Quantitative Precipitation Forecast
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105
FOUS30 KWBC 240051
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
751 PM EST Sat Nov 23 2024

Day 1
Valid 01Z Sun Nov 24 2024 - 12Z Sun Nov 24 2024

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

Pereira

Day 2
Valid 12Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
NORTHWEST CALIFORNIA INTO FAR SOUTHWEST OREGON...

...1930 UTC Update... No changes made to the Day 2 ERO based on
the 12Z guidance. This includes the array of HREF probabilistic
data, which continues to depict spotty elevated probabilities of
0.50+ in/hr rates during day 2 in the Marginal Risk area, and the
south- north axis of 250-500 kg/ms IVT aligns along the NorCal
coast.

Hurley

...Previous discussion...
Locally heavy rain will be possible Sunday into Sunday night over
northwest CA into far southwest OR. A front slowing near the coast
and a weak low to mid level wave riding along it will support a
modest uptick in IVT and plentiful low level convergence. Moisture
is a limiting factor for heavy rainfall with the better moisture
plume situated well south and not really getting drawn into this
system. Thus this is not expected to be a significant atmospheric
river, and normally would not expect a system of this nature to
pose a flood risk. However given the significant rainfall over the
past few days, this additional rainfall may be enough to cause some
localized additional flooding as the area will be more
hydrologically sensitive than normal.

We are still seeing some spread in model QPF output over this
period. If some of the more aggressive solutions pan out then we
could see 1-3" of rainfall is spots with hourly totals peaking
around 0.3-0.4" in the typically more favored upslope areas.

Chenard


Day 3
Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS MUCH OF
THE CALIFORNIA CENTRAL VALLEY AND WESTERN SLOPES OF THE SIERRA,
ALONG WITH PORTIONS OF NORTHERN CALIFORNIA INTO FAR SOUTHWEST
OREGON...

...1930 UTC Update...
Have made a fairly sizable southward expansion to the Marginal Risk
area on Day 3, to now encompass the CA Central Valley and adjacent
Sierra foothills. This was based on the latest guidance trends,
which are fairly consistent in showing a developing modest, W-SW to
E-NE oriented AR coming ashore south of the Bay area Mon and Mon
night. IVTs of 400-600 Kg/ms are expected to push ashore per both
the GFS and ECMWF, with a corresponding TPW anomaly of 2.5-3
standard deviations above normal. Global QPFs, along with the NAM
and GEM regional guidance, have come up, especially across the
western Sierra slopes given the strengthening upslope enhancement.
24 hour rainfall totals of 2-5+ are noted from the models, again
with the heaviest totals across far eastern portions of the Central
Valley into the western slopes of the Sierra.

Hurley

...Previous discussion...
Showery conditions will likely continue across portions of northern
CA and southwest OR Monday into Monday night as a deep layered low
and shortwave energy moves inland. Certainly not a classic setup
for flooding with limited moisture and dropping IVT values...
however this area remains more hydrologically sensitive than normal
following the significant atmospheric river event the past few
days. Additional rainfall over far northwest CA into southwest OR
is generally 1-3" in the guidance, with isolated totals of this
magnitude also possible over the western slopes of the northern
Sierra Nevada.

A more significant plume of moisture/IVT will actually move into
portions of southern/central CA...generally from portions of San
Luis Obispo and Monterey counties into the western slopes of the
southern Sierra Nevada. QPF has been trending up with this system,
although still seeing some model spread. The latest model consensus
suggests a possible 2-3" of rain along the coastal ranges and the
southern Sierra below snow levels. Not seeing any instability with
this system and large scale forcing is only modest, so hourly
rainfall rates are generally not expected to exceed 0.5". Given
this, and the fact that most of this area has not seen significant
rainfall recently, will continue with no areas in the excessive
rainfall outlook for now.

Chenard

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