm front denoted with wind shift
from south to southwest and increase to 40kts at surface, 50-60kts
near boundary layer and over 70kts at 700mb, this has lead to IVT
values very near 1000 kg/m/s analyzed just offshore while
remaining above 700 kg/m/s into the Cascades currently, but is
expected to rise as the effective warm sector moisture intersects
the terrain fairly orthogonally as well.  As a result, broad
.25-.33"/hr rates have been seen near the coastal areas so far and
will expand into W WA over the next few hours.  The nose of the
upper-jet remains north for a bit and height-falls on the western
side of the core will not start to dip southward until well after
00z as the cold front reaches the mouth of the Juan de Fuca Strait
around 00-01z.

The strong flux, upslope flow will allow for rates of .5"/hr
regularly through the mid to upper slopes of the Cascade Range as
the WAA has freezing heights above all but the highest peaks.  As
such upper slopes will see 3-5" totals by 03z with foothills
likely to be 2-4" and lower valleys less than 2"...with isolated
totals to 6-7"+ along the core of the AR in the southern WA
Cascade Range.  Coastal Ranges including the Olympics may see
4-6".   Overall, FFG values are high enough even to encompass all
but those highest extreme values being greater than 3.5/3hrs and
over 5"/6hrs.  However, NASA SPoRT LIS 0-40cm soil saturation
values are generally above 70-80% across which is generally a bit
above normal for the Coastal Range, but near 75-85th percentiles
for the Cascade Range; however, full saturation should occur
through the late afternoon, and as the main surge occurs toward
00z, much of the rainfall should convert to run-off.  This should
swell the rivers fairly quickly with compounding issues resulting
in riverine flooding (please refer to discussion and graphical
products from NW River Forecast Center and National Water Center
for further details).

Given this is the first surge of a prolonged AR event, the
potential for flash flooding is likely limited to the highest
slopes/reaches of the watersheds or some localized urban flooding
due to poor drainage.  With the saturation of the soils through
depth, there will be a slowly increasing potential for mudslides
especially near/downslope any recent burn scars.

Gallina

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

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

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

LAT...LON   49112107 48812060 48292030 47312040 46882082
            45752140 45492147 45512177 45902229 46102264
            46062298 45822333 45732387 46362422 47182433
            47832456 48222487 48462487 48292398 47902348
            47362331 46992302 46942246 47252212 47702194
            48252209 48572212 48962190 49092160