723
FOUS30 KWBC 190817
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
417 AM EDT Tue Aug 19 2025

Day 1
Valid 12Z Tue Aug 19 2025 - 12Z Wed Aug 20 2025

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR A PORTION OF
THE SOUTHERN APPALACHIANS...

...Southern Appalachians...
Old frontal boundary strengthened by `cold` air damming,
northeasterly flow has pooled enhanced low to mid-level moisture
through the eastern slopes of the Southern Appalachian mountains.
Another day of slow southward sag of the boundary, clearing skies
for ample insolation across the terrain and strong upslope should
trigger solid convective response through early afternoon. A narrow
mid-level ridge to the northwest will keep weak mid to upper level
steering in place to allow for slow cell motions with potential of
continued upslope convergence resulting in scattered clustering
and potential cell mergers. Hi-Res CAMs continue to paint the
eastern slopes of the NC and NE GA Appalachians with 2-4" totals
resulting in HREF neighborhood probability of 3" at 40-50% and
indications of 5"+ at 20%. Total moisture is not overly anomalous,
but is still above average, though persistent upslope flux in
complex terrain suggests a small Slight Risk of Excessive Rainfall
appears prudent at this time.

...Coastal Plains of Southern Maryland to Georgia...
Reinforcing cold front has stalled across the southern Mid-
Atlantic Monday that has pooled enhanced moisture near the mouth of
the Chesapeake Bay through the Coastal Plains of the Carolinas,
connecting back to an older cold front and pool across east-central
GA. Total Pwats of 2" are pooled along the boundary. As such, this
will tighten moisture/theta-E gradients and with ample early
morning insolation should make for an unstable convective
environment by afternoon. Additionally, slow approach and influence
of Hurricane Erin, will maintain/tighten moisture/instability
gradient and strengthen sea-breeze development. Given strong low
level convergence; weak,parallel to coast mid to upper level
steering; especially near intersection with front extending out
from SE VA, localized rates of 2-2.5"/hr are probable and with deep
layer steering generally parallel to the Coastal Carolinas, some
short- term repeating is possible before propagating inland
resulting in some localized totals of 2-4" with highest
probabilities further north in SE VA/NE NC.

...Great Lakes to eastern Central Plains...
Mid to upper level progression depicts ridge building across the
Northern Plains as broader shortwave feature rolls into a weakness
across the Great Lakes later today (Tuesday); with the amplifying
ridge, further elongation of a positive trough weakness expands
southward toward the Ozark Plateau and eastern Central Plains. The
associated surface low and cold front will progress through the
central Great Lakes into the Northeast toward end of day supporting
relatively stronger moisture flux along it and through the eastern
Midwest/Northern Ohio Valley with slightly above average PWat
values of 1.5-1.75"; locally higher to provide ample moisture for
efficient rainfall and localized scattered to widely scattered
flash flood risk. Deeper layer moisture is slightly reduced further
southward along the sagging frontal zone into the Mid-MS River
Valley and Ozarks, but to counter-act the reduction in
coverage/rainfall efficiency; cell motions will be slower and more
chaotic in nature to allow for increased duration/random mergers
to occur than farther north across W NY/MI/OH and IND. Hi-Res CAM
signals remain on course for random/scattered incidents of 2-3"
totals consistent with a broad Marginal Risk area across the Great
Lakes/Ohio Valley into the Central Plains. This remains on track to
prior forecasts, with only small adjustment to connect risk areas
to the Marginal Risk across the Southern High Plains and western
Central Plains (see below).

...Southwest to Southern High Plains...
As noted in section above, broad 1.5 to 2 standard anomaly ridge
continues to dominate the central Rockies, spreading into the
northern High Plains. This shift allows for shortwave `ridge-
rider` energy to further erode and retrograde along its southeast
quadrant. As such, another day of active thunderstorms within an
above average low-level moisture plume will unfold centered over
NM. Hi-Res CAMs and HREF show highest signal along the early
afternoon upslope/convergence along the Sangre de Cristo,
Sacramento, Mogollon and Black Ranges. Eventually spreading out
with scattered to widely scattered slow moving pulse cells across
the southern Central Plains and southern High Plains into the
evening hours. High rainfall production and slow/stationary cell
motions will result in scattered instances of 2"+ resulting in
widely scattered possible flash flooding consistent with a Marginal
Risk of Excessive Rainfall.

Gallina


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).
For now the Marginal is expanded per 00Z guidance consensus
(connecting to the previous Marginal on the Appalachians) with a
note that there very well may be upgrades needed once the target is
more clear.

...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. The Marginal Risk is retained.

...Eastern North Carolina...
Moisture plume from Erin with 2" PW converges on surface trough
over far eastern NC providing a corridor of enhanced rainfall.
Also, uncertainty in the track of Erin may bring the outer bands
close enough to warrant an excessive rainfall risk. This rainfall
looks to be on the order of a couple inches, but also comes during
an approaching new moon with swell that may locally worsen
drainage. A Marginal Risk is introduced for the Outer Banks.

Jackson


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 with some overall expansion 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. For now, there Marginal Risk over the eastern Dakotas
and much of MN is expanded south to include more of SD per the 00Z
RRFS and Canadian Regional output.

...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.

Dolan/Jackson


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