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FOUS30 KWBC 110730
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
230 AM EST Tue Nov 11 2025

Day 1
Valid 12Z Tue Nov 11 2025 - 12Z Wed Nov 12 2025

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

Wegman

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 3
Valid 12Z Thu Nov 13 2025 - 12Z Fri Nov 14 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
CENTRAL AND SOUTHERN CALIFORNIA...

The inherited Marginal Risk was left mostly unchanged across
California with this update. A small expansion up the western
foothills of the Sierras was added given the flashy nature of some
of the smaller streams and creeks to a period of heavy rain. The
cold front that will barrel into the coast Wednesday evening will
continue eastward into Thursday through the Sacramento Valley. The
front will push southward down the coast with time as well, with
heavy rain beginning in L.A. Thursday afternoon and evening. Within
the Marginal Risk area, the greatest threat for flash flooding will
be on previous burn scars in southern California`s Transverse
Ranges, as well as in any urban areas at the foot of those
mountains. Upslope flow from an increasingly parallel front will
cause heavy mountain snow amounting to a few feet in spots, but at
the lower elevations, heavy rain may cause localized flash
flooding.

Fortunately, the cold front causing the heavy rain will remain
progressive through this period. This will limit the time any heavy
rain will impact any one area of California. Meanwhile, the forcing
for heavy rain will weaken as the front pushes south, which should
somewhat offset (due to diminishing rain rates) any flash flooding
potential, which would otherwise be increased due to urbanization
and burn scars in and along steep terrain. The Marginal Risk may
need to be expanded further north up the Sacramento Valley into
portions of northern California with future updates if guidance
trends of slowing the speed of the front continue. Due to the
aforementioned weakening of the front, a Slight Risk was not
considered with this update, but could be possible in the most
sensitive areas, especially if forecast rainfall amounts increase
further with future updates. Should a Slight Risk be needed, it
appears most likely in the Transverse Ranges northwest of Los
Angeles.

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