FOUS30 KWBC 010752
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
252 AM EST Wed Jan 1 2025

Day 1
Valid 12Z Wed Jan 01 2025 - 12Z Thu Jan 02 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL FOR SOUTHWEST=20
OREGON AND NORTHWEST CALIFORNIA...

The 00Z HREF guidance has come in with a somewhat wetter footprint
for today across areas of southwest Oregon and northwest California
as a new round of offshore shortwave energy and proximity of a=20
front helps to bring in a new round of moderate to heavy rainfall.=20
The GFS and ECMWF solutions suggest IVT values increasing to near=20
500 kg/m/s along the immediate coast from Curry County, Oregon down
through Del Norte and Humboldt Counties in northwest California.
Offshore CMORPH2/microwave data shows heavier rainfall rates=20
associated with this next modest surge in atmospheric river
activity that will be arriving, and the HREF guidance suggests=20
rainfall rates will generally peak in the 0.25" to 0.50"/hour range
with perhaps an occasional instance or two of 0.50"+/hour rates.=20
Going through 12Z/Thursday, the HREF consensus suggests some=20
24-hour rainfall amounts of 2 to 4 inches with a couple of spotty=20
5+ inch amounts possible, and especially for northwest California
where some of the higher IVT values will persist a bit longer. The
antecedent conditions are wet across the region, and these rains=20
may cause some isolated runoff concerns. As a result, a Marginal=20
Risk of excessive rainfall has been depicted for this area.

Orrison


Day 2
Valid 12Z Thu Jan 02 2025 - 12Z Fri Jan 03 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF=20
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

Multiple impulses of upper level energy racing along a 150 kt WSW=20
jet will move into the Oregon and California coast near the state=20
line during this Day 2/Thursday period. The first is a front that=20
will have already moved into the area tonight, and will diminish in=20
intensity and slowly shift north through the early morning. A second=20
impulse moves in Thursday afternoon, resulting in a much more=20
concentrated area of rain about 50 miles or so to either direction=20
of the state line. Finally, a third and the strongest impulse will=20
move in late Thursday night and Friday morning. Each of these 3=20
impulses of energy will cause rainfall rates to pick up on the=20
Oregon and far northern California coast. None of them are expected=20
to advect in much instability, so upslope will likely be the primary=20
forcing mechanism for rainfall. With nearly continuous rainfall,=20
albeit to varying intensity with each impulse over already very=20
saturated ground, runoff and small stream flooding will be
possible. Since the heavier/steadier rainfall will be ongoing at=20
the start of the period over northern California, and guidance has=20
been creeping up as far as total rainfall amounts go, the Marginal=20
Risk extends south into Humboldt County to account for continuing=20
rainfall and isolated runoff problems that may be ongoing from the
Day 1/Wednesday period.

Wegman


Day 3
Valid 12Z Fri Jan 03 2025 - 12Z Sat Jan 04 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR FAR=20
SOUTHWESTERN OREGON AND PORTIONS OF NORTHERN CALIFORNIA...

A strong shortwave and associated surface low will plow into the=20
coast of California and Oregon on Friday. Locally heavy rain with=20
rates around 1/2 inch per hour will be ongoing Friday morning along=20
the coast. The shortwave and low will move into the coast Friday=20
evening and through the overnight. This will shift the heaviest=20
rains both southward and inland, bringing northern portions of the=20
Sacramento Valley, which has been largely missed with the previous=20
days' rains into the potential for heavy rain and higher elevation=20
snow. The Marginal Risk inherited was left unchanged with this=20
issuance. The heaviest rains are likely to be Friday morning as the=20
aforementioned shortwave and surface low approach. Once they move=20
into the coast Friday evening and overnight, rainfall rates should=20
diminish quite a bit as the associated moisture moves inland. Any=20
flooding from the Day 2/Thursday period will likely continue into=20
Friday morning along the Oregon and California coasts...with the=20
flooding for the Sacramento Valley more likely Friday afternoon.

Wegman


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!-WVL2toOY_f6DpCmDYn4q231pvomp3QYT5Cwb9KIKkZZ=
tsY3DtJLC6pTvCvWBGomVZFy3FlLkIOfHQ260eZhIG5sCT8$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!-WVL2toOY_f6DpCmDYn4q231pvomp3QYT5Cwb9KIKkZZ=
tsY3DtJLC6pTvCvWBGomVZFy3FlLkIOfHQ260eZhkujEBcM$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!-WVL2toOY_f6DpCmDYn4q231pvomp3QYT5Cwb9KIKkZZ=
tsY3DtJLC6pTvCvWBGomVZFy3FlLkIOfHQ260eZhS3rTmnY$=20

=3D =3D =3D
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