Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1032 AM EST Tue Nov 11 2025
Day 2
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 SOUTHWESTERN OREGON...

A deep digging positively tilted upper level trough will approach
the West Coast on Wednesday night. As it approaches the coast, a
strong shortwave rounding the base of the trough will initiate
cyclogenesis off the California coast. The strengthening low will
track north along the coast. The low's trailing cold front will be
the impetus forcing plentiful Pacific moisture (2-2.5 sigma above
normal) northeastward along a corridor drawn by a 100 kt jet into
the northern California and southwestern Oregon coast Wednesday
evening. 850 mb flow out of the SSW will upslope along the coastal
ranges on the order of 50-70 kt. MUCAPE values will remain under
100 J/kg, so any convection will likely be tied to locally
increased lift along the mountains associated with the upslope,
resulting in a period of heavy rain.

Soil moisture levels in this area are around normal for this time
of year, which should support much of the rainfall in the area
being beneficial. However, given potential for a bit of
elevated instability as well as the upslope, there remains some
possibility for isolated instances of flash flooding, especially
between the coastal ranges and the coast. The inherited Marginal
Risk was left unchanged. Some uncertainty remains as to the timing
of the heavy rainfall, namely how far east the rain can get before
the 12Z Thursday start of the day 3 period. The latest guidance has
been slower, so the Marginal Risk may be able to be pulled from
the western foothills of the Sierras with future updates should
those trends continue.

Wegman

Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt