FOUS30 KWBC 010757
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
257 AM EST Sat Feb 1 2025

Day 1
Valid 12Z Sat Feb 01 2025 - 12Z Sun Feb 02 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF 
NORTHERN CALIFORNIA...

A nearly stationary positively tilted longwave trough will be 
anchored across central British Columbia today. It will direct a 
zonal but powerful 150+ kt jet streak into the Pacific Northwest. 
The jet streak will in turn guide an atmospheric river (AR) moving 
eastward after soaking Hawaii into northern California today. The 
support of the zonal jet will allow the AR's moisture to extend well 
inland from the coast, including into Nevada, Utah, Idaho, and 
Wyoming. The areas getting the greatest rainfall will be the 
northern Sierra Nevada mountains and secondarily the coastal ranges 
around and north of the Bay Area. For just this Day 1 period, expect 
4-6 inches of rain from west of Lake Tahoe north and west to about 
Paradise, CA. 1-3 inches are expected for the northern Coastal 
Ranges from the Bay Area through southern Trinity County.

IVT values for this atmospheric river event peak at around 850 kg/ms 
around 18Z/10am PST in the Bay Area this morning according to both 
GFS and EC ensemble output. While this alone is notable, it's far 
from extreme. It will be the duration of this event that will make 
it stand out from most ARs. While IVT values will drop closer to 500 
kg/ms in the following days, they will remain elevated near that 
level for 3 consecutive days after today. Using IVT as a proxy for 
heavy rain, this would suggest the heaviest rain of the event will 
be this afternoon and evening, but smaller disturbances responsible 
for brief upticks in precipitation rates become increasingly 
difficult to predict at extended time ranges, so it's very possible 
additional heavy rainfall will occur in the subsequent days.

PWATs advecting into the coast with the AR will get as high as 1.25 
inches today. NAEFS mean PWAT probabilities put this at the 98th 
percentile as compared with a 30 year average. Additional flooding 
concerns are also forecast around the 2024 Park Fire east of Red 
Bluff and north of Chico. Here, hydrophobic soils and a lack of 
vegetation will convert most, if not all of the rainfall to runoff, 
resulting in locally worse flooding as compared with surrounding areas.

Changes from the previous forecast were minimal, with expansions of 
the Marginal Risk drawn in around Lassen N.P. The highest elevations 
will see snow, but with such high snow levels (6,000-7,500 ft), the 
large majority of the expected precipitation in northern California 
will be in the form of rain. NASA Sport soil moisture imagery shows 
much of Northern California's soils are below to well below normal, 
so much of the rain, especially in the populated valleys should 
start out as beneficial...whereas the flooding threat will be the 
most impactful in the foothills of the Sierras.

Wegman


Day 2
Valid 12Z Sun Feb 02 2025 - 12Z Mon Feb 03 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF 
NORTHERN CALIFORNIA...

The slow moving atmospheric river will continue to bring a heavy
rainfall and flood threat to portions of northern CA Sunday into
Sunday night. The IVT core and max rainfall axis should shift a bit
north compared to day 1 (Saturday). PW anomalies will gradually 
drop, however the persistent IVT should support a prolonged 
moderate to locally heavy rain. Snow levels are quite high, and 
thus the Slight risk area goes into the northern Sierra, and even 
a bit to the east of the crest given the strong inland IVT anomalies.

The highest rainfall rates will likely be across portions of the
northern Sierra where more anomalous IVT will focus into the
terrain. 48hr rainfall amounts of 8-12" are forecast over portions
of the northern Sierra (generally from Placer up to Butte and 
Plumas counties) through 12z Monday. This portion of the Sierra 
has some of the highest 48hr ARI thresholds in the country, so 
this region is clearly used to heavy rainfall amounts. With that 
said, these forecast totals are still exceeding the 2 to 5yr 
thresholds. Seems like it'll be a cold rain so not sure how much 
SWE loss we'll get in the higher elevations where there is 
snowpack (rain might mostly get absorbed into the snowpack). Given 
these totals and periodic rates around 0.5" per hour, at least some
flood impacts appear likely, with higher end Slight risk 
probabilities justified. The lack of convective rainfall rates, 
combined with the fact this area can generally take quite a bit of 
rain (as seen by the high ARI thresholds), suggests these higher 
end Slight probabilities should suffice for now.

A prolonged steady rain along coastal to valley areas will result
in a widespread 1-3" of additional rainfall over the Slight risk 
area, brining 48hr totals into the 3-6" range. A lack of strong 
frontal convergence should keep rainfall rates lower here, but the
prolonged nature of the rain should still allow for an uptick in 
flood impacts as conditions continue to saturate.

Chenard


Day 3
Valid 12Z Mon Feb 03 2025 - 12Z Tue Feb 04 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF 
NORTHERN CALIFORNIA...

Not much change to the IVT axis is forecast into Monday, meaning 
the prolonged moderate to heavy rainfall event will continue across
northern CA. Snow levels should begin to drop by this time 
resulting in less impacts over the higher terrain of the Sierra, 
but a continued uptick in flood impacts over coastal areas into the
foothills where conditions will continue to saturate. By Monday 
afternoon model guidance indicates we should see a strengthening of
frontal convergence across northern CA, which should help drive an
uptick in rainfall rates along the coast. 

Model spread begins to increase by day 3 with the 00z NAM and GFS 
a bit further north with the heavier rainfall axis than the 00z 
ECMWF, UKMET and GEM. Interestingly the 18z ECMWF AIFS is closer 
to the GFS/NAM camp. Big picture wise the differences do not end 
up having a major impact on QPF or the ERO (differences get bigger
on day 4), but if the further north GFS/NAM end up being right the
Slight risk area could end up shifting a bit north with future 
updates. There is some potential for higher end Slight risk, or 
maybe even MDT risk, probabilities along coastal areas given the 
expected uptick in rainfall rates over what should be fairly 
saturated ground conditions by this point. However a lack of 
stronger dynamics and lack of instability interacting with the IVT 
plume may prevent rates from getting high enough to cause more 
widespread significant impacts. So will just maintain the Slight 
risk for now, but either way anticipate at least some continued 
flood impacts Monday and will continue to monitor trends.

Chenard
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