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FOUS30 KWBC 250757
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
257 AM EST Mon Nov 25 2024

Day 1
Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE SOUTHERN SIERRA NEVADA...

Moderate to locally heavy showers are expected to continue across
portions of northern CA and southwest OR today and tonight as a
deep layered low and associated shortwave energy moves inland. The
deep layer cyclonic flow coupled some steepening of the mid-level
lapse rates and orographic ascent/upslope flow over the coastal
ranges should favor some occasional 0.25" to 0.50"/hour rainfall
rates. Some of these heavier showers will also spread inland
across the northern Sierra Nevada. Overall, the additional rainfall
amounts here should be relatively modest, but portions of the
coastal ranges of northwest CA and far southwest OR may see some
1-3" rainfall totals going through early Tuesday morning. Given the
wet/saturated soil conditions and high streamflows, these
additional rains may foster some renewed runoff problems and
localized flooding. A Marginal Risk will be maintained for these
areas.

Farther south into the upslope areas of the central and especially
southern Sierra Nevada, and also the coastal ranges of Monterey,
San Luis Obispo and Santa Barbara Counties, a moderately strong
atmospheric river event will be arriving today and continuing
through tonight and early Tuesday. This will be associated with the
arrival of a southern stream shortwave impulse which will be
embedded within the transport of deeper layer subtropical moisture
extending well offshore of southern CA and upstream over the
eastern Pacific. The GFS and ECMWF support increasing IVT
magnitudes especially by 00Z across the coastal ranges and into
higher terrain of the southern Sierra Nevada with magnitudes rising
to 500 to 700+ kg/m/s. The orientation of this will be generally
orthogonal to the terrain, and this should yield enhanced
orographic ascent that will be conducive for heavier rainfall
rates.

The 00Z HREF guidance favors some occasional 0.50"+/hour rainfall
rates with the higher probabilities of this focused over the
foothills of the Sierra Nevada involving Fresno and Tulare
Counties, and a small portion of northern Kern County. A consensus
of the 00Z HREF guidance supports 24-hour rainfall totals
(generally below 9000 feet/snow level) of 3 to 6 inches, with a
couple of models indicating some spotty 6 to 8+ inch amounts.
Meanwhile, over the coastal ranges from Monterey County down
through San Luis Obispo and Santa Barbara Counties, some 1 to 3+
inch rainfall totals are supported in the guidance. One key factor
in general that is supporting these higher rainfall totals, aside
from the upslope flow into the terrain, is the fact that much of
the moisture will be of subtropical origins, and this will yield PW
anomalies as high as 2 to 4 standard deviations above normal for
this time of the year. Given the forecast rainfall amounts and
associated rainfall rates, there may be some flooding concerns that
materialize, and this will include at least a localized threat for
some burn scar flash flooding/debris flow impacts. As a result, a
Slight Risk for excessive rainfall has been introduced for portions
of the southern Sierra Nevada, with some localized expansion of
the prior Marginal Risk area to account for rainfall over the
coastal ranges and parts of the San Joaquin Valley.

Orrison

Day 2
Valid 12Z Tue Nov 26 2024 - 12Z Wed Nov 27 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL AND SOUTHERN SIERRA NEVADA...

Model differences remain in the 25/00Z guidance with respect to
the timing of a shortwave moving inland from the California/Oregon
border with the ECMWF being faster to bring the shortwave trough
across the Sierra Nevada range and into the Great Basin by
27/00Z...although most of the 00Z suite of guidance...including the
GFS...trended trended faster and a bit weaker. With the exception
of the Canadian GEM and Regional...the 25/00Z GFS remained heaviest
across the central and southern Sierra Nevada for the 24 hour
period ending Wednesday morning. Still expect some increased
hydrologic sensitivity that could result in at least some localized
minor flooding issues given that this will be the second
consecutive day of steady rainfall for these areas. Even though the
non-25/00Z GFS/Canadian consensus is for 2 to 4 inches across the
central/southern Sierra Nevada over the 24 hour period, the amounts
tend to be more in the 1 to 3 inch range below the expected snow
levels (approx 9000 ft per the WPC Winter Weather Desk forecaster)
which may mitigate some of the concern for flooding. Even so...the
concern remains that those values may be too low at least locally
considering the coarser resolution of the available guidance at
longer lead times so an upgrade may still be needed later. Rainfall
rates and areal coverage of rainfall should decrease late in the
period as the flow of moisture becomes disrupted and the better
dynamics pull away from the area.

Bann

Day 3
Valid 12Z Wed Nov 27 2024 - 12Z Thu Nov 28 2024

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

Bann

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