FOUS30 KWBC 140810
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
410 AM EDT Fri Mar 14 2025

Day 1
Valid 12Z 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 strong mid-to-upper level shortwave currently crossing the 
Southwest will move east of the Rockies today, with a negatively- 
tilted low developing over the central Plains early in the period. 
Powerful forcing is forecast to interact with a relatively modest 
plume of moisture advecting north ahead of the system. Storms are 
expected to develop initially over the lower to mid Mississippi 
Valley this afternoon before moving east into the Tennessee and 
Ohio valleys during the evening and overnight. Cell motions will be
quite progressive given the very strong wind fields 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). Still, this 
appears to be a lower-end flood threat, and probably confined to 
localized impacts over more sensitive urban or low lying areas that
are impacted by a series of 2-3 training cells in a short period.

Churchill/Pereira


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

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
TENNESSEE AND OHIO VALLEYS...

...Ohio and Tennessee Valleys...

Even as the previously noted low lifts to the north through the
Great Lakes into Ontario, the upper pattern will remain amplified
across the central U.S. as an upstream shortwave dives southeast
into the southern Plains on Saturday morning and spreads eastward
over the Lower Mississippi Valley into Saturday afternoon/evening.
A surface wave developing along the leading system's trailing cold
front will help focus deeper moisture tapped by strengthening
southerly winds and with increasing upper-level dynamical support
as the southern shortwave approaches. This will help lead to the
continued development and maintenance of an intense line of
thunderstorms pushing eastward along the front. The general
consensus of the most recent 12Z guidance indicates that widespread
accumulations of 2-4 inches centered over parts of the Tennessee
and lower Ohio Valleys are likely (with locally higher amounts). 
A large Slight Risk was once again maintained across the region, 
reflecting the higher probabilities (50-70%) for 2 inches or more 
shown by the GEFS/ECENS (and available CAMs) where this favorable 
combination of deeper moisture, interacting with strong forcing, 
along a relatively slow moving front, is expected to raise the 
potential for heavy amounts late Saturday into early Sunday from 
training storms. This remains a higher-end Slight Risk for parts of
the area, and a more focused corridor with the potential for an 
upgrade to a Moderate Risk with the addition of more CAM data
extending fully into the period later today. The latest 00z 
guidance indicates this potential remains highest from northeast
Mississippi/northwest Alabama northeast through central Tennessee 
into south-central Kentucky where locally higher totals of 5-6" 
could be realized (as the FV3 in particular highlights). However, 
have opted to remain at a higher-end Slight Risk for now as it is
difficult to pin down the targeted area that may need a Moderate
risk upgrade. The threat will be evaluated once again with the 12z
CAM suite with the potential for a targeted Moderate risk upgrade. 

...Pacific Northwest...

Introduced a Marginal risk to portions of the Pacific Northwest
(mostly coastal/western Oregon and a bit of far northwestern CA)
with a notable uptick in QPF with this forecast cycle, suggesting
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 (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 from 21z Sat well into Day 3. 

Churchill/Putnam/Pereira


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 00z GEFS
and ECENS both show areas with the potential for 2-3" totals, but
they are not in agreement on the regions (GEFS showing potential
for the Mid-Atlantic to Northeast, while ECENS has lower probs
overall and centered from the Mid-Atlantic to the Southeast). 

...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 for a period. Model consensus suggests
an additional 2-4" QPF through Day 3, and that could necessitate a
targeted upgrade to Slight risk in future updates (depending on
forecast consistency and trends in the next cycle or two). 

Churchill

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