FOUS30 KWBC 250832
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
432 AM EDT Tue Aug 25 2026

Day 1  Valid 12Z Tue Aug 25 2026 - 12Z Wed Aug 26 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN NORTHEAST
FLORIDA THROUGH THE GEORGIA COAST...

In coordination with WFO JAX, a SLGT Risk was introduced along the
I-95 corridor in Northeast Florida through the coast along Georgia,
with heavy rainfall in the form of seabreeze thunderstorms expected along
the coast. Low-level flow suggests an axis of convergence situating
along the coast, with an onshore flow component also aiding as a
lifting mechanism. The latest run of global models are in agreement
in terms of PWATs forecast to exceed 2", with the American globals
(NAM & GFS) suggesting PWAT value could approach 2.25", fueling
efficient thunderstorms. HREF and RRFS suggest medium-to-high
chances (65 and 55% respectively) of hourly rainfall rates
exceeding 2". Also noted was an uptick in neighborhood FFG
exceedance probs, particularly in the 3hr & 6hr FFG guidances, with
now low chances (~25-30% in both the HREF & RRFS) of FFG exceedance.

...Southwest to Central Plains...

Showers and thunderstorms will once again redevelop across the
central and southern Rockies, albeit less expansive than previous
days. An upper-level ridge will continue to remain situated over
much the Southwest, expanding westward, which will inhibit moisture
from moving into the southern portions of the Southwest. In the
Central Plains, a shortwave trough sliding along the northeast
periphery of the aforementioned ridge will support a complex of
thunderstorms in the late afternoon and evening. Latest CAMs
guidance suggest a quasi-linear structure which is expected to be
fairly progressive. Thus, while rain rates could exceed 1"/hr at
times, storms will be very short-lived, and as such, the MRGL was maintained.

Elsewhere, heavy rainfall is expected to occur over the southern
portions of Louisiana in association with an offshore wave. An
introduction of a MRGL area was considered in this area, but
ultimately was not added in coordination with WFO LIX due to high
FFGs, and the region largely consisting of difficult-to-flood wetlands.

Blanco-Alcala
 

Day 2  Valid 12Z Wed Aug 26 2026 - 12Z Thu Aug 27 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHWEST AND ROCKIES TO THE CENTRAL PLAINS AND THE LOWER
MISSISSIPPI VALLEY...

...Lower Mississippi Valley to the Tennessee Valley...

A shortwave trough is expected to ride the eastern periphery of a
ridge located in the southwestern U.S. and generate showers and
thunderstorms of the Central Plains and Lower Mississippi Valley.
Global models have begun to hone in the Ozark/Lower Mississippi
Valley for the heaviest QPF core. PWAT values in the Lower
Mississippi Valley are expected to exceed 2", particularly in
Alabama and parts of eastern Louisiana. At the surface, a
stationary boundary will remain in place as an influx of deepening
moisture converges along the boundary. Further south, a continued
influx of moisture from the Gulf will move north, further
amplifying convergence of moisture. While global models are
beginning to come into a bit more agreement as to the placement of
the heaviest precipitation, spread is still high enough (spanning
from northeast Arkansas to western Alabama) to limit confidence. As
CAMs begin to become available, an upgrade to a SLGT may become necessary.

...Southwest and Rockies to the Central and Southern High Plains...

Diurnal thunderstorms over the southwest will continue to be the
theme, although a southwest upper-level ridge will continue to
move moisture away from the Arizona and New Mexico area. As such, the
inherited MRGL Risk area was merely adjust to match the latest
suite of model guidance. Further east, a westward shift to the
aforementioned ridge will bring a return of deeper moisture over Southern
Rockies and High Plains. Convective activity will recommence in
this area during this period and could bring isolated instance of
flash flooding.

Blanco-Alcala
 

Day 3  Valid 12Z Thu Aug 27 2026 - 12Z Fri Aug 28 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL SOUTHEAST, MID-
ATLANTIC, AND PORTIONS OF THE SOUTHWEST AND ROCKIES...

Showers and thunderstorms capable of producing heavy rainfall will
focus along a stationary front extending over the Southeast US/Gulf
coast region into parts of the Mid Atlantic. Height falls
associated with weak shortwave energy will embedded within a
broader trough will aid and focus the convection in vicinity of the
front. In addition, the approach of a front in the northern stream
trough will tap into some deeper moisture being drawn northward
ahead of the above-mentioned front to bring an isolated threat of
flooding on Thursday before the northern stream front sweeps off
shore. Latest run of global models indicate a surprising amount of
agreement regarding a QPF concentration located over the Lower
Mississippi Valley, specifically Louisiana, Mississippi, Alabama,
and the Florida panhandle. High FFGs, and an area characterized by
wetlands ultimately resulted in an upgrade to a SLGT not being
introduced. Future updates may warrant the need for a SLGT upgrade,
especially if trends continue to hone in on the aforementioned region.

Further west, across portions of the Southwestern US, monsoonal
moisture will again fuel diurnally driven widely scattered
convection covering portions of Arizona and New Mexico. Given the
plume of precipitable water values nosing back into to the Arizona
desert on the western side of a broad mid- and upper level high.
Local downpours leading to instances of flash flooding are
possible. As such, the MRGL Risk area over the Southwest was merely
adjusted to match latest run of global model and ensemble guidance.

Blanco-Alcala/Otto
 

Day 4 and Day 5  Valid 12Z Fri Aug 28 2026 - 12Z Sun Aug 30 2026

...THERE ARE MULTIPLE MARGINAL RISK AREAS ACROSS PARTS OF THE
WESTERN STATES...SOUTHEAST/MID ATLANTIC AND UPPER MIDWEST/NORTHERN
PLAINS EACH DAY...

...Southwest and Intermountain West...

Maintained a Marginal Risk both days across portions of the
Southwestern US as monsoonal moisture fuels diurnally driven
widely scattered convection covering parts of the desert Southwest
and the Intermountain region. A plume of precipitable water will
continue to get drawn northward on the western side of a broad
mid- and upper level high. This could lead to isolated flash
flooding on Friday and Saturday...with favored terrain, urbanized
areas and burn scars being the most prone to flooding.

..Southeast to Mid-Atlantic Region/Southern New England Coast...

Showers and thunderstorms capable of producing heavy to excessive
rainfall each day should be focused along a quasi-stationary front
extending over the Southeast US/Gulf coast region into parts of the
Mid Atlantic region. Height falls associated with weak and nearly
impossible to time shortwave energy embedded within a large scale
trough to the north will focus the convection in vicinity of the front.

...Upper Midwest/Northern Plains...
There is a risk of excessive rainfall developing across parts of
the Upper Midwest/Northern Plains on Friday as low level flow of
Gulf moisture is drawn into the region on 30 to 45 kt low level
jet and rides over a surface warm front over the region on Friday.
This area shifts eastward on Saturday as the mid and upper level
flow backs to more southwesterly as the upper ridge axis gets
flattened by shortwave energy entering the Pacific Northwest.

Bann
$$
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