Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1156 AM EDT Sat Mar 15 2025

Day 1
Valid 16Z Sat Mar 15 2025 - 12Z Sun Mar 16 2025

...THERE IS A MODERATE RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE MID-SOUTH INTO THE TENNESSEE VALLEY...

...16Z Update...

The shortwave trough over Texas and Oklahoma, which is south of
the primary low circulation now over Wisconsin continues to drive
much of the convection, severe, and flash flooding across the Lower
Mississippi and Tennessee Valleys this morning. This shortwave is
stronger than much of the previous guidance had suggested, which in
turn has kept the primary area of storms that stretches from
Louisiana to western Tennessee as of this writing further west.
Thus, the heaviest rainfall totals this morning have been along the
Mississippi River, with a secondary area of storms over northern
Alabama into portions of eastern Tennessee.

As the shortwave trough continues to get its act together, it will
begin to shift eastward, resulting in the line of storms also
moving east, with increasing forward speed. This in turn will
gradually diminish the flash flooding threat as the line of storms
will not have time to drop prodigious amounts of rain in these more
eastward areas of Alabama, Georgia, and the Southern Appalachians.
Given this, ERO upgrades were proposed along the Mississippi River
where rainfall amounts over 3 inches in far southeastern Arkansas
have already occurred, and the storms will continue to impact the
area for another few hours before the significant eastward shift of
the line begins. Conversely, because the storms have stayed
further west than expected, there has been less rain into the
southern Appalachians as the stronger storms with the eastward
extent of the rain shield arm have stayed further west towards
Chattanooga, and impacted the mountains far less. While all of
these areas will see the main line of storms go through later today
into tonight, the lack of prior rainfall should reduce the flooding
impacts from the line significantly. For western North Carolina,
there should still be an upslope component to the southerly flow
along the line of storms as it moves through late tonight, which
may locally enhance the rainfall as well as begin the rain sooner
due to the added lift. The Slight Risk for this area has been
maintained, while the Moderate for extreme southwest North Carolina
and northern Georgia was downgraded with this update.

The 12Z HREF guidance suite has also followed suit with the
westward shift in the rainfall. A higher end Moderate has been
extended southwestward to include northwestern Mississippi along
the Mississippi River, Memphis, and extends northeastward into the
Nashville area. Neighborhood probabilities in this area have
increased to as high as 80% for 5 inches or more of rain. However,
with the chances for 8 inches or more of rain only at 10-20% at
most, we remain shy of criteria necessitating a High Risk upgrade.

...Pacific Northwest...

No changes were made to the previous forecast. The heaviest rain
will begin late this afternoon and persist through much of the
night. With most of the expected rainfall capable of isolated
flash flooding not having begun yet, there was no need determined
for any changes.

Wegman


Day 2
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 still 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 (though some
2-3" totals are possible in the coastal Carolinas region). The
Marginal risk was maintained as these relatively limited totals
should only present a localized flash flood risk 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 2 period 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 2, and a targeted upgrade to
Slight risk remains possible (depending on the evolution of the
forecast QPF in the Day 1 period).

Churchill/Putnam


Day 3
Valid 12Z Mon Mar 17 2025 - 12Z Tue Mar 18 2025

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

Churchill


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt