FOUS30 KWBC 030816
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
316 AM EST Mon Feb 3 2025

Day 1
Valid 12Z Mon Feb 03 2025 - 12Z Tue Feb 04 2025

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

The atmospheric river event across much of northern California will 
continue through the entire Day 1/Monday period. A narrow (100-200 
mile) wide plume of rainfall rounding the northwestern periphery of 
a broad subtropical high is also being steered around a positively 
tilted upper level trough centered over British Columbia near the 
northern tip of Vancouver Island. The jet stream is rounding the 
upper level trough from the northeastern side of the trough around 
to its west and south then ejecting around the southeastern side of 
the trough. The trough will retrograde southwestward, allowing a 
cutoff upper level low to form as the jet stream eventually cuts off 
from the upper level low and maintains a trough over much of eastern 
Canada.

The upper low will drift southwestward through 12Z Tuesday, which 
will work to reorient the atmospheric river from its current west-to-
east to more of a southwest-to-northeast orientation. This will have 
several ramifications: First, it will maintain the plume of rain 
into northern California as the reorienting atmospheric river pivots 
over this area. Rates are expected to remain largely steady based on 
the latest HiRes guidance. Second, the "pivot" will be associated 
with a subtle surface low tracking along the atmospheric river. This 
will locally increase rainfall rates from midday through this 
afternoon area-wide...though localized convergence in the northern 
Sacramento Valley will keep rainfall rates elevated there throughout 
the period. Third, the southwesterly flow may support the ongoing 
low level jet in the Sacramento Valley better since the broader flow 
will align better with the low-level jet. The result will be 
continued heavy rain for the northern and eastern Sacramento Valley 
and the foothills of the northern Sierras with a widespread 3-5 
inches of rain expected.

The southwesterly reorienting will then begin to push the 
atmospheric river southward from this afternoon through tonight, 
reaching the Bay Area this evening. Upslope flow into the Sierras 
will keep heavy rain ongoing into the foothills of the Sierras all 
the way up the Sacramento Valley, however, the rain will finally 
taper off and give the northern coastal ranges a much needed break 
by tonight as the core of the AR moves south of the Bay Area and 
weakens with time. The faster southerly movement of the AR from late 
this afternoon through the overnight should significantly lower the 
flooding threat with decreasing latitude, so the southern end of the 
Slight risk is largely the same, stopping just north of the Bay 
Area, as amounts from the Bay Area south should only necessitate a 
Marginal Risk since they've been relatively dry in recent days.

With this expected evolution, a few minor changes were made to the 
Day 1 ERO from continuity, namely to begin expanding the Marginal 
Risk southward to include the Santa Cruz mountains through Monterey 
Bay. Otherwise, only minor tweaks were made to better match both 
Marginal and Slight risk areas with the local higher elevations that 
will see mostly snow, while including some of the valleys in between.

Wegman


Day 2
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 event will continue into Tuesday across
central and northern CA. Model solutions continue to indicate the 
approach of a mid level shortwave and a stronger upper jet after 
18z Tuesday. The approach of these features should help slow/stall
and increase the magnitude of the IVT axis during the day. The 
increase in forcing/IVT and a strengthening of the low level 
convergence along the front, will both support an uptick in 
rainfall rates from near San Francisco into the Sacramento Valley 
and foothills of the Sierra. Hourly rainfall could approach and 
locally exceed 0.5" along this front, which should drive an 
increase in the flood threat. The good news is that most of these 
areas in line for the heaviest rainfall Tuesday have been south of 
the heaviest rain the past few days. This combined with the fact 
that the front/IVT axis (while briefly stalling), will start to 
make quicker southward progress by 00z, should limit the threat of 
more widespread significant flood impacts. Nonetheless, do 
anticipate some flooding and landslide potential to accompany this 
rainfall Tuesday.

The Slight risk extends south into coastal Monterey county, where
stronger orographic effects will help drive heaver rainfall totals
in the 4-7" range. As the front drops into this area there will 
also be an uptick in hourly rainfall, with high probabilities of 
0.5" in an hour and localized totals approaching 0.75" in an hour 
probable. Heavy rainfall is common along this corridor, and 
antecedent conditions this far south are quite dry. Both these 
factors should help put a cap on the magnitude of flood impacts 
from this rain, however would still expect to see at least some 
flood and landslide potential.

The Marginal risk extends further down the coast into Santa
Barbara, Ventura and far western Los Angeles counties. The front 
should be quick moving by this time, and the IVT axis should be
weakening. The GFS is the weakest model, but most other models have
come around to at least a short period of locally heavy rainfall 
getting into the favored upslope terrain areas. Even these wetter 
models (and the WPC forecast) 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.

Chenard


Day 3
Valid 12Z Wed Feb 05 2025 - 12Z Thu Feb 06 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS 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, weak mid level shortwaves 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 through the period. Model guidance also indicates decent 
instability 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, with the 
ECMWF the furthest north solution and the GFS on the southern side
of the model spread. 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. At this time generally favor something in the 
middle, which would favor an axis from KY into southern IN/OH and 
into WV for the greatest rain rate potential, and the Marginal risk
encompasses these areas. However, if a warmer/further north 
solution becomes more likely, then the Marginal may need to be 
expanded further north, with the opposite being true if a 
colder/south scenario comes to fruition. 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

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