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 
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 
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
$$
--- SBBSecho 3.20-Linux
 * Origin: capitolcityonline.net * Telnet/SSH:2022/HTTP (618:250/1)