FOUS30 KWBC 231546
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1046 AM EST Tue Dec 23 2025

Day 1
Valid 16Z Tue Dec 23 2025 - 12Z Wed Dec 24 2025

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

...16Z Update...

Only minor tweaks were needed for the Slight and Marginal Risk
areas across much of California today. The atmospheric river (A.R.)
is making its long expected turn from westerly to south-
southwesterly. For now, that has resulted in the steadiest rain to
shift north to over far northwestern California and southwestern
Oregon. However, even here rates are generally below a half inch
per hour, so flooding concerns look to be minor for much of the
rest of the day.

A rapidly deepening low and its attendant cold front will approach
the California coast tonight. The low and its cold front will
greatly increase rainfall rates over much of California. To the
north rain will be associated with the low itself, as it tracks NNE
along the coast. To the south, the rain will be associated with the
trailing cold front. With the transverse ranges aligned generally
west-east, and the flow SSW to NNE, there will be a significant
orthogonal component to the mean flow which will further enhance
rainfall rates on the south facing slopes of the western Transverse
Ranges tonight, including the Santa Ynez and San Rafael ranges near
Santa Barbara. Late tonight, rain rates could approach an inch per
hour, which when combined with the numerous burn scars in the area,
will increase the flooding threat significantly. Thus, have opted
to highlight those ranges in a higher-end Slight Risk for this
update. A Moderate Risk may need to be considered in this region
for the evening update.

Wegman

...Previous Discussion...

The S/SW to N/NE orientated atmospheric river is expected to
maintain scattered to widespread rainfall along the coastline and
inland to the Sierra Nevada range with rain for the lower
elevations and snow for the higher elevations. Cyclogenesis will
occur along the greatest gradient/boundary between the cold air
mass within the trough and the much warmer air mass in the
ridge/atmospheric river. As the low deepens the onshore flow will
align parallel to the coastline thus significantly increasing the
areal extent heavy rainfall and the higher terrain snow
accumulations.

The areas of greatest impact will be into the
northern Coastal Ranges and the Transverse Ranges. Both these
ranges are sufficiently aligned perpendicular to the south-
southwesterly flow to have a significant local upslope contribution
to rainfall rates. In the Transverse Ranges, local burn scars will
also worsen impacts from resultant flooding over and downstream of
any burn scars north and west of the Los Angeles Basin. The
renewed round of heavy rain will be in areas getting hard hit by
the current atmospheric river and will continue with the deepening
low offshore into the northern coastal ranges. A Slight Risk was
maintained for the Oregon border down through Los Angeles along
with a Marginal Risk encompassing much of the same area.

Campbell/Wegman


Day 2
Valid 12Z Wed Dec 24 2025 - 12Z Thu Dec 25 2025

...THERE IS A HIGH RISK FOR EXCESSIVE RAINFALL ACROSS SOUTHERN
CALIFORNIA...

Highly anomalous 850-700 mb moisture flux on the order of 5+
standard deviations above the mean will yield extremely heavy
rainfall ahead of the amplifying mid/upper trough located offshore
of the California coast. The heavy rainfall totals for Southern CA
will likely occur over an 18 hour period, with widespread 4-7 inch
totals, with isolated max amounts of 9"+. Expect a well defined
south to north precip band in this anomalous moisture flux axis
with hourly rainfall rates of 0.50-1.5"+ likely. The potential for
high hourly rainfall totals will accentuate runoff over soil that
is relatively dry and or over recent burn scar areas. This will
pose a significant threat of flash flooding, landslides, rock falls
and mud slides. The greater than normal travel during the
Christmas Holiday will likely expose a potentially larger number of
people to these lift threatening hazards and was taken into
consideration with the upgrade to high risk.

Although snow levels will be lowering across the Sierra Nevada and
adjacent locations and much of the highest elevations will have
significant snow, some of the lower elevations could receive
additional rainfall of 1 to 4 inches, which is covered by a
sprawling Slight Risk area. Deep-layered moisture will also track
eastward across eastern California, southern Nevada and into
southern Utah and western Arizona where areal averages up to 1 to
2+ are forecast. A new Slight Risk area was raised for much of
southern Nevada, along the California/Nevada border and for western
Arizona. Rainfall rates will easily surpass 0.5-1.0 inch/hr rates,
particularly for California.

Campbell/Oravec


Day 3
Valid 12Z Thu Dec 25 2025 - 12Z Fri Dec 26 2025

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

The atmospheric river will continue to direct anomalous Pacific
moisture into California resulting in heavy rainfall across the
state. With the trough progressing inland, the Sierra Nevada Range
should see an significant/extended snow event in the higher
elevations with heavy rainfall expected in the lower elevations. A
Slight Risk area seems appropriate for this period given some of
the highest QPF forecast will be in the form of snow thus reducing
the threat for rapid runoff and flooding concerns. Southern
California can be considered on the higher end of Slight for this
period since it will be trying to recover from several inches of
rain expected for the Day 2 period.

Campbell


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