665
FOUS30 KWBC 200058
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
858 PM EDT Tue Aug 19 2025

Day 1
Valid 01Z Wed Aug 20 2025 - 12Z Wed Aug 20 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FROM PARTS OF THE
LOWER GREAT LAKES REGION TO THE SOUTHWEST AND ACROSS PARTS OF THE
SOUTHEAST...

01Z Update...
Primary updates to the ERO were based on current observation
trends and recent hi-res guidance runs and included the removal of
the Slight Risk area centered over the southern Appalachians and
the Marginal Risk area over southern Texas. Activity across these
areas has diminished or moved out, with no additional widespread
redevelopment expected overnight.

Elsewhere, in addition to lingering monsoon activity in the
Southwest, scattered activity is expected to continue ahead of a
surface-to-low level boundary that currently extends from the lower
Great Lakes region back through the Ohio Valley and Ozarks into the
southern Plains. PWs remain high (1.75-2 in) along much of
boundary, fueling the potential for heavy rates. Overall, activity
is expected to wane through the evening with the loss of daytime
heating. However, locally heavy amounts resulting in isolated
runoff concerns cannot be ruled out. Therefore, the previous
Marginal Risk was maintained, but adjusted based on recent guidance
and radar/satellite trends.

In the Southeast, reduced the previous Marginal Risk to a small
portion of central Georgia and the South Carolina Lowcountry, where
slow-moving storms fueled by ~2 inch PWs continue. Any runoff
concerns produced by these storms are expected to be short-lived as
they too are expected to continue diminishing over the next couple
of hours.

Pereira


Day 2
Valid 12Z Wed Aug 20 2025 - 12Z Thu Aug 21 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL IN EFFECT FROM
SOUTHERN NEW ENGLAND DOWN THE APPALACHIANS, THE MID-SOUTH AND TEXAS,
AS WELL AS EASTERN ARIZONA...

...Northeast...
Stationary front lingering over the northern Mid-Atlantic looks to
form a convergent focus to moisture streaming north ahead of
Hurricane Erin, particularly Wednesday night. This front and
enhanced moisture (The 00Z ECMWF retains PW around 2"), along with
the area from Long Island/NYC to southern New England in
a right entrance region of a Wly jet that extends over the Gulf of
Maine continue to point to a possible predecessor rain event (PRE).

With the 12Z guidance, there remains some signal for a PRE over
portions of the Northeast into southern New England, with some of
the hi-res guidance indicating pockets of locally heavy rainfall.
However, there remains considerable uncertainty on the placement
and development of this, so there wasn`t enough confidence to
introduce any higher risk areas to the ERO for now, but something
that bears watching in the next update.

...Southern Plains through the Appalachians...
Slow migrating cold front is the continued focal point for
convective development Wednesday leading to widely scattered
thunderstorms capable of localized flash flooding. Environmental
conditions are conducive for pockets of heavy rain that would
cause some flash flood concerns in urban centers of the Southern
Plains through the Appalachians. Enhanced rainfall is expected in
the mountains where complex terrain and convergent flow will aid in
upslope forcing capable of slow- moving cell motions and heavy
rainfall. A broad low-end Marginal persists over a broad area with
potential for enhanced areas pending how the broad synoptic and
localized mesoscale (boundaries) pattern evolves.
The Marginal risk was expanded west through the Sacramento
Mountains where CAMs are once again focusing enhanced diurnal
convection.

...Southwest...
Monsoonal moisture presence (1.5 sigma above normal) and general
diurnal instability will aid in traditional afternoon and evening
convective output across east and southeast AZ into west-central NM
on Wednesday and the inherited Marginal Risk was maintained.

...Eastern North Carolina...
Moisture plume from Erin with 2" PW converges on surface trough
over far eastern NC providing a corridor of enhanced rainfall, most
likely across the Outer Banks. There`s enough of a signal for the
outer rain bands to produce locally heavy rain to keep the Marginal
Risk in place, with only minor adjustments this cycle. The most
likely scenario is for 1-2 inches.


Jackson/Taylor


Day 3
Valid 12Z Thu Aug 21 2025 - 12Z Fri Aug 22 2025

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE
SOUTHWEST, SOUTH-CENTRAL TEXAS THROUGH THE SOUTHEAST, AND THE
NORTHERN PLAINS INTO MINNESOTA...

...Southwest...
Lingering monsoonal moisture (PW one sigma above normal) and
diurnal instability will trigger afternoon and evening showers and
storms across most of central and eastern Arizona, southern Utah,
and northwestern New Mexico. Heavy precipitation should be
localized, but could cause flash flooding concerns for sensitive
areas (steep terrain, burn scars, urban areas). The Marginal Risk
is retained for the above mentioned areas for this isolated flash
flood threat.

...South-Central Texas through the Southeast and Southern
Appalachians...
A slow-moving frontal boundary will sag farther into the southern
states on Thursday and continue to serve as a focus for scattered
showers and storms from south-central TX to western NC. Diurnal
heating/instability and a broad swath of anomalous moisture will
support locally heavy rain that could lead to isolated instances of
flash flooding, especially in urban, poor drainage, and/or low-
lying areas. Overall, convective organization looks like it will be
lacking, but heavy rain may be more concentrated over the southern
Appalachians with upslope enhancement. Furthermore, the greatest
moisture anomaly is over Louisiana, so that area may be a target
for an upgrade should it become apparent in CAM solutions. A broad
Marginal Risk is in place to cover this activity from the southern
Plains to the southern Appalachians with some expansion east to the
central Carolinas.

...Northern Plains and Upper Midwest...
An energetic upper-level shortwave is forecast to move east across
the northern Plains and Upper Midwest Thursday afternoon and night.
This will be accompanied by a strong surface cold front that moves
through moisture and instability to support at least locally heavy
rain rates with showers and storms. The front should be fairly
progressive, limiting heavy rain potential, but convective mode
will play a role in determining heavy rain/flash flood threats in
this area.

...Eastern North Carolina...
Moisture convergence from Erin and possibly outer bands continue
over the Outer Banks into Thursday warranting a continuation of the
Marginal Risk from Wednesday, at least for the first half of the
day on Thursday before any rain bands move well off-shore.

Dolan/Jackson/Taylor


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