FOUS30 KWBC 241557
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1057 AM EST Sun Nov 24 2024

Day 1
Valid 16Z Sun Nov 24 2024 - 12Z Mon Nov 25 2024

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

...16Z update...

No significant changes were needed for this update and forecast
reasoning described below remains valid. The 12Z hires guidance
showed the greatest threat for heavy rainfall arriving between
21-00Z, and continuing until 03-06Z associated with the low level
wave west of the northern California coast. A localized excessive
rainfall threat will continue through 12Z Monday and given 850-700
mb winds will waver between SSW and S over the next 12-24 hours,
the highest rainfall totals should tend to be favored into the=20
terrain of southwestern Oregon with the highest rainfall totals
given the track of the low level wave. Spotty 1-2 inch totals=20
through 12Z Monday are expected from northern California into=20
southwestern/western Oregon, with perhaps an isolated 3 inch total
from northern CA into coastal Oregon.

Otto

...previous discussion follows...


Locally heavy rain will be possible today and especially tonight
over portions of northern CA into far southwest OR as an offshore
shortwave and attendant wave of low pressure arrives near the
coastal ranges. Generally the IVT magnitudes out ahead of this
surface wave will be modest with values increasing to as high as
250 to 400 kg/m/s. However, with a front nearby and favorable
upslope flow/orographic ascent involving the coastal ranges, there
will likely be some concentrations of heavier shower activity with
rainfall rates capable of occasionally reaching into the 0.25" to
0.50"/hour range. Some pockets of heavier rain may also make it
into the far northern Sacramento Valley and adjacent foothills of
the northern Sierra Nevada and Shasta/Siskiyou ranges. The 00Z HREF
guidance supports some spotty 1 to 3 inch rainfall amounts going
through early Monday morning, and thus this is not expected to be a
particularly heavy event. However, given the significant rainfall
and lingering hydrological impacts from the most recent atmospheric
river, these additional rains may result in at least some
localized runoff problems and flooding.

Orrison

Day 2
Valid 12Z Mon Nov 25 2024 - 12Z Tue Nov 26 2024

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF CALIFORNIA AND SOUTHWEST OREGON...

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 upwards of 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 move into portions of
southern/central CA...generally from portions of San Luis Obispo and
Monterey counties into the western slopes of the southern/central
Sierra Nevada. IVT values are forecast to be around 500-600 kg/ms,
which at this latitude is starting to approach climatological max
values for late November. PWs are also forecast to increase to
around the 99th percentile for late November, and thus expecting
plenty of moisture and upslope flow to allow for decent rainfall
totals. Not expecting any instability, so rainfall intensity will be
solely driven by the aforementioned moisture transport and upslope
enhancement...and these factors alone are unlikely to support
rainfall rates high enough for true flash flooding. However we will
have an extended period of time with elevated IVT values, and so a
prolonged steady rainfall could eventually result in some localized
minor flood concerns. Overall expecting 1-3" of rain over the
coastal ranges in San Luis Obispo and Monterey counties (with
localized totals towards 4" possible)...with totals ranging from
near 1" in the eastern valley locations to 2-4" over the western
Sierra below snow levels.

Chenard

Day 3
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...

The atmospheric river event described in the day 2 discussion
continues into Tuesday over portions of the central and southern
Sierra Nevada. There is a bit more model spread by this time with
regards to the persistence of the IVT and strength of the mid/upper
forcing. A majority of the 00z deterministic models have trended a
bit weaker and faster with the shortwave energy pushing across CA
and into NV...and this trend would result in less additional QPF
over central CA on day 3. However the 00z GEFS and recent NBM
probabilities suggest there is still a chance for a slower/stronger
wave resulting in more robust rainfall amounts.

This will be the 2nd consecutive day of steady rainfall for these
areas, and so do expect some increased hydrologic sensitivity that
could result in at least some localized minor flooding issues. When
summing the two days of rainfall together, the latest WPC
deterministic QPF is calling for around 2" in Fresno to Visalia,
increasing to 3-6" in the foothills/western slopes of the southern
and central Sierra Nevada. Given the magnitude of IVT forecast even
even forecasting 1-2" spilling over to the east of the crest towards
Bishop and Independence. These forecast values average around a 2yr
ARI for 48 hr amounts, also supportive of a localized minor flood
threat. As mentioned above, there is both higher and lower end
potential with this forecast rainfall. The higher end scenarios
could even result in Slight risk level impacts, while the lower end
outcomes would be a minimal flood threat. So will need to continue
to monitor trends, and for now still think the Marginal risk is
appropriate.

Chenard

Day 1 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/94epoints.txt__;!!DZ3fjg!4eaJBXxxn1lEdiJLIpPoGpS3Y_tzVkKWdrthu0LnUzab=
DvH9um2dVI-hYaiy9J4I1v6_nj12LfBM0GjnkXBhmZAMwG8$=20
Day 2 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/98epoints.txt__;!!DZ3fjg!4eaJBXxxn1lEdiJLIpPoGpS3Y_tzVkKWdrthu0LnUzab=
DvH9um2dVI-hYaiy9J4I1v6_nj12LfBM0GjnkXBh4lX3hjQ$=20
Day 3 threat area: https://urldefense.com/v3/__https://www.wpc.ncep.noaa.go=
v/qpf/99epoints.txt__;!!DZ3fjg!4eaJBXxxn1lEdiJLIpPoGpS3Y_tzVkKWdrthu0LnUzab=
DvH9um2dVI-hYaiy9J4I1v6_nj12LfBM0GjnkXBhgZ1s9eM$=20

=3D =3D =3D
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