Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
400 AM EDT Sat Mar 29 2025

Day 1
Valid 12Z Sat Mar 29 2025 - 12Z Sun Mar 30 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE CENTRAL GULF COAST...

A lingering shortwave trough over the central Gulf Coast will
support a continuation of the decreasing flash flood risk (relative
to the past couple of days), as WPC QPF calls for areal average
totals on the order of 1-3" (concentrated in the vicinity of
coastal MS and adjacent portions of southeast LA). The 00z HREF
indicates surprisingly high odds of 5" exceedance (as high as
20-40% per a 40-km neighborhood method), especially considering
the overall decreasing trend in the forecast over the past couple
of days. Much of this is attributable to convective feedback from
the FV3 (large area of 9"+ totals, and not an uncommon phenomenon
from this particular CAM), but both the ARW and ARW2 also indicate
4-6"+ totals (though both of these models have been less than
stellar with the convection associated with this trough over the
past couple of days). Meanwhile, the 00z HRRR (as well as the NAM-
nest, for what it's worth) depicts much less significant totals
over land, indicating that some of the highest totals stay just
offshore (as well as a secondary maxima well north into central MS
with 2-4" localized totals). All this is to say, spatial
uncertainty remains higher than usual (as depicted by relatively
low probabilities of 2" exceedance, 10-20%, via 10-100 km Ensemble
Agreement Scale neighborhood probabilities). Given 3-6 hr FFGs
mostly in the 4-5" range along the coast (where the most intense
convection is most likely to occur, if it doesn't stay offshore
altogether), the limited inherited Marginal risk area was
maintained for this update (owing to the CAM model trends described
above, with forcing and instability continuing to look fairly
lackluster). Any localized flash flood threat should mostly be
confined closer to the Gulf Coast (and mostly limited to more
sensitive urban areas, which could be locally significant if
impacted by 5"+ totals).

Churchill


Day 2
Valid 12Z Sun Mar 30 2025 - 12Z Mon Mar 31 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE MISSISSIPPI, TENNESSEE, AND OHIO VALLEYS...

A much broader inherited Marginal risk area (relative to Day 1)
has been largely maintained, as a pair of shortwave troughs
emerge from the Intermountain West into the Central U.S. on
Sunday, eventually partially phasing with the polar jet (and an
associated longwave trough) by Sunday night. This complex
interaction is expected to result in a broad area of convection
across much of the eastern half of the CONUS, including portions of
the MS, TN, and OH Valleys. Despite the complexity of the synoptic
and mesoscale details, models remain in fairly good agreement in
indicating convection breaking out across a broad warm sector
(characterized by PWATs of 1.5-2.0", SB CAPE of 1000-2000 J/kg, and
effective bulk shear of 40-60 kts) supportive of discrete and
multi-cell clusters initially, and likely transitioning to more a
more linear QLCS orientation along the front into the evening (as
best depicted by the 00z FV3 and NAM-nest, which are the only CAMs
which extend through the full period at 00z). While WPC areal
average QPF calls generally for only 1-2" totals, most
deterministic solutions from the global models are likely well
underdone on localized totals, as CAMs will be needed to better
model the discrete convective initiation that is anticipated prior
to the main QLCS passage (with the FV3, NAM- nest, and CMC-reg all
indicating the potential for 3-5" totals, with probabilities
likely maximized in the vicinity of the Mid-South and MS Delta
regions where instability and moisture are more ideal... and this
is where a targeted Slight Risk is most likely to be introduced in
future updates, possibly as early as later today).

Churchill


Day 3
Valid 12Z Mon Mar 31 2025 - 12Z Tue Apr 01 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHEAST...

The aforementioned pair of shortwave troughs are progged to fully
separate by Day 3, as the northern stream shortwave continues to
phase with the polar jet (and associated parent longwave trough)
into the Mid-Atlantic/Northeast, while the southern shortwave
likely hangs back into the Mid-South and Lower MS Valley. This is
anticipated to result in a weaker setup overall for excessive
rainfall on Monday into Monday night, as the best dynamics and
upper-level forcing over the Northeast and Mid-Atlantic is
displaced from the best moisture and instability over portions of
the Southeast (where forcing from the secondary shortwave should
still be sufficient for organized convective activity). The
inherited Marginal risk was maintained for this portion of the
Southeast (including much of AL/GA with surrounding portions of
TN/NC/SC and MS), where GEFS/ECENS exceedance probabilities suggest
the best potential for 2-4" localized totals. Farther northeast
into the Mid-Atlantic and southern portions of New England, while
total tropospheric moisture will certainly be lower (~1.25" versus
1.5"+), these values are either equally or more anomalous than in
the Southeast (90th percentile or higher). Despite this, have
decided not to introduce a Marginal risk at this time, due to the
combination of marginal instability and progressive storm motions.

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