Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
848 PM EDT Mon May 12 2025

Day 1
Valid 01Z Tue May 13 2025 - 12Z Tue May 13 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
SOUTHEAST AND SOUTHEAST FLORIDA...

...01Z Update...

In coordination with MFL/Miami, FL forecast office, the Moderate
Risk was downgraded to a Slight for the I-95 corridor from West
Palm Beach south. Much of the convection in the area has moved
offshore, with only one remaining training band south of Homestead.
HiRes models continue to suggest occasional storms forming across
south Florida overnight, but agreement is poor on their coverage.
Thus, given the expected isolated or widely scattered nature of the
overnight convection, flash flooding should be isolated for most
areas.

Elsewhere, the Slight across the Southeast remains largely
unchanged, though coverage of heavy rainfall from storms is quite
low, so most areas are in a low-end Slight. That said, a few lines
of training storms are locally causing flash flooding, so the
potential for additional line formation and training over other
areas justifies the continuation of the Slight.

The Marginal was trimmed on the southern end across the Southeast
behind the most persistent convection, with the upper low and its
associated forcing moving north, resulting in a sub-Marginal threat
for areas further south.

Wegman

Day 2
Valid 12Z Tue May 13 2025 - 12Z Wed May 14 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE MID-ATLANTIC...

No significant adjustments needed to the Day 2 Update, other than
expanding the Slight further across portions of central MD based on
the latest QPF trends.

---previous discussion---


The slow-moving upper low and occluded surface low will continue
to move north, and showers and storms will focus across the Mid-
Atlantic on Tuesday. Deep moisture ahead of the system will push
PWAT values above 1.5 inches, and there will be enough instability
(MUCAPE > 1500 J/kg) across portions of North Carolina and Virginia
to support high rain rates (1-2 inch per hour), which may rival 1
and 3 hr FFGs. Upslope flow along the Appalachians from southern
Pennsylvania through Virginia will also help to enhance rainfall,
with generally 1.5-2.5 inches of rain expected across this area
with locally higher amounts possible. After heavy rain on Monday,
these areas may be more sensitive to flooding given higher soil
moisture, and isolated to scattered flash flooding concerns are
expected. This will warrant a continuation of the Slight Risk area
from northeastern North Carolina through Virginia and portions of
West Virginia, Maryland, and southern Pennsylvania.

Isolated flash flooding may also be possible for portions of the
Ohio Valley and Appalachians. Moisture wrapping around the northern
side of the low pressure system combined with sufficient
instability near the surface low will support scattered convection
with relatively slow storm motion. Therefore, a Marginal Risk is in
place for these regions.

Dolan

Day 3
Valid 12Z Wed May 14 2025 - 12Z Thu May 15 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL OVER PORTIONS OF
THE NORTHERN PLAINS AND MID-ATLANTIC...

...Northern Plains...

A deep upper level trough will push across the Northwest to the
Plains on Wednesday, resulting in a strengthening low pressure
system in the northern Plains. Stratiform rain with likely some
embedded convection on the northern and western side of the low are
forecast to produce 0.5-1 inches of rain, with locally higher
amounts possible, in eastern Montana and northeastern Wyoming. The
main threat will be due to the persistent nature of the
precipitation in this area, starting up Tuesday night and
continuing through Wednesday into Wednesday night. Rainfall totals
will likely come close to the 6 hour FFGs in eastern Montana.

For the Dakotas, convective precipitation is forecast to develop
ahead of the low along a strengthening frontal boundary. Models are
in good agreement on the axis of heaviest precipitation setting up
over central North Dakota and eastern South Dakota. PWAT values
are expected to be at least 2 standard deviations above normal
(1-1.5 inches), and there should be enough instability (MUCAPE >
1500 J/kg) during the day on Wednesday to support at least an
isolated flash flood threat.

...Mid-Atlantic...

Soils are expected to be thoroughly saturated across the Mid-
Atlantic after heavy rain on Monday and Tuesday, which may create a
low end flood threat with lingering showers on Wednesday. The
slow-moving surface low is forecast to track right over the Mid-
Atlantic with scattered precipitation, and there should be enough
instability near the low to support some convection capable of
producing locally heavy rain. Therefore, a Marginal Risk area was
introduced for portions of the Mid-Atlantic. The northern extent of
the Marginal was nudged further up into portions of southeast PA
and central NJ based on the 12Z guidance.

Dolan/Taylor


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