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QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Sun Mar 23 2025

Day 1
Valid 12Z Sun Mar 23 2025 - 12Z Mon Mar 24 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
WESTERN WASHINGTON STATE...

...Western Washington...

In coordination with SEW/Seattle, WA forecast office, a Slight Risk
upgrade was introduced with this forecast update. A strong cold
front and trailing atmospheric river plume are moving into the
coast of Washington State now, and will continue to push west into
the Olympic Mountains and Cascades through the day today. As the
front moves into the mountains, the combination of forcings from
the front, the upslope flow, and abundant Pacific moisture both
with and behind the front will increase the potential for half-inch
per hour rainfall rates into the mountains. Meanwhile, an 
increasingly southerly component of the onshore flow associated 
with the following atmospheric river will advect warmer air into 
the region. This will result in rising snow levels above 7,000 ft,
which will increase the potential for resultant flooding from
higher elevation snowmelt and the heavy rainfall. 

IVT values associated with the trailing atmospheric river have
risen a bit in the latest guidance, peaking in the 625 to 675 kg/ms
range today. This increase along with slightly greater amounts of
atmospheric moisture have resulted in higher forecast rainfall
today, especially during the daylight hours as the front moves
ashore. The front being either primary or a significant component
of the forcing will result in the heaviest precipitation into the
Olympics and Cascades today. Once the front dissipates as it is
shredded by the mountains, the atmospheric river and upslope flow
will not have nearly the amount of forcing alone as they had with
the front. Thus, the potential for heavy rainfall rates above a
quarter inch per hour should diminish by tonight. 

Soils in this area remain nearly saturated from repeated rounds of
rain across western Washington in recent weeks. Therefore most, if
not all the rainfall today should convert to runoff. Expect 4 to 6
inches of rain, with locally higher amounts west of the Olympics,
and 3-5 inches into the northern Cascades northeast of Seattle. For
the Seattle area, significantly less rain, on the order of a
fraction of an inch is expected, as rain shadowing off the Olympics
will greatly cut into rainfall totals. This will be partially
offset by the urbanization in the area, so the Marginal Risk
remains largely unchanged. The Slight Risk upgrade is focused on
the wettest areas, namely west of the Olympics and the western
foothills of the northern Cascades into Whatcom and Skagit Counties.

...Lower Mississippi and Tennessee Valleys...

Very few changes were needed with this update across the Lower
Mississippi and Tennessee Valleys. A progressive cold front moving
across the middle of the country will continue south and east into
the Southeast today. This evening through tonight, the front will 
run into a strengthening low-level jet (LLJ), which will align
along the frontal interface as it moves into the Mid-South.
Scattered showers and thunderstorms will develop ahead of the front
as a result, which in a few instances may train over the same
areas, but modest Gulf moisture and instability to 2,000 J/kg
should limit storm strength, notwithstanding their fast motion. The
inherited Marginal Risk is largely unchanged, stretching northeast
from northern Louisiana to the southern Appalachians. Soils have
largely dried since the last heavy rain event in this area, so any
flash flooding will be isolated to typical flood prone and urban areas.

Wegman


Day 2
Valid 12Z Mon Mar 24 2025 - 12Z Tue Mar 25 2025

The probability of rainfall exceeding flash flooding guidance is
less than 5 percent.

In coordination with MOB/Mobile, AL and TAE/Tallahassee, FL
forecast offices, the inherited Marginal Risk for portions of the
central Gulf Coast was downgraded with this update. The front
moving across the Southeast will continue across the Gulf Coast
Monday morning, and is now largely expected to continue into the
northern Gulf, as well as towards the Georgia coast during the day.
Any storms from the overnight period Monday morning will continue
eastward and remain progressive enough that training is unlikely.
There will remain some Gulf moisture and instability, but these too
will be modest, and generally unsupportive of the heavy rainfall
rates needed to overcome the high FFGs along the Gulf Coast. With
areal rainfall now generally under a half inch, and in many areas
under a quarter inch (outside of any heavier rain cores), and the
expected continued movement of the front and associated storms, the
flash flooding potential has continued the trend of decreasing from
previous days. For these reasons, the Marginal was dropped.

Wegman


Day 3
Valid 12Z Tue Mar 25 2025 - 12Z Wed Mar 26 2025

The probability of rainfall exceeding flash flooding guidance is
less than 5 percent.

Very late Tuesday night (generally from around 06Z Wed on), the
collision of return flow off the Gulf in the form of 15-25 kt SSW
flow over Texas will run into a slow-moving warm front along the
Red River. Model guidance has hinted for a few days now that an
area of storms, possibly a small MCS, will develop over either
southeast Oklahoma or northeast Texas along this front. While there
has been model consistency, the area of storms remains small, and
somewhat uncertain as to where it will set up. This area of the
country has been very dry of late, which too precludes much
flooding. Given the small area that would be impacted, the area
remains without a Risk area for now, but will continue to be
monitored for possible introduction of a Marginal with CAMs
guidance or improved agreement on a wetter forecast.

Wegman

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