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QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
229 PM EST Mon Dec 22 2025

Day 1
Valid 16Z Mon Dec 22 2025 - 12Z Tue Dec 23 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL IN EFFECT FOR
PORTIONS OF CENTRAL AND NORTHERN CALIFORNIA...

...16Z Update...

The atmospheric river impacting California is in a bit of a lull
today. IVT values, a measure of the moisture advection from off the
Pacific, will be dropping rapidly through the day. This means that
the wind will be less and less cooperative with advecting moisture
into the Sierra Nevada, resulting in diminishing upslope
enhancement of the rain. That said, the plume of moisture moving up
I-80 from the Bay Area into the central Sierra Nevada range will
trend north as a digging trough in the upper levels realigns the
flow to more out of the SSW. This too will gradually diminish the
rainfall rates in the hard-hit upslope regions.

As the plume of rain drifts north this afternoon, expect a renewed
round of rain into the hardest hit portions of the Sierra Nevada
from Oroville east. This is likely to cause additional or renewed flooding...however the diminishing rainfall rates should keep
impacts below Moderate Risk criteria. The rain will continue north
into the northern Sacramento Valley by tonight due to the
increasingly southerly flow. While soils are also saturated from
recent rainfall in this area, since the predominant flow prior to
today was westerly, it should be different slopes of the northern
coast ranges and the Klamath Mountains that get the heaviest rain
today, and even then, due to the diminishing pressure gradient,
should not be anywhere near as heavy as prior days overnight tonight.

Given the transient nature of the rain plume today, the Moderate
Risk has been downgraded to a Slight, with no changes to that
Slight or the surrounding Marginal. Impacts from the rain that does
fall today could still include land slides, mud slides, and flash
flooding in smaller creeks that have had some time to drain a bit
this morning.

Wegman


Day 2
Valid 12Z Tue Dec 23 2025 - 12Z Wed Dec 24 2025

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

2100 UTC update: No changes made to the previous

Previous discussion...

The trajectory of the atmospheric river will become more S/SW to
N/NE as it shifts southward during this period as the trough
offshore in the Pacific also digs to the south. In contrast to
the air mass containing the atmospheric river, which will be warm
and moisture-laden, this contrast will support cyclogenesis 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 onshore
flow associated with the low itself will continue very heavy rain
and mountain snow across northern California, while the trailing
cold front moves into southern California starting Tuesday night.

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.

Campbell/Wegman


Day 3
Valid 12Z Wed Dec 24 2025 - 12Z Thu Dec 25 2025

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

2100 UTC UPDATE:

The primary change to the previous day 3 Excessive Rainfall Outlook
was to add a high risk area along the west to east Transverse
Range region of Southern California after collaboration with WFO
LOX. Anomalous 850-700 mb moisture flux...5+ standard deviations
above the mean...will impact Southern CA late day 2 into day 3
ahead of the amplifying mid to upper level trof off the CA 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
.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.

Oravec

Previous discussion...Widespread heavy rainfall will ramp up even
further across the Transverse Range and surrounding locations of
southern California. Areal average of 3 to 5 inches are expected
but isolated local maximums may reach as much as 9 inches. A
Moderate Risk is in effect for a majority of Santa Barbara,
Ventura, Los Angeles, range, San Bernardino, Riverside and San
Diego Counties. 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
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