FOUS30 KWBC 031537
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1037 AM EST Fri Jan 3 2025

Day 1
Valid 16Z 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...

...16z update...
Current observational trends show the core of Atmospheric=20
River/warm conveyor belt has pressed ashore across northwest=20
California and will continue a steady southeastward trend through
the next 6-12 hours, eventually weakening and diminishing across
the southern Sierra Nevada Range toward 06z tonight. Behind cold
front, continued modest onshore flow and steepening lapse rates may
allow for modest instability and more scattered in nature shallow
convective cells. Rates may locally increase with those cells but
will be short-duration and limited in overall coverage. All in all,
an additional 1-2" are likely across the favored terrain, with
highest totals across the lower slopes of the northern Sierra
Nevada Range due to longest duration to remain over the next 6 or
so hours. As such, limited changes have made to the Marginal Risk
area.

Gallina

---Prior Discussion---

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


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!9gXgc0WYHrTYGYq6vjSKzaYIiecj1CuNnvsTzxUvo_qv=
uP_gVuem-SPKAjZ5Vu0z3jpb2r4sRrDBqRgqEdohT6joqPs$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!9gXgc0WYHrTYGYq6vjSKzaYIiecj1CuNnvsTzxUvo_qv=
uP_gVuem-SPKAjZ5Vu0z3jpb2r4sRrDBqRgqEdohcOubOR8$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!9gXgc0WYHrTYGYq6vjSKzaYIiecj1CuNnvsTzxUvo_qv=
uP_gVuem-SPKAjZ5Vu0z3jpb2r4sRrDBqRgqEdohJdtodq8$=20

=3D =3D =3D
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