FOUS30 KWBC 301034
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
534 AM EST Thu Jan 30 2025

Day 1
Valid 12Z Thu Jan 30 2025 - 12Z Fri Jan 31 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FROM NORTHEAST
TEXAS INTO ARKANSAS AND PORTIONS OF KENTUCKY AND TENNESSEE...

Not much change to the inherited Slight risk stretching from
northeast TX, across central AR and into portions of TN and KY.
Both PWs and IVT are approaching climatological maximum values for
late January across this corridor, so a very dynamic system with
plenty of moisture to work with. Instability will be a limiting
factor, keeping rainfall rates and the overall flash flood
potential lower than it otherwise could be. The highest rates will
probably be early in the period over the southwestern portion of
the Slight risk (northeast TX into southwest AR), where hourly rain
could approach 1.5". Further downstream over AR into KY and TN
rainfall rates off the 06z HRRR and 00z HREF peak closer to 0.5-0.75"/hr.

Much of the Slight risk area is expected to see at least 2" of
areal averaged rainfall today into this evening, with the areas
from northeast AR into far western TN/KY having the highest
probabilities of 3"+ amounts. There is good overlap between the
higher 3" EAS probabilities from the HREF and the 06z HRRR max QPF
axis...adding confidence in the potential for a 3-5" rainfall
swath from northeast AR into far western KY/TN. Rainfall rates here
will not be all that high to start out, with these higher totals
driven more by a persistent moderate rain. However by 00z guidance
indicates these areas could move into the warm sector and see more
robust convection with higher rate potential. If this occurs then a
greater coverage of flash flooding could evolve.

Stronger convection should develop over portions of southern AR
into MS by this evening. However this activity will be progressive
enough to limit the flash flood risk.

A Marginal risk was maintained in the developing comma head from
eastern KS into northern MO, southern IA and west central IL. Low
topped convection could locally increase rainfall rates, with a
broad 1-3" of rain forecast (highest probs of approaching 3" are in
far northeast KS into northwest MO).

Chenard


Day 2
Valid 12Z Fri Jan 31 2025 - 12Z Sat Feb 01 2025

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

...West Virginia...
A Slight risk was issued for portions of central and eastern
WV...primarily for areas where significant snowpack still exists.
The latest NOHRSC analysis indicates upwards of 2-4" of snow water
equivalent over the terrain of eastern WV and the latest model
consensus is for 1-3" of rainfall over this area. With
temperatures and dewpoints forecast to at least get into the 40s
(maybe 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 flash variety. However there are indications in both the
06z HRRR and 00z HREF that embedded convective elements Friday
could locally push hourly rainfall a bit over 0.5". Given the
amounts 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...
A Marginal risk was maintained across this area as comma head
rainfall drops up to 1.5" of rain in a 6hr period Friday. Typically
this degree of rain would not result in impacts, however frozen
ground will likely enhance runoff potential. Given the extra
runoff this will generate, amounts around 1.5" in 6 hrs could be
enough to trigger at least some flood concerns across this corridor.

...West Coast...
An atmospheric river will bring heavy rainfall from western WA
southward into northern CA Friday into Friday night. The IVT plume
is generally progressive and weakening through the period, so not
expecting major rainfall totals or impacts. Generally forecasting a
1-3" 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 more in the 2-4" range across the
terrain here. Will need to monitor for a potential Slight risk
upgrade across portions of the northern CA coast, but for now will
stick with the Marginal. Leaning towards this day 2 rain mainly
helping saturate conditions leading towards a greater flood risk
on later days. However should things trend quicker and/or wetter
an upgrade may need to be considered.

Chenard


Day 3 
Valid 12Z Fri Jan 31 2025 - 12Z Sat Feb 01 2025

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

A pretty significant model change has been noted over the past
couple cycles across the eastern Pacific into the west coast. The
earlier model consensus was for a stalling of a moderate to strong
AR over northern CA and southwest OR resulting in significant
rainfall amounts. However recent model runs have unanimously
shifted further south with this AR, now stalling it more over
central to north central CA. There has also been a weakening trend
in the AR, so QPF values are not as high as earlier forecast.
However, with the southward shift the max rainfall axis is now over
areas that tend to be more sensitive to heavy rainfall and related
flood impacts. Thus even with the drop in QPF, think a southward
shift of the Slight risk is warranted. The Slight risk now
stretches from Mendocino county south into the San Francisco area
and as far south as Santa Cruz county. It also extends inland
across the Sacramento Valley and into the Sierra Nevada.

Rainfall along the coast will start on Friday and continue into
the day Saturday. A gradual saturating of soils are expected to
lead to potential greater impacts on Saturday, warranting the
Slight risk. Two day rainfall totals are currently forecast in the
2-4" range. Two day rainfall amounts of 1-3" are expected within
Sacramento valley locations, with upwards of 4-6" within the
Sierra Nevada (most of which will be rain given high snow levels).

Chenard
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