401
FOUS30 KWBC 122026
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
326 PM 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...

At the start of Day 2, a strong frontal band is forecast to cross
the coastal ranges of central and northern CA, Central Valley, and
portions of the Sierra Nevada foothills. Hi-res guidance continues
to advertise heavy showers and potentially a few thunderstorms
containing 0.50" to 1.0"/hour rates along and ahead of the front,
amid 800 to 1000 kg/m/s IVT values in the core of the atmospheric
river. Gradually, a series of shortwaves pivoting around the main
low center will result in the trough digging southward and slowing
down as it is cut off from the northern stream.

Overall rainfall forecast remains fairly steady with this update
cycle, where 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. Accordingly, the Slight
Risk area remains unchanged for the Bay Area, eastern portions of
the Central Valley, and Sierra Nevada foothills. The experimental
WPC Urban Rain Rate Dashboard reflecting the 12Z HREF ensemble
suite maintains 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
tomorrow afternoon 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.

Asherman/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 gradually decrease in
association with the closed low moving toward central and northern
CA. While uncertainty remains with the evolution of the vertically
stacked low and subsequent heavy rain threat, a general consensus
still supports a resurgance of IVT values into the 500-800 kg/m/s
range Saturday afternoon/night. Thus, 2 to 3+" rainfall totals are
still on deck for the coastal ranges from Santa Barbara, Los
Angeles, and Organge Counties. Accordingly, a Slight Risk has been
maintainted with minimal changes owing to concerns for scattered burn
scar and urban flooding.

Asherman/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