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FOUS30 KWBC 041938
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
238 PM EST Sun Jan 4 2026

Day 1
Valid 16Z Sun Jan 04 2026 - 12Z Mon Jan 05 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SIERRA AND FOOTHILLS...

16z Update: Locally heavy rainfall rates into coastal Monterey
County will continue to result in an isolated flash flood and
mudslide risk through ~00z. Hourly rainfall between 0.5"-1" will
continue to locally occur, with a slow southward shift expected in
these heavier rainfall rates through the day. Rainfall totals of
2-4" are expected along the coastal portion of Monterey County.

Another focus for heavy rainfall will be across the eastern valley
into the foothills of the central Sierra. Southerly flow up the
valley, combined with westerly flow behind an approaching cold
front, will locally enhance convergence and support some periodic
low topped convection capable of rainfall rates over 0.5"/hr. The
combination of upslope enhanced rainfall and these locally higher
rates will result in a continued isolated to scattered flash flood
risk through today.

Showers into southern CA through today may periodically drop ~0.5"
of rain within an hour...but the coverage and magnitude of rainfall
is not expected to match what occurred yesterday over Santa
Barbara and Ventura counties. By tonight the approaching cold
front should increase shower coverage, but again hourly rainfall
should generally peak around 0.5" or less. HREF FFG exceedance
probabilities stay below 15%, and while there is a risk of
isolated flash flooding and mudslides, the magnitude of the threat
is lower than yesterday and also mainly focused farther south.

Chenard

...Previous Discussion...

Precipitation is expected to increase in coverage and intensity
Sunday morning across northern to central California as the next
batch of shortwave energy rotates through the mean west coast trof
and inland across central CA. PW values and 850-700 mb moisture
flux are expected to increase Sunday associated with this next
batch of shortwave energy. This will support locally heavy precip
totals along much of the California coast Ranges and into the
upslope of the Sierra. Hourly rainfall rates in the .25-.50" range
possible along the central CA coast and into the upslope of the
northern Sierra. A small slight risk area was maintained in the
upslope of the northern Sierra where model consensus is for
potential of additional 1 to 2" of rain. The HREF probabilities of
3 hr precip exceeding 3 hr FFG values shows an area of 15-30%
probabilities sinking southward along the upslope of the northern
Sierra Sunday where the slight risk area is drawn. Overall, no
changes of note made to the previous outlook for the upcoming day 1
period.


Oravec

Day 2
Valid 12Z Mon Jan 05 2026 - 12Z Tue Jan 06 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ALONG THE CENTRAL
TO NORTHERN CALIFORNIA COAST AND NORTHERN CALIFORNIA...

19Z Update: Our model preference is shifting towards the closer to
the coast low track and thus wetter scenario of the
ECMWF/GEM/UKMET/AIFS. Still not a high confidence forecast, but=20
this scenario seems to have more support than the farther offshore
GFS/NAM solution. The upward trend in QPF supports a solid=20
Marginal risk of excessive rainfall, with amounts of 1-3" and some=20
flood impacts likely across central/northern CA. We gave some=20
consideration to a Slight risk upgrade, but it remains unclear=20
whether rainfall rates will get high enough for that level of flash
flood risk. So for now we will keep things at the Marginal level=20
and continue to monitor. It's worth noting that most 12z HREF=20
members appear too far offshore with the closed low, with this poor
synoptic scale forecast negatively impacting their QPF forecast.=20
Thus we were not really able to rely on the high res hourly=20
rainfall forecasts this cycle...so hopefully the 00z HREF will=20
trend more in line with the global model consensus, which should in
turn result in more useful hourly rainfall rate data to diagnose=20
the flash flood risk.

Chenard

...Previous Discussion...

There are large scale differences developing day 2 with the
amplification of a closed low in the base of the mean trof off the
Central to Southern CA coast. The NAM and GFS are showing a more
southwest solution compared to the EC, CMC and UKMET. This farther
southwest solution is resulting in a trend of less precip along
the central CA coast to the south of San Francisco and into the
Northern Sierra in the GFS and NAM as the low level flow has a
much weaker onshore component compared to the EC, CMC and UKMET.
Models often struggle with closed formation downstream of
flattening upper ridges with overall low confidence at the moment.
The latest WPC qpf did trend towards less precip across the central
CA Coast Range and over the Northern Sierra, but not as dry as the
GFS and NAM. Subsequently, the previous marginal risk area was
decreased in size. There may be additional changes needed to the
risk area with future model cycles, depending how the evolution of
the closing off mid to upper level center evolves.

Oravec


Day 3
Valid 12Z Tue Jan 06 2026 - 12Z Wed Jan 07 2026

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

Chenard


Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!9mKkEMKK1V8M4zUKSgH6brhInGUvJrsfJNRgyc6yrHdB=
hmT5qpXhBzD7_HSqkR5_oeRRZ59xDmlLnO6pQklnG0pkbzk$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!9mKkEMKK1V8M4zUKSgH6brhInGUvJrsfJNRgyc6yrHdB=
hmT5qpXhBzD7_HSqkR5_oeRRZ59xDmlLnO6pQklnRFHk8RU$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!9mKkEMKK1V8M4zUKSgH6brhInGUvJrsfJNRgyc6yrHdB=
hmT5qpXhBzD7_HSqkR5_oeRRZ59xDmlLnO6pQkln4TO8T2E$=20

=3D =3D =3D
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