FOUS30 KWBC 040827
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
327 AM EST Tue Feb 4 2025

Day 1
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...

A large and energetic cutoff low that will remain largely stationary 
off the coast of Vancouver Island will be the upper level animus for 
the active weather ongoing across California today. An atmospheric 
river event that has been ongoing for the past several days will 
come to an end (or at least take a significant break) during this 
period. However, to get to the break, one last potent area of low 
pressure moving along the atmospheric river still has to move across
California, resulting in another period of heavy rain. The cold air 
following behind its cold front will push the AR far enough south 
but also far enough away from the forcing to cause it to mostly 
dissipate tonight.

The atmospheric river currently is located along the I-80 corridor 
from San Francisco through Lake Tahoe. It is expected to shift south 
to about a Monterey to Modesto line, leaving the I-80 corridor in 
periodic shower activity. It will not move much along the foothills 
of the Sierras, where upslope flow will keep the steady rain going 
well north of where the core of the atmospheric river is. From the 
Bay Area north along the coast, expect a break from the rainfall 
through until about 15Z/7am PST. That all changes beyond 15Z, when 
the aforementioned low begins to move ashore, resulting in steady 
rain for the almost the entire northern half of California. As in 
previous days, localized convective enhancement and convergence 
bands are expected to develop within the broader rainfall shield. 
This will be supported by a strengthening LLJ through the Sacramento 
Valley, which will intensify due to the approach of the southerly 
flow ahead of the low. Colder and drier air following behind the low 
will begin to clear the precipitation out from northwest to 
southeast starting around 22Z/2pm PST. Until the back edge clears, 
locally heady rain will continue, which will be enhanced by the 
passage of the surface cold front itself, bringing a brief period of 
heavy rain for most of northern California during the late afternoon 
and evening hours, potentially impacting this evening's commute.

After about sunset or so, the entire atmospheric river will weaken, 
though the prevailing southwesterly upslope flow may continue much 
lighter rain into the Transverse Ranges, the southern Sierras, and 
eventually the Los Angeles Metro area. Despite the recent fires in 
that area, the rain should be light enough east of Santa Barbara 
County that there should not be enough to cause flooding concerns, 
so the light rain from the showers should be beneficial, if not 
nearly enough to dent the ongoing drought conditions in southern California.

All of the guidance has shifted significantly northward with the 
track of the incoming low for the day today. This resulted in a 
northward shift in the ERO risk areas, with the Slight Risk being 
expanded well up the coast to Mendocino County and all of the 
northern Coast Ranges and trimmed out of the southern Central 
Valley. An internal higher-end Slight was added to most of the 
foothills of the northern Sierras since this area stands to pick up 
an additional 3-5 inches of rain on top of the foot+ they've seen 
the past few days. 

The ERO trimming was due to rain shadowing from the southern Coast 
Ranges. Unlike previous days, the southern Coast Ranges from the 
Santa Cruz Mountains south through the western Transverse Ranges 
should all get a healthy soaking today from the atmospheric river, 
though since those areas have seen significantly less rainfall in 
recent days compared to areas further north, have left most of these 
areas in the Marginal.

Wegman


Day 2
Valid 12Z Wed Feb 05 2025 - 12Z Thu Feb 06 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF 
THE OHIO VALLEY...

Strong southwesterly flow will advect abundant moisture northward
into the OH Valley on Wed into Wed night, with PWs forecast to
increase over the 90th percentile for early February. Increasing
WAA, a mid level shortwave moving east within the quick zonal 
flow, and upper level divergence in the right entrance region of a 
jet to the north and left exit region of a strong jet over the MS 
Valley, will broadly support an expansion of showers across the 
region. Model guidance continues to indicate upwards of 500-1000 
j/kg near and south of a northward moving warm front, thus expect 
we will see embedded thunderstorms as well. Given the forcing, 
moisture and instability forecast, do anticipate we will see some 
locally heavy rainfall rates materialize, especially by Wed evening
into the overnight near the lifting warm front.

Still some uncertainty on the location of this front, although
models are in much better agreement tonight compared to last 
night. Precipitation north of the front will likely be either sleet
or freezing rain, and thus the position of this feature will be 
important to delineate the northern edge of the flood risk area. 
The current consensus for the axis of heaviest rainfall is across 
portions of KY, southern IN/OH into much of WV. Some of these areas
may start as sleet/freezing rain, but will transition to locally 
heavy rain as the warm front lifts north. Areal averaged rainfall 
is currently forecast in the 0.5"-1" range, although given the 
forecast instability, locally higher totals ~2" appear possible. 
Portions of central/eastern KY into WV have above average soil 
saturation and streamflows, and so this area is likely most 
susceptible to some flooding impacts if higher rainfall rates do materialize.

Chenard


Day 3
Valid 12Z Thu Feb 06 2025 - 12Z Fri Feb 07 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE OHIO VALLEY AND CALIFORNIA...

...OH Valley...
Another round of locally heavy rain is expected Thursday across
portions of KT/TN and WV along and ahead of an approaching cold
front. Locally heavy rainfall should be ongoing at 12z Thursday
across portions of WV, before a relative lull in activity by late 
morning into the early afternoon. However by later in the afternoon
into the overnight hours another round of showers and embedded 
thunderstorms should develop near the cold front over KY and 
spread eastward into WV. Seeing enough instability forecast to 
support locally heavy rainfall rates. Some of this corridor already
has above average soil saturation and streamflows, and there is 
also some overlap between this day 3 rainfall and the day 2 rains. 
Two day rainfall totals of 1-2" are forecast, but given the 
instability both days, some areas could locally exceed 3". This 
rainfall should be enough to result in at least some flooding impacts.

...California...
After a one day break another system is forecast to move into CA 
on Thursday. This is expected to be a pretty dynamic system, with a
strong mid level shortwave and upper level jet. These dynamics 
should support a short duration period of enhanced rainfall 
rates...although the quick forward motion of the system will limit 
overall rainfall magnitudes. Not seeing much in the way of 
instability currently forecast, and IVT and PWs are not overly 
anomalous with this system either. These limitations will be 
somewhat made up for by the stronger dynamics, however still not 
expecting this to be a high end event. Rainfall totals around 1" 
are forecast along coastal areas with 1-2" in the foothills of the 
Sierra (snow levels will be lower with this system limiting 
impacts). Given the saturated conditions over the region, this 
quick shot of locally heavy rainfall may still be enough to 
generate localized flood impacts, supporting the Marginal risk.

The Marginal risk extends southward into Santa Barbara, Ventura and
Los Angeles counties as well. Less rainfall forecast here, but
there is an uptick in IVT into the terrain, and so would not be 
surprised if QPF trends up a bit as the event nears. Still not 
expecting significant rainfall, but possibly enough for localized impacts.

Chenard
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