FOUS30 KWBC 030830
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
330 AM EST Fri Jan 3 2025

Day 1
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 long duration atmospheric river has resulted in 2-4" rainfall
totals over portions of far northwestern CA over the past several
days, with 1-3" occurring over the past 48 hours (extending into
portions of far southwestern OR as well). A final push of low-level
moisture transport is beginning to reach the coastline early this
morning, as a potent shortwave and associated surface cyclone
brings the most significant source of uplift yet (as well as
elevated instability with the HREF indicating MU CAPE of 250-500
J/kg in association with height falls aloft). The shortwave and 
low will move into the coast this evening and through the 
overnight shifting, shifting the heaviest rainfall both southward 
and inland with time (bringing northern portions of the Sacramento
Valley, which has been largely missed with the previous days' 
rainfall, into the Marginal risk for excessive rainfall). Areas
most at-risk for rainfall rates exceeding 0.25"/hr (which can
become problematic for particularly sensitive terrain, such as 
burn scars) are along the upslope portions of the Coast Range,
Klamath Mountains, Cascade Range, into the northern Sierra Nevada
(per HREF Ensemble Agreement Scale, 10-100 km, neighborhood
probabilities of 60-90% for 1-hr, 0.25" exceedance). Additional
24-hr totals of 1-3" are expected (with a slight majority of the 
rainfall occurring prior to 00z). 

Churchill


Day 2
Valid 12Z Sat Jan 04 2025 - 12Z Sun Jan 05 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Churchill


Day 3
Valid 12Z Sun Jan 05 2025 - 12Z Mon Jan 06 2025

...THERE IS A MARGINAL RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST AND CENTRAL GULF COAST...

An impressive deep layer cyclone is anticipated to develop over 
the central US into Day 3 (originating from the aforementioned 
shortwave impacting the Pacific Northwest on Day 1), as strong DPVA
via a digging shortwave (cutting off into an upper low) interacts
with the right-entrance region of a broad phased jet structure
(~130 kt jet streak @ 250 mb over the Mid-Atlantic). A substantial
low-level jet (850 mb of 40-50 kts) is expected to form as a
result, ushering in highly anomalous (for this time of year)
precipitable water values nearing 1.5" (near the 90th percentile to
max moving average, per JAN/BNA/BMX sounding climatology). While 
downscaled global guidance (GFS/ECMWF) suggests only localized 1-2"
totals at this stage, the impressive dynamics of this system 
(along with the anomalous moisture) suggest the potential for this
guidance to be exceeded (as hi-res CAMs will shed a light on as 
the event gets closer in range). That said, both the progressive 
nature of the system and a distinct lack of instability should be 
limiting factors for organized heavy rainfall. Maintained an 
inherited Marginal risk area for much of the Southeast (with some 
expansion towards the central Gulf Coast, where instability will be
more abundant) encompassing the expected warm sector of the system
(while excluding areas farther north where more impressive broad 
QPF exists, as this precipitation will mostly occur in the form of 
ice and snow). 

Churchill

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