Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
321 AM EST Tue Dec 9 2025
Day 1
Valid 12Z Tue Dec 09 2025 - 12Z Wed Dec 10 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
PACIFIC NORTHWEST...

...Western Oregon and Washington...

The atmospheric river (A.R.) event is well underway across the
Pacific Northwest this morning. The plume of anomalous moisture
associated with the A.R. is currently aligned across northwestern
Oregon. Radar indicates rainfall rates are between 1/4 inch and 1/2
inch per hour. As is fairly typical of most A.R.s, the impact from
the rain isn't necessarily from the instantaneous rates from heavy
rain, but rather the long duration of steady rain, which can be
heavy at times on the windward (west-facing) sides of the coastal
ranges and the Cascades. A strong and vertically stacked
subtropical high is parked well off the California coast.
Meanwhile, frequent lows are tracking northeast across the
northeastern Pacific and slamming into the Canadian coast. The
trailing cold fronts are tapping into the plume of moisture that is
riding the gradient between the strong high to the south and the
lows to the north.

A Pacific low moved into the British Columbia coast earlier this
evening. This shoved the A.R. to its current position into
northwestern Oregon. As the next low tracks a little further north,
the subtropical high will begin ridging towards the north in
response. This will push the A.R. north with time. A moving A.R. is
usually a very good thing, as it reduces the time it's heavily
raining in any one area. Unfortunately this time, it will simply
move the A.R. back to the already hard hit areas of western
Washington that were hard hit yesterday. This northward shift is
expected this evening as the low moves ashore near Haida Gwaii, or
just south of the southern Alaska Panhandle. As the trailing front
follows into the Washington coast tonight, rainfall rates will
increase significantly as the A.R. aligns into Washington. This
will allow the rain rates to diminish substantially in most of
Oregon, though light rain near the Columbia River is likely to
continue to some much lesser degree. The A.R. is characterized by
PWATs to 1.25 inches, which is between 2 and 3 sigma above normal
for this time of year. Orthogonal westerly flow into the mountains
at 40-50 kt, which is plenty strong enough for substantial upslope
enhancement.

In Oregon, expect the heavy rain, ongoing at present, to continue
for much of the day, while gradually shifting north and weakening.
In Washington the heaviest rain will be overnight tonight. The
Slight Risk area was expanded north with this update to include all
of the Olympic Peninsula as well as for a portion of the foothills
of the Cascades east of Seattle. The surrounding Marginal was
trimmed on the southern end in central Oregon as the trend
throughout the period will be a northward readjustment of the A.R.

...Northern Idaho and Adjacent Areas...

The Marginal Risk into northern Idaho, a small sliver of far
western Montana, and the northeast corner of Oregon is largely
unchanged with this update. Over the last 24 hours, the area has
seen anywhere from 1-1.5 inches of rain. At least that much is
expected over many of the same areas of the Sawtooths, Bitterroots,
and west into northeastern Oregon. Since the A.R. is originating
from the tropics, in addition to the anomalous moisture, much
warmer air will also accompany the rain, causing snow levels to
rise. Expect a significant snowmelt contribution to the rising
streams and rivers as a result. A Slight Risk upgrade may become
necessary if rainfall forecasts increase further through the day.
NASA Sport soil moisture analyses show that soils all across
northern Idaho are at saturation, so all new rainfall today will
convert to runoff. The impact of this runoff is unclear, hence the
possibility of a Slight Risk upgrade.

Wegman

Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt