FOUS30 KWBC 142026
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 PM EDT Fri Mar 14 2025

Day 1
Valid 16Z Fri Mar 14 2025 - 12Z Sat Mar 15 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE LOWER TO MID MISSISSIPPI VALLEY INTO PARTS OF THE OHIO AND
TENNESSEE VALLEYS...

A deepening mid-level upper low will move northeastward today from
the southern Plains and over the Middle Mississippi Valley. At the
same time, strengthening southerly flow will help to advect a
moist, unstable airmass increasingly northward and ahead of an
eastward moving cold front. Storms are expected to rapidly develop
along the cold front by late afternoon possibly as far west as the
Middle/Lower Missouri Valley before increasing in coverage as the
front reaches the Middle Mississippi Valley this evening, with
eventual development also extending southward into the Tennessee
Valley. Both the front and accompanying storm motions are expected
to remain rather quick moving given the strong dynamics in place.
Therefore, this convection will almost certainly pose more of a
severe risk than a flash flood threat. However, given the strong
dynamics, along with sufficient instability for deep convection,
expect peak rainfall rates will be briefly intense enough
(~1-2"/hr) to produce localized amounts that exceed current model
forecasts (2-3 inches over relatively short period) which could
lead to some isolated flash flooding particularly in urban areas.
There is also an increasing signal in the guidance that some
additional storms may develop ahead of the front as an 850 mb jet
ramps up from the Lower Mississippi Valley into the Tennessee
Valley in the evening/overnight hours. These storms would also pose
a threat for some locally heavy rain rates and the potential for a
quick 2-3 inches of rainfall.

Putnam


Day 2
Valid 12Z Sat Mar 15 2025 - 12Z Sun Mar 16 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF 
THE TENNESSEE VALLEY...

...Ohio and Tennessee Valleys...

The upper-level pattern will remain amplified Saturday across the 
central U.S. as one upper-low lifts northward into Canada while a 
second upstream shortwave/upper jet dives southeast into the 
southern Plains on Saturday morning and spreads eastward over the 
Lower Mississippi Valley/Southeast into Saturday afternoon/evening.
A surface wave developing along the leading system's trailing cold
front will help focus deep Gulf moisture tapped by a very strong 
850 mb jet while supported aloft by increasing upper-level 
divergence ahead of the approach of the upper jet. This will help 
lead to the continued development and maintenance of both open warm
sector convective development along the leading edge of the moist 
warm air advection/850mb jet as well as along the slowing eastward 
moving cold front. The updated 12Z CAM guidance supports the most 
likely corridor of overlap between the warm sector development as 
well as potential repeated/training rounds of storms along and just
ahead of the cold front across portions of the Tennessee Valley 
from northeastern Mississippi east-northeastward into northern 
Alabama, south-central/southeastern Tennessee, and northwestern 
Georgia. This region also coincides with areal average QPF amongst 
the CAM guidance of 3-6", with locally higher totals of 8"+, and 
HREF probabilities of greater than 5" of rainfall over 50%. These 
very heavy rainfall totals, supported by downpours of 2"+/hour, 
will have the potential to lead to widespread, significant 
instances of flash flooding, with an upgrade to a Moderate Risk now
included for the region. While there is greater uncertainty, the 
latest 12Z guidance also shows support for similar totals further
northeastward into portions of south- central/southeastern Kentucky
as well as further eastward into western North Carolina, which may
require an expansion of the Moderate Risk if confidence increases.
Areas of south-central/southeastern Kentucky as well as western 
North Carolina also remain more sensitive to additional rainfall. 
Regardless, a broader area of average rainfall totals of 2-3", 
locally higher, will contribute to additional scattered to 
potentially numerous instances of flash flooding across the 
Tennessee and Ohio Valleys within the Slight Risk. 

...Pacific Northwest...

A Marginal Risk remains in place for portions of the Pacific 
Northwest (mostly coastal/western Oregon and a bit of far 
northwestern CA) where guidance continues to indicate the potential
for 2-5" localized rainfall totals along the coast (with the 
highest amounts indicated in the OR/CA coastal border area). This 
is due to a faster/stronger AR reaching into the Day 2 period from 
Saturday evening into early Sunday morning (as an inherited 
Marginal risk has been maintained for this update for Day 3) with 
IVT of 400-600 kg/ms indicated perpendicular or nearly 
perpendicular to the coast that will continue into Day 3. A 
targeted upgrade to a Slight Risk may be necessary on Day 2 and/or 
Day 3 as confidence increases in the timing of the heaviest 
rainfall given it may very well split different portions of both 
periods. 

Putnam/Churchill


Day 3
Valid 12Z Sun Mar 16 2025 - 12Z Mon Mar 17 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE EAST COAST, FROM THE SOUTHEAST TO NEW ENGLAND, AND FOR PORTIONS
OF THE PACIFIC NORTHWEST...

...East Coast...

As the aforementioned low lifts northward into Canada, what's left
is a weakening cold front as the upper-level pattern becomes a bit
less amplified (shortwave over the Southeast lifting a bit and 
cutting off from northern/polar stream). The result looks to be 
scattered localized totals of 1-2" over a large region from the 
Southeast (central FL, southeast GA, and Carolinas) into the Mid- 
Atlantic and portions of the Northeast/New England. The 12z 
GFS/ECMWF as well as GEFS and ECENS both show the potential for 
locally higher totals of 2-3", with more confidence in these totals
being realized over portions of the eastern Carolinas, and then 
less likely further north into the Mid-Atlantic/Northeast/New 
England as well as further south into northern Florida. Regardless,
these totals look to remain somewhat limited as the front begins a
faster progression towards the Atlantic Coast. 

...Pacific Northwest...

The aforementioned AR looks to bring in stronger moisture 
transport into Day 3 with the potential for IVT in the 600-800 
kg/ms range near the OR/CA border region, peaking for a period 
early in the day on Sunday. Model consensus suggests an additional 
2-4" of QPF through Day 3, and a targeted upgrade to Slight risk 
remains possible in the day 3 period (as well as day 2) in future 
updates depending on forecast consistency and greater confidence on
the exact timing of the window of heaviest rainfall, which may not
evenly split the day 2/3 period.

Putnam/Churchill

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