FOUS30 KWBC 030826
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
426 AM EDT Sun Aug 3 2025

Day 1
Valid 12Z Sun Aug 03 2025 - 12Z Mon Aug 04 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR PORTIONS OF
THE SOUTHEAST/GULF COAST AND SOUTHERN PLAINS...

...Southeast/Gulf Coast...

The cold front will continue to waver along the Gulf Coast as a 
stationary front in response to subtle additional height falls as 
the trough extending from the Northeast continues to subtly 
amplify. The trough axis may reach the Gulf Coast by evening, but 
flatten at the same time in response to an elongated ridge draped 
across the Gulf. A constant pool of anomalous PW values (above 
2.25 inches with deep column saturation) noted via moist- 
adiabatic lapse rates through the depth of the column indicating 
tall-skinny CAPE which is progged to reach above 1000 J/kg during 
peak heating. Convergence along the front into these thermodynamics
will support another day of slow moving convection noted by 0-6km 
mean winds of just 5-10 kts pointed to the east, with rainfall 
rates likely exceeding 2 inches/hour at times (HREF 50-60% chance) 
supported by warm cloud depths nearing 15,000 ft to produce 
efficient warm- rain collision processes.
 
Shortwave energy is expected to rotate beneath the trough, tracking 
from the Florida Panhandle into eastern Georgia. This features may
support an increase in areal coverage of convection and possibly
organizing into clusters/multi-cells. With weak storm motions in 
place, any collisions/organization during this time could briefly 
pulse up rain rates even more significantly, while also leading to 
longer temporal duration or repeating rounds of these rain rates. 
Where this occurs, the HREF and REFS both indicate a 70-90% chance 
of 3 inches/24 hours and locally a 40-70% chance of more than 5 
inches/24 hours. These accumulations are forecast to occur where 
recent rains have been heavy and increased soil saturation. The 
inherited Slight Risk was expanded west/northwest to cover more of
central/eastern Alabama and western Georgia.

Otherwise, scattered thunderstorms with intense rain rates could
produce instances of flash flooding. Models are depicting a 
secondary area of maximum rainfall potential along a westward 
advancing surface trough through northern Alabama into central
Tennessee which may require an upgrade in the ERO with later 
issuances since 3 inches/24hr probabilities peak at 40-50%, but 
this correlated with 3-hr FFG exceedance that is just 10-15%. 

...Plains...

The stationary front stretching from Montana to Texas is expected
to weaken, leaving behind a weak convergence boundary as it 
decays. Mid-level flow upstream of this feature will gradually 
become more W/NW as it gets squeezed between an amplifying ridge 
over the Southwest and a trough across Canada. Within this pinched 
flow, multiple shortwaves/vorticity impulses will shift eastward, 
interacting with the front to cause another day of convective 
development across the High Plains and into the Plains, with 
several MCSs possible beneath any of the stronger impulses.

The exact location and track for any of the MCSs that develop
remain uncertain however there is a decent signal for storms to
have rainfall rates of 1-2 inches/hour. While the CAMs are 
generally suggestive of forward propagating features that will 
limit the duration of heavy rainfall, locally any of these could 
produce some short-term training, especially where any MCVs and 
lingering boundaries can track. There are hints that a narrow swath
of higher QPF will streak from the Dakotas to Minnesota but the
spread was too great to consider a Slight Risk upgrade at this but
may be considered for updates later today. The Slight Risk over
Texas and southwest Oklahoma was maintained as it highlights the
areas where there will be an elevated threat for flash flooding.

Campbell/Weiss


Day 2
Valid 12Z Mon Aug 04 2025 - 12Z Tue Aug 05 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL FOR THE SOUTHEAST...

...Southeast...

A surge in moisture ahead of the longwave trough axis will 
maintain unsettled conditions and widespread showers/thunderstorms 
from southern Alabama through Georgia and coastal South Carolina.
While there is a degree of uncertainty, a signal arises for an 
axis of heaviest QPF to streak across the Gulf Coast into Georgia.
PW values likely continuing at 2-2.25 inches overlapping MUCAPE of
more than 1000 J/kg, rainfall rates of 1-2+ inches/hour are 
likely within any convection that develops and then tracks 
northeast. This setup may be conducive for some repeating rounds. 
A Slight Risk was hoisted for this period for the Florida Panhandle
and west-central Georgia.

...Northern Plains...

The closed mid-level low from the Day 1 period will continue on is
northeastward track as it weakens, leaving a lingering boundary in
its wake. Local rainfall rates in excess of 1 inch/hour are 
possible, with a secondary surge in convection possible late in 
response to 850mb winds reaching 20-30 kts converging into the 
region. Rain accumulations during the Day 1 periods will likely
lower FFG across the region, thus there will be isolated excessive
rainfall impacts.

...Northern Rockies...

During this period a shortwave advancing through the Intermountain
West will aid in the amplification of a trough shifting over the
Northern High Plains. In response, this will drive enhanced ascent
into the already broad synoptically forced lift, helping to spawn 
a weak wave of low pressure to track into the High Plains by late
afternoon/evening. Fairly progressive showers and thunderstorms
will move from the Northern Rockies to the Northern High Plains.
PW anomalies of +1 to +1.5 sigma above the climo mean is forecast,
which may lead to rainfall rates reaching 1 inch/hour sporadically
thus increasing the threat for local flash flooding concerns.

Campbell/Weiss


Day 3
Valid 12Z Tue Aug 05 2025 - 12Z Wed Aug 06 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL IN THE SOUTHEAST...

...Southeast...

Gulf moisture will continue to surge north/northeast over the
stalled west-east orientated frontal boundary. Showers and
thunderstorms will continue across much of the Southeast with the
highest QPF expected to focus over eastern Alabama and central
Georgia where accumulations of 2 to 3 inches will be possible. This
multi-day event will result in lowered FFG and increased threat for
flash flooding. A Slight Risk area was raised for this period
covering eastern Alabama and much of central/northern Georgia.

...Northern Plains and Minnesota...

The upper-level trough and surface frontal system mentioned during
Day 2 will continue to track across the northern tier, shifting 
showers and thunderstorms to the east. Guidance is showing areal
averages of 1 to 2 inches across the region with isolated maximums
possibly up to 3 inches, with the higher totals closer in 
proximity to the International border. A broad Marginal Risk was 
maintained for this period. First guess fields hint a small area 
may need a Slight Risk, but confidence on where is low at this time
so opted to stick with just a Marginal for this issuance.

Campbell
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