FOUS30 KWBC 310815
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
315 AM EST Fri Jan 31 2025

Day 1
Valid 12Z Fri Jan 31 2025 - 12Z Sat Feb 01 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WEST VIRGINIA AND NORTHERN ILLINOIS...

...West Virginia...
Only minimal changes to the Slight risk area over portions of WV. The
latest NOHRSC analysis indicates upwards of 2-4" of snow water 
equivalent over the terrain of eastern WV and the latest model 
consensus continues to depict 1-3" of rainfall over this area. 
With temperatures and dewpoints forecast to at least get into the 
40s (50s in spots) the rain on snow should trigger a fairly 
efficient melting of the snowpack. This is likely to result in 
numerous instances of areal flooding and at least some river flooding.

Rainfall rates are generally forecast to stay in the 0.25" to 0.5"
an hour range, which is probably not enough to generate flooding
of a more flashy variety. However there are indications in the 00z
HREF that embedded convective elements could locally push hourly 
rainfall a bit over 0.5". Given the amount of runoff that will be 
occurring from the combined snow melt and rain, any of these higher
rates could locally result in a flash flood threat as well.

...Northern Illinois and Indiana...
Only minor changes to the inherited Slight risk area within the
comma head rainfall axis over northern IL. Despite little to no 
instability, strong frontogenesis within this axis is resulting in 
moderate HREF probabilities of rainfall exceeding 0.5" in an hour. 
Rainfall amounts upwards of 1.5" in 6 hrs should be enough to at 
least trigger some minor flood impacts given the extra runoff 
generated by the mostly frozen ground.

...West Coast...
An atmospheric river will bring heavy rainfall from western WA
southward into northern CA today into night. The IVT plume is 
generally progressive and weakening through the period, so not 
expecting major rainfall totals or impacts. Generally forecasting
1-3" of rainfall (locally higher in the favored terrain), which 
should mainly drive an isolated minor flood threat.

The IVT axis does start to stall as it shifts into northern CA, and
so could be looking at totals locally in the 2-4" range across the
terrain here. Given the generally dry antecedent conditions still
think this initial batch of rain will mainly just set the stage 
for a greater flood threat in the following days, so will keep the 
risk at Marginal. However we may see at least some uptick in flood
related impacts over central to northern CA by tonight.

Chenard


Day 2
Valid 12Z Sat Feb 01 2025 - 12Z Sun Feb 02 2025

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

The atmospheric river is forecast to stall over central/northern 
CA on Saturday. Most model solutions indicate an increase in IVT 
moving into the CA coast near San Francisco just after 12z 
Saturday, which should bring an uptick in rainfall rates onshore. 
PWs will be quite impressive for the time of year, with values 
forecast to exceed the 99th percentile. A lack of strong dynamics 
and frontal convergence, combined with only moderate IVT values, 
will cap the magnitude of rainfall rates. Nonetheless, expecting to
see enough of an uptick in rainfall Saturday with the stalling AR 
to support an increase in flood impacts, warranting the Slight risk.

While IVT values along coastal areas will be moderate, what this
AR will have going for it is an impressive inland extent of IVT
magnitudes...with values exceeding the 99th percentile (and around
climatological max values) within the Sierra. This supports the 
inland extension of the Slight risk across the Sacramento Valley 
and into the mountains. Snow levels will be high, so the Slight 
risk extends pretty far into the terrain. We also made sure to 
extend the Marginal risk over the crest and into the eastern slopes
of the Sierra, as given the IVT, we should see enough rain make it
over the crest to support at least a minor flood risk.

Two day rainfall totals are currently forecast upwards of 2-4"
along coastal areas, 1-3" within Sacramento valley locations, and 
upwards of 4-7" within the Sierra Nevada (most of which will be 
rain given high snow levels).

Chenard


Day 3
Valid 12Z Sun Feb 02 2025 - 12Z Mon Feb 03 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF 
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 2 (Saturday). PWs drop off compared to 
earlier in the event, however the persistent IVT should support a 
prolonged moderate to locally heavy rain. Snow levels are quite 
high, and thus the Slight risk was expanded into the terrain, and 
even a bit to the east of the crest given the inland IVT 
anomalies. Rainfall rates within the higher terrain of the Sierra 
and just downwind of the crest will more be driven by the 
persistent IVT over the entire period, and thus flood impacts will 
be possible through Sunday and into the overnight hours.

Along coastal and valley areas, rainfall rates should increase by 
later Sunday night as convergence increases along a front moving 
into the CA coast. Given what should be saturated conditions by 
this time, this uptick in rainfall rates along coastal areas into 
portions of the northern Sacramento Valley may result in an uptick 
in flood impacts by later Sunday night. 

Chenard

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