FOUS30 KWBC 011546
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1046 AM EST Wed Jan 1 2025

Day 1
Valid 16Z Wed Jan 01 2025 - 12Z Thu Jan 02 2025

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

..16Z Outlook Update...
Current observational/model trends support continued, yet spotty=20
areas of ~0.25 inch/hr rain rates at times within the Marginal Risk.
These rain rates are falling on wet soils from recent rainfall=20
events, and could prompt minor/localized excessive runoff and flood
issues. The bulk of the heavier rainfall should persist through=20
about 03Z today before tapering off thereafter. Rainfall totals of
1-2 inches (locally higher) are possible. These trends continue to
support a Marginal Risk, and no changes are made to the previous=20
outlook. See the discussion below for more information.

Cook

...Previous Discussion...

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
front helps to bring in a new round of moderate to heavy rainfall.
The GFS and ECMWF solutions suggest IVT values increasing to near
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
associated with this next modest surge in atmospheric river
activity that will be arriving, and the HREF guidance suggests
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.
Going through 12Z/Thursday, the HREF consensus suggests some
24-hour rainfall amounts of 2 to 4 inches with a couple of spotty
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
may cause some isolated runoff concerns. As a result, a Marginal
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
SOUTHWESTERN OREGON AND NORTHWESTERN CALIFORNIA...

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

A strong shortwave and associated surface low will plow into the
coast of California and Oregon on Friday. Locally heavy rain with
rates around 1/2 inch per hour will be ongoing Friday morning along
the coast. The shortwave and low will move into the coast Friday
evening and through the overnight. This will shift the heaviest
rains both southward and inland, bringing northern portions of the
Sacramento Valley, which has been largely missed with the previous
days' rains into the potential for heavy rain and higher elevation
snow. The Marginal Risk inherited was left unchanged with this
issuance. The heaviest rains are likely to be Friday morning as the
aforementioned shortwave and surface low approach. Once they move
into the coast Friday evening and overnight, rainfall rates should
diminish quite a bit as the associated moisture moves inland. Any
flooding from the Day 2/Thursday period will likely continue into
Friday morning along the Oregon and California coasts...with the
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!9yL5Nn3Ix453KcAe2wTcqkAyuGUxeEkYAUImcd06P8Sa=
DJ52Ty3Rcl_0B0qP_LU7aK1TIl7MnGsKJEmfkq6sSyBbODo$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!9yL5Nn3Ix453KcAe2wTcqkAyuGUxeEkYAUImcd06P8Sa=
DJ52Ty3Rcl_0B0qP_LU7aK1TIl7MnGsKJEmfkq6sItD9tPc$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!9yL5Nn3Ix453KcAe2wTcqkAyuGUxeEkYAUImcd06P8Sa=
DJ52Ty3Rcl_0B0qP_LU7aK1TIl7MnGsKJEmfkq6sl-mBWOY$=20

=3D =3D =3D
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