FOUS30 KWBC 240816
QPFERD

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

Day 1
Valid 12Z 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...

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

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