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FOUS30 KWBC 011503
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1003 AM EST Thu Jan 1 2026

Day 1
Valid 16Z Thu Jan 01 2026 - 12Z Fri Jan 02 2026

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

...16Z Outlook Update...
Changes to the Slight Risk area in southern California currently
reflect convective trends, wtih deep convection persisting along
and ahead of a surface cold front over Pacific waters approaching
that area. Mesoanalyses depict a pool of weak surface-based=20
instability just offshore of San Diego County that has enabled=20
deeper convection to materialize and repeat/train into portions of=20
San Diego Metro. This regime, along with persistent orographic lift
against the Transverse and Peninsular coastal ranges, will=20
continue to support flash flooding in addition to mud/rock slides=20
and occassional near-stream flooding for at least another 6 hours=20
or so. The Slight Risk has been expanded southward to the=20
international border and trimmed on its westward extent across=20
Santa Barbara and Ventura Counties based on aforementioned trends.=20
The Marginal Risk areas remain in tact as a large upper low=20
approaches the state of California, spreading areas of moderate to=20
heavy rainfall especially across coastal/Sierra front ranges=20
through the evening.

See the previous discussion for additional information.

Cook

...Previous Discussion...

A 500mb trough with a small, yet potent, closed circulation will
become part of a negatively tilted 200-500mb upper-trough axis
that is partially responsible for the slug of highly anomalous
moisture that will engulf much of the West today. The focus remains
on Southern California where the Transverse Ranges soils remain
quite sensitive in wake of last week's significant flooding. IVT
values are encroaching upon 500 kg/m/s and embedded within SSWrly
flow that is aligned orthogonally to the Transverse Ranges.
Additional forcing will be located along the frontal boundary that
will trigger additional convection over the East Pacific upon
approach. PWATs are forecast to be just over 1.25" off the Southern
California coast through this morning, but around 18Z, a dryslot
in the 700-300mb layer should result in the conclusion of today's
heaviest rainfall. Some lingering heavy rainfall may be possible in
the Peninsular Range east of San Diego beyond 18Z, but most
rainfall around the L.A. region should be just about over (aside
from lingering upslope into the Transverse Ranges).

Latest HREF guidance does show minor instability present (<250
J/kg on average) between 09-18Z, which may be just enough when
taking into account sensitive soils to result in additional flash
flooding in not just the Transverse Ranges, but also the greater
Los Angeles metro area as well. The new 00Z HREF 24-hour QPF
probabilities for surpassing 3" of rainfall peaks around 50% over
the San Bernadino and low chances (15-25) for localized totals
eclipsing 5". Snow levels remain very high, making just about all
of the expected QPF to be in the form of rain. The lack of higher-
end instability is what is keeping the Slight Risk in place, but
make no mistake-- there remains a heightened risk for flash
flooding this morning in Southern CA with rock slides and debris
flows in/around burn scars possible, including urbanized flash
flooding.

Farther north, most area have recovered a bit better from last
week's significant rainfall, but the copious amounts of moisture
combined with sufficient synopitc-scale forcing aloft and
supportive upslope flow along the Coastal Range and as far inland
as the Sierra Nevada does suggest a Marginal Risk for flash
flooding is still warranted. The 00Z HREF does show a small area
between Santa Maria and Big Sur where MUCAPE >250 J/kg could
materialize between 15-18Z and foster more potent instantaneous
rainfall rates as convection moves through. Still, the line of
showers and thunderstorms would be progressive and keep the flash
flooding potential to a very localized area.

Mullinax

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...

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 MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CALIFORNIA...

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://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!5ohuCikjrKAwZAFRpsatOeIB9cgsRlzzU_30OLjzZccq=
oyu_Bb9OjVOeaihELQFY74ilH-zYdxFic9EyYCLWlv8LMxQ$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!5ohuCikjrKAwZAFRpsatOeIB9cgsRlzzU_30OLjzZccq=
oyu_Bb9OjVOeaihELQFY74ilH-zYdxFic9EyYCLWaDaGg68$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!5ohuCikjrKAwZAFRpsatOeIB9cgsRlzzU_30OLjzZccq=
oyu_Bb9OjVOeaihELQFY74ilH-zYdxFic9EyYCLWp8TtQ8I$=20

=3D =3D =3D
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