FOUS30 KWBC 230748
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
248 AM EST Sat Nov 23 2024

Day 1
Valid 12Z Sat Nov 23 2024 - 12Z Sun Nov 24 2024

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

A cold front and the remnants of the long duration, multi-day
atmospheric river that has been impacting the West Coast will be 
dropping south across the central Sierra Nevada early this morning.
As the deeper layer moisture flux parameters/IVT magnitudes 
continue to weaken and thin out, the rainfall rates will be 
continuing to drop off. However, there still may be a sufficient
level of upslope flow/orographic ascent in the foothills of the 
central Sierra Nevada for a few more hours early this morning of
spotty 0.25" to 0.50"/hour rainfall rates. Some weak post-frontal
instability is also still forecast which could allow for some 
scattered pockets of heavy shower activity going through midday to 
impact some of the western slopes of the central Sierra Nevada. 
This could result in some isolated quick- hitting 0.25" to 0.50" 
rainfall amounts, but these convective cells are forecast to be 
progressive. Overall, the additional storm total amounts based off 
the 00Z HREF guidance may reach as high as 1 to 2 inches.

Given the very wet antecedent conditions, the additional rains may
foster some additional localized runoff/flooding concerns. Thus, 
the Marginal Risk area over portions of the Sierra Nevada will be 
maintained.

Orrison


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...

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 OVER PORTIONS OF 
NORTHERN CALIFORNIA INTO FAR 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 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

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