Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
232 PM EST Mon Nov 10 2025

Day 1
Valid 16Z Mon Nov 10 2025 - 12Z Tue Nov 11 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Gallina

Day 2
Valid 12Z Tue Nov 11 2025 - 12Z Wed Nov 12 2025

The probability of rainfall exceeding flash flood guidance is less
than 5 percent.

Gallina

Day 3
Valid 12Z Wed Nov 12 2025 - 12Z Thu Nov 13 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
NORTHERN CALIFORNIA AND FAR SOUTHWEST OREGON...

...20z update...
12z guidance suite shows a quite similar evolution and general
timing, if a touch slower, resulting in a slightly northward shift
to the overall forecast rainfall placement. As such, a few dozen 
mile northward expansion of the Marginal Risk across SW Oregon was 
made, though intersection of the moisture plume/IVT transport is
fairly oblique to the SW Oregon coastal ranges, as the deepening
triple point lifts northward.

Gallina


~~~~Prior Discussion~~~~
A potent cold front and mini-atmospheric river (A.R.) will move
into the northern half of California from west to east Wednesday
night. This feature will be quite progressive, however the
intensity of the A.R. will peak as it moves into California.
Guidance suggests 850 mb flow will be southerly into the northern
Sacramento Valley at 50-70 kts early Thursday morning. This
extremely fast flow will support Integrated Vertical Transport
values peaking between 800-1,000 kg/ms. This will support localized
rainfall rates to an inch per hour when factoring in upslope
effects, especially on south facing slopes and despite a lack of
supporting instability. Soils in this area are a bit above normal
for this time of year already. The quick hit of heavy rain could
very quickly fill local streams and creeks, especially immediately
downstream of the mountains, resulting in isolated instances of
flash flooding. Snow levels will be between 7,000 and 8,500 ft,
which will support only the highest elevations getting snow. Once
the cold front driving the heavy rain barrels through around
midnight, snow levels will crash to between 3,000 and 5,000 ft.
This will be accompanied with much drier air moving in, ending the
heavy rain threat.

The Marginal Risk area was expanded east with the latest update to
include all of the ranges surrounding the northern Sacramento
Valley, while maintaining the Marginal threat for the coastal
ranges of northern California.

Wegman


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