FOUS30 KWBC 260053
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
753 PM EST Mon Nov 25 2024

Day 1
Valid 01Z Tue Nov 26 2024 - 12Z Tue Nov 26 2024

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

...01Z Update...

The outlook remains much the same, with model consensus showing=20
deep moisture transport remaining centered from the central=20
California coast trough the San Joaquin Valley into the southern=20
Sierra Nevada. Rainfall rates through the day have remained modest,
but models do suggest periods of heavier rain will develop,=20
supported by an uptick in moisture and forcing overnight. Heaviest=20
rainfall amounts will likely fall along the upslope regions of the=20
Sierra Nevada Foothills, where the consensus of the CAMs, including
recent runs of the HRRR, indicate local amounts of 2-4 inches are=20
likely overnight.

...16z Update...

Little change needed to the inherited ERO with this update, as the
details of the 12z HREF are virtually identical to the prior runs,
including expected 3-6" totals (and possible localized 6-8+") in
association with the targeted Slight Risk area along upslope
portions of the southern Sierra Nevada. See below discussion for
more information.

Churchill


...Previous Discussion...

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

...19Z Update...

Have upgraded a portion of the Marginal Risk area to Slight,
essentially the same area where the slight is in effect during Day
1 (western foothills/slopes of the southern Sierra). This is
essentially a continuation of the current D1 Slight Risk, based on
the 'ongoing' nature of this latest AR event beyond 12Z Tue. Based
on the CW3E AR Scale, this AR tops out as 'moderate' with IVT
values peaking between 500-600 Kg/ms. TPW anomalies continue
between 3-4 standard deviations above normal during the first half
of the Day 2 period (through 00Z Wed), then weaken overnight as
does the AR and IVT anomaly.

Latest (12Z) high-res CAMs show additional 24hr QPF totals in the
2-4"+ range across the Slight Risk area where the precipitation is
expected to fall as rain (lower elevations), though once again the
NAM CONUS-Nest remains rather bullish with the coverage of 4-8"
totals. 12Z HREF probabilities of 0.50+ in/hr rates are most
elevated within the Slight Risk area, mainly before 00Z Wed.

Hurley


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.

Hurley

Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!__LQaNA7DrRcMVIRdVvwK8ilEOxSvlalZaipFwZVLyPR=
0Eg1klC4j89jEIdQdj-Hi46l23d9qE9_PAyvOjeVagoqp7w$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!__LQaNA7DrRcMVIRdVvwK8ilEOxSvlalZaipFwZVLyPR=
0Eg1klC4j89jEIdQdj-Hi46l23d9qE9_PAyvOjeV2Yiwd6k$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!__LQaNA7DrRcMVIRdVvwK8ilEOxSvlalZaipFwZVLyPR=
0Eg1klC4j89jEIdQdj-Hi46l23d9qE9_PAyvOjeVY-YRrfk$=20

=3D =3D =3D
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