AWUS01 KWNH 181400
FFGMPD
ORZ000-WAZ000-190200-

Mesoscale Precipitation Discussion 1265
NWS Weather Prediction Center College Park MD
859 AM EST Thu Dec 18 2025

Areas affected...Northwest Oregon...Far Southwest Washington...

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

Valid 181400Z - 190200Z

SUMMARY...Extreme surge of moisture flux into terrain as warm
front passes will result in mid to upper slopes of coastal and
Cascade Ranges to receive up to 1"/hr rates. Localized 5-7"
totals are possible and may result in rapid rises and possible
flooding in/along streams/rivers.

DISCUSSION...GOES-W WV suite depicts a strong well
anti-cyclonically curved ridge continue to dominate the eastern
north Pacific with strong core of 150-180kt westerly 250 jet
crossing the Juan de Fuca Strait attm. At the trailing edge
increased divergence and subtle sheared shortwave is streaking
through just west of 130W. This will focus surface to boundary
layer cyclogenesis with a low 990mb low expected to lift northward
toward the Strait by 21z. The energy through the atmospheric
column is highly dynamic along and behind a surging warm front.
This warm front has yet to reach the Pacific Coast, but is
expected around 16-17z from south to north along the Oregon coast.
Weaker southerly flow will quickly shift with 925mb flow swinging
WSW to align through nearly the entire column to 500mb. Winds
will increase to over 60kts with occasional speeds up to 70kt.

CIRA LPW denotes the WAA aloft as just recently reached the coast
though the 850-700mb layer with core of 99th and Maximum record
values in this an 700-500mb layers (displaced further east over
central OR attm due to the gentle slope of the deeper AR as a
whole). As the warm front passes the total PWats will be over
1.25" and fluxed on that strength of wind (50-80kts from
925-700mb) will result in IVT values over 1000 kg/m/s with higher
resolution guidance suggesting localized peaks nearing 1200 kg/m/s
placing the surge toward the extreme range. While orientation to
the terrain is not fully orthogonal, the magnitude of flux
convergence and orographic ascent will support .75"/hr rates in
the mid to upper slopes of the central to northwest Oregon Coastal
Range and perhaps an hour later into the upper slopes of the
Cascades north of Lane county. These rates are expected to surge
and ebb through 00z, with an isolated 1"/hr rate possible. As
such, rainfall totals will quickly total toward 5"+ in the terrain
with rain-shadowed areas perhaps not even reaching .5". Early
arriving 12z Hi-Res CAMs suggest isolated totals over 7" by 03z
are not out of the realm of possibility. These values are pushing
24hr ARI (Average Return Intervals) definitely into the 25 year
range, with some suggestions of exceedance of 50 to 100 year
values, in the upper slopes of the central Oregon Cascades. So
the rainfall is highly atypical and therefore flooding is possible.

FFG values in the region are static and are not likely to be
exceeded at 1 or 3hr periods but may push the 6hr+ time range.
Yet, NASA SPoRT LIS 0-40cm soil saturation remain above 70% given
the connection to last week's event that was focused further
north, and may result in above average run-off as upper profiles
will quickly saturate. As such, even the glancing blow across into
W WA could exacerbate ongoing flooding there and has been included
in the area of concern, though rates/totals would be reduced to
the core in central to northern Oregon. So any flooding is not
necessarily going to be flashy in nature but notable surge in
river levels will be noted. As such, please continue to monitor
River Forecasts from local forecast offices, Northwest River
Forecast Center, and products/discussion from the National Water Center.

Gallina

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

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

LAT...LON 46812171 46592138 46062138 44512157 43922193
43642242 43972308 44012371 44032419 44232423
44802415 45412406 46042402 46202370 46122328
45572281 45862238 46232230 46612213

$$
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