FOUS30 KWBC 020803
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
303 AM EST Sun Feb 2 2025

Day 1
Valid 12Z Sun Feb 02 2025 - 12Z Mon Feb 03 2025

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

A prolonged atmospheric river (AR) event is underway across almost 
all of northern California this morning. A large area of high 
pressure off southern California is teaming up with a pair of 
lows...one well north of Hawaii and the second near the northern tip 
of Vancouver Island, BC. These features will interact to form a long 
corridor where abundant tropical moisture can continue riding 
northeastward from the tropics and into northern California. 
Meanwhile, the lows are limiting the northward extent of the 
moisture, especially the one off Canada, which is supplying colder 
air into Oregon and Washington State. 

In the upper levels, a stationary upper level low will guide upper 
level disturbances around its southern periphery and into northern 
California throughout this period, perhaps even retrograding a bit 
west by tonight which will only further lengthen the fetch over the 
Pacific. The strong high in place near southern California will form 
a tight gradient between the two features, resulting in a 
hyperactive zonal jet which will support continuous forcing for lift
supporting the rainfall within the AR.

Nasa Sport Soil Moisture imagery shows much of the soils of northern
California have become nearly saturated as a result of the 1-4 
inches of rain that fell yesterday through the time of this writing. 
Thus, expect much of the rainfall going forward associated with this 
AR to largely convert to runoff. 

A pair of upper level disturbances are moving along the atmospheric 
river. The first will clear California and move into the interior 
this morning. Heavy rain ongoing now over far northern California 
will gradually drift south through the day in response to this first
disturbnace. This will give coastal regions a brief break in the 
rainfall behind the disturbance from the start of this period 
through late morning/midday. Then the second disturbance will renew 
the heavy rainfall beginning midday/early afternoon and continue 
right through tonight. The rainfall plume with this second round of 
rain will be very slow to move, only crawling northward with time. 
Most areas of far northern California will see several hours of 
continuous, steady rain. Rates don't look to be quite as high with 
this second disturbance, likely averaging between 1/4 and 1/2 inch 
per hour. Thus, the primary hazards with it will be associated with 
the long-duration of the rain.

Potential impacts from this AR today and tonight include possible 
mudslides and rock slides in the foothills and mountains below snow 
level (~6,500 ft), as well as the potential for localized flooding 
in flood sensitive and low-lying areas with poor drainage. 

Wegman


Day 2
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. Model 
guidance continues to show a strengthening of the frontal 
convergence by this time over northern CA to go along with an 
uptick in IVT values moving into the coast. This should result in 
an increase in rainfall rates by late morning into the early 
afternoon, with this axis of higher rates shifting south through 
the afternoon and evening.

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,
should put a cap on upward vertical motion and rainfall rates.
Thus while some flooding impacts appear likely, more widespread
significant impacts may be harder to come by, and thus not 
planning on any upgrade at this time.

Chenard

Day 3
Valid 12Z Tue Feb 04 2025 - 12Z Wed Feb 05 2025

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

The atmospheric river will continue dropping southward on Tuesday,
bringing another day of moderate to heavy rainfall and some
flooding risk. Still some uncertainty with the latitude of the AR 
by this time, although models do appear to be converging on a 
solution covered by the Slight risk area. Some trimming on the 
northern end of the Slight risk may eventually be warranted, but 
with some lingering uncertainty and the saturated conditions over 
that area, we were a bit conservative with the northern extent of 
the risk area.

Model solutions indicate the approach of a mid level shortwave
and a stronger upper jet after 18z Tuesday. These features will 
help drive an increase in IVT, and may also help briefly stall the
southward progression of the front. Thus it appears likely that we
will see an uptick in rainfall rates after 18z Tuesday from 
approximately San Francisco south into Monterey county. Still some 
uncertainty on the exact location of this enhanced rainfall rate 
activity, as well as exactly how intense the rain will be...but 
current model solutions support upwards of 1.5" in a 3 hour 
period, and 2-4" in total. This rainfall will likely drive an 
increase in the flood threat over these areas. Luckily most of the 
heavier rainfall on the preceding days will have been north of this
corridor, so not expecting conditions to be too saturated leading 
into the uptick in rain. This fact, combined with the overall 
progressiveness of the system, keeps the flood risk at a Slight 
level. Although will need to continue to monitor, as if rainfall 
rates trend higher can not rule out a localized corridor of MDT 
risk level impacts.

The Marginal risk extends further down the coast into Santa
Barbara, Ventura and Los Angeles counties. The front should be
quick moving by the time it makes it this far south, but a 
quick shot of heavy rainfall is possible. The 00z GFS and NAM do 
not bring the better moisture transport this far south, however the
00z ECMWF, UKMET and GEM do. Even these latter models only get 
rainfall totals into the 1-2" range given the fast frontal motion.
So generally not expecting widespread or significant impacts at 
this time, although rainfall rates could be briefly high enough to 
result in some localized minor flood concerns. We will keep a close
eye on the sensitive burn scars over Los Angeles county, but at 
this time even the more aggressive ECMWF would suggest rainfall 
rates will be on the decline by the time they get that far south. 
Will need to continue to monitor, but think a Marginal risk will 
suffice for now.

Chenard

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