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FOUS30 KWBC 121526
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1026 AM EST Wed Nov 12 2025

Day 1
Valid 16Z Wed Nov 12 2025 - 12Z Thu Nov 13 2025

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

...16Z Update...
Forecast thinking remains unchanged from the overnight cycle, with
no changes made to the Marginal Risk area.

Asherman

...Previous Discussion...

A digging upper-level trough will approach the West Coast late
today through tonight. Deepening surface low pressure offshore of
northern CA will drive a strong frontal band and axis of enhanced
IVT into the coastal ranges of northwest CA. A southerly low-level
jet of 50 to 70 kts is progged by a consensus of the latest
guidance to nose inland near and to the north of the Bay Area
including many of the coastal ranges extending as far north as
Humboldt County. The GFS and ECMWF support IVT magnitudes reaching
800 to 1000 kg/m/s along and just ahead of the front, and the
latest hires guidance suggests some 2 to 4 inch rainfall totals for
the more orographically favored upslope areas by 12Z/Thursday. This
will include some potential for rainfall rates to reach upwards of
0.50" to 0.75"/hour as potential will also exist for some embedded
convection to arrive with the front. Some localized runoff
problems including potential for some urban flooding near and just
north of the Bay Area will be possible by early Thursday morning.
The Marginal Risk area was tweaked to focus on northwest CA where
the axis of stronger IVT and orographic support for heavier
rainfall totals should be.

Orrison


Day 2
Valid 12Z Thu Nov 13 2025 - 12Z Fri Nov 14 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN AND CENTRAL CALIFORNIA INCLUDING THE BAY AREA AND PARTS
OF THE CENTRAL VALLEY AND SIERRA NEVADA...

A strong frontal band and associated mid-level trough will cross
the coastal ranges of central and northern CA this period and
advance into the Central Valley and portions of the Sierra Nevada
foothills. Heavy showers and potentially a few thunderstorms will
be accompanying the front as it moves inland, and the latest hires
guidance suggests bands convection with rainfall rates peaking into
the 0.50" to 1.0"/hour range which will be aided by the nose of 800
to 1000 kg/m/s IVT values as strong low to mid-level southerly
flow punches inland with the arrival of the height falls. Gradually
the bulk of the shortwave energy/forcing by later in the period
will begin to lose latitude as the latest guidance supports the
bulk of the offshore upper trough slowing down and digging farther
south. Regardless, there will be heavy rainfall overrunning the
coastal ranges and impacting large areas of the Central Valley and
the northern Sierra Nevada.

The more orographically favored areas may see as much as 2 to 4
inches of rain, with some spotty 5 inch amounts not out of the
question. Generally 1 to 3 inches is expected for the lower
elevations including the interior valleys. A Slight Risk area has
been introduced for the Bay Area and also eastern portions of the
Central Valley into the Sierra Nevada foothills. The experimental
WPC Urban Rain Rate Dashboard reflecting the 00Z HREF ensemble
suite suggests elevated probabilities of the 1 and 3-hour QPFs
exceeding the 2 to 5-year ARIs Thursday morning around the San
Francisco and San Jose areas, and even inland around Sacramento as
the aforementioned bands of convection cross through the region.
Small stream and urban flooding will the primary concern as a
result. A localized flash flood threat involving burn scar
locations cannot be ruled out as well.

Orrison


Day 3
Valid 12Z Fri Nov 14 2025 - 12Z Sat Nov 15 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR THE TRANSVERSE
RANGE OF SOUTHERN CALIFORNIA...

Heavy rainfall associated with a front slowing down and pivoting
across southern CA is expected this period, and this will drive a
threat for burn scar flash flooding and even some localized debris
flow/landslide potential for the southwest facing slopes of the
Transverse Range. IVT values will be gradually coming down with the
original advance of Pacific moisture along the front as it slowly
comes down the coast in association with the original height
falls/troughing moving into central and northern CA. However, the
models agree in allowing for a closed low evolution offshore of
southern CA by Friday night and early Saturday. Additional
shortwave energy rounding the base of this will be interacting with
the front, and should allow for at least some persistence of
locally heavy shower activity. However, an uptick in shower coverage
and intensity is expected at the end of the period as stronger IVT
values begin to resurge northeastward into southern CA from
offshore. There is model spread with the overall details of the
heavy rainfall threat, but a general consensus of the guidance
supports areas of 2 to 3+ inch rainfall totals impacting the coastal
ranges from Santa Barbara County down through at least Los Angeles
County and potentially Orange County. A Slight Risk of excessive
rainfall has been introduced to address the concerns for impacts
primarily to the burn scar locations as heavy rains arrive, but
there may also be some urban flooding concerns involving the
broader Los Angeles Basin.

Orrison


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