FOUS30 KWBC 020021
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
721 PM EST Thu Jan 1 2026

Day 1
Valid 01Z Fri Jan 02 2026 - 12Z Fri Jan 02 2026

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

The upper low associated with the heavy rain that fell earlier
across portions of California is now inland, centered over the
Sierra Nevada. In its wake, mesoanalysis shows a shortwave ridge
moving onshore, which recent runs of the RAP indicate will continue
to build and remain centered over the state through the remainder
of the period. Precipitation coverage has been on the decrease
across the state for the past few hours and apart from a few
isolated showers, estimated rates are under 0.25 in/hr. Isolated to
scattered showers are forecast to continue into the overnight;
however, the consensus of the guidance indicates that apart from a
few localized areas, most likely centered over the northern
Sacramento Valley and adjacent terrain, additional accumulations
will be under 0.5 inch.

Therefore, the previous outlook areas, including the Slight Risk
for Southern California, were removed.

Pereira

Day 2
Valid 12Z Fri Jan 02 2026 - 12Z Sat Jan 03 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA...

...19z Update...
Only minor changes made to the inherited Marginal risk area as the
forecast remains on track.

Chenard

...Previous Discussion...

An expansive upper level cutoff low over the Pacific well west of
the northern California coast will approach the northern California
coast Friday night. The occluded front wrapping around a surface
low will be the primary forcing for heavy rainfall into northern
California Friday night. The occluded front will turn the flow a
bit more onshore (southeast before the front to south-southwest
behind it). This will allow the coastal ranges to upslope some of
that moisture, locally enhancing rainfall rates, particularly into
the northern Coastal Ranges and the Klamath Mountains. Some heavy
rain may impact the Bay area and the coastal ranges south from
there in the waning couple hours of the period, assuming the timing
of the front moving into the coast remains unchanged. IVT values
between 500 and 750 kg/ms are far from impressive, and moisture
anomalies rise to around 2 sigma above normal, a common occurrence
in this part of the world, especially the past few weeks. Thus,
with the rainfall event just getting going, coming in in the middle
of the night, and with less-than-impressive dynamics, the Marginal
Risk for much of the central and northern California coast remains
largely unchanged with this update, and the Marginal looks on
track.

Wegman

Day 3
Valid 12Z Sat Jan 03 2026 - 12Z Sun Jan 04 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
SOUTHERN CALIFORNIA...

...19z Update...
A Slight risk was added across Santa Barbara and Ventura counties
in southern CA with this update. A line of locally heavy rainfall
rates is expected to traverse this corridor between 12z Sat and
00z Sun along/ahead of a cold front. Forecast rainfall amounts have
trended up over this area, with totals of 1-3" now expected,
mostly falling in ~6 hours or less. The quick duration will be a
limiting factor for more significant flood impacts, but with some
weak instability possibly working into the front and low level flow
aligned orthogonal to the terrain...hourly rainfall over 0.5" is
probable on a localized basis. Given the saturated soil over this
area due to recent heavy rainfall, these rates may be high enough
to result in isolated to scattered flash flooding, along with
localized mudslides and debris flows.

Chenard

...Previous Discussion...

A series of upper level disturbances rotating around a large upper
level low will impact much of California and the West Coast this
weekend. With each upper level disturbance, there will be a
commensurate surface front that also makes its way ashore into
northern California. The result will be an almost non-stop rain,
especially for upslope areas on southerly flow, as well as for all
of the coastal ranges. That said, each disturbance will briefly
increase rainfall rates, with those rates decreasing back down in
between disturbances. Thus, Saturday's rain looks to be a long-
duration relatively light (for most areas) rain, rather than there
being big concerns on any one area seeing prolonged heavy rain.
Given the area's good ability to drain rainwater quickly, a
Marginal Risk remains in effect for northern California. The area
will continue to be monitored for a possible Slight Risk upgrade
with future updates.

Wegman


Day 1 threat area: https://www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: https://www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: https://www.wpc.ncep.noaa.gov/qpf/99epoints.txt