437
FOUS30 KWBC 060057
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
857 PM EDT Tue Aug 5 2025

Day 1
Valid 01Z Wed Aug 06 2025 - 12Z Wed Aug 06 2025

...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE SOUTHEAST AND INTO THE MID-ATLANTIC AND IN THE UPPER MIDWEST...

01Z Update...Few changes made to the recently-added Slight Risk
area in parts of the Upper Midwest/Northern Plains. 00Z sounding
from Aberdeen showed 40 to 45 kt southerly low level winds feeding
directly into a complex that has a history of 3 to 6 inch rainfall
amounts. The changes which were made were based on trends in
satellite and radar imagery.A baroclinic leaf seen in water vapor
satellite imagery was crossing the Marginal risk area placed over
the Ohio Valley...which was resulting in an uptick in showers and
a few thunderstorms. Felt is was a bit too early to remove the
Marginal risk area but the expectation is that convection should
weaken an dissipate by late evening or the early-morning hours.
Likewise...convection should gradually weaken and decrease in areal
coverage this evening across parts of the Southeast US but enough
upper support and low level flow of moisture precludes removal of
the Slight risk area altogether. There was enough support to break
the broader region into two distinct areas...one being parts of
coastal Carolinas and the second being near the Gulf coast.

Bann

...Southeast and Mid-Atlantic...

16Z UPDATE: Most changes for this outlook were made to the Slight
Risk area over the Southeast. They were not big overall, but it
did recenter the risk area a bit to the northeast, expanding it
about 70 miles to the northeast, and trimming about 25 miles off
the northwest side. The ongoing round of convection in central
South Carolina has progressed further northeast than guidance over
the past several cycles had indicated, and 12Z hi-res models show
an additional round of convection developing overnight from central
North Carolina into central South Carolina. The overnight round
would be due to a restrengthening of a LLJ and the unstable air
mass expanding inland as a result. No other change in the reasoning
described in the previous discussion; a complex pattern of cell
mergers and colliding boundaries may allow storms and heavy
rainfall to maximize in a couple localized areas, with potential
for greater than 5 inches of rain in a short period of time.

The 12Z HRRR shows a scenario late tonight that would be of
greater concern: a training and backbuilding convective band within
30 miles or so of the Raleigh-Durham metro area, which leads to
locally extreme rainfall. Other models show hints of a similar
scenario but do not hit similar rainfall extremes. This will need
to be monitored in the event of greater consensus or observational
trends, in case probabilities may need to be increased.

PREVIOUS DISCUSSION: A conducive overall setup for locally heavy
downpours exists, with a low-level front/convergence zone and the
right entrance region of a strong upper-level jet streak
superimposed over the same area (along with anomalously high
moisture and PWs over the 90th percentile). The main limiting
factor for organized heavy rainfall is certainly instability,
though a modestly strengthening low-level jet (15-25 kts) will
provide some isentropic upglide to kick things off this morning
(with hourly totals likely limited to 1-2"). Most hi-res models
depict a break in the precipitation during the day, but another
round may ensue later tonight over some of the same areas, as the
frontal zone and parent trough to the west move very little.

Further diurnally-driven development is expected within the broader
warm sector from the central Gulf Coast to south-central Georgia,
and the rest of South Carolina, with stronger instability building
today with daytime heating and PWs in excess of 2 inches. Hi-res
models continue to indicate fairly high probabilities of 2"/hr
rain rates, and a complex pattern of cell mergers and colliding
outflow boundaries and sea breezes. This may lead to periods of 1-2
hours where convection is maximized in particular areas, and
scattered instances of flash flooding. The inherited Slight Risk
was expanded a bit to cover coastal portions of SC/GA with the
highest probabilities for localized 5" exceedance.

...Northern Plains and Minnesota...

16Z UPDATE: No major changes were made with this outlook update,
other than to remove a small portion of the northern side of the
risk area due to observational trends and the latest guidance.

PREVIOUS DISCUSSION: There continues to be a fairly strong signal
for storms to initiate in the northern extent of an instability
plume in the Plains, near the nose of a moisture transport maximum,
in the afternoon today. Consensus among the hi-res models is this
will rapidly organize into an MCS and then likely propagate quickly
to the southeast in the general direction suggested by the
thickness contours. The potential for relatively fast forward
motions may mitigate some of the flash flood risk. However,
precipitation anomalies over the past 14 days indicate the pattern
has been relatively wet of late in the Northern Plains, and
precipitable water values will be anomalously high. Deep moisture
combined with strong instability should support high instantaneous
rain rates. Therefore, any areas where the duration of heavy rain
could be lengthened (such as backbuilding along the periphery of a
cold pool, cell mergers with antecedent convective development) may
lead to flash flooding. The Marginal Risk was maintained and
adjusted southward based on the latest guidance (factoring in
upwind propagation favoring southern propagation late).

...Ohio River Valley...

16Z UPDATE: No major changes were made with this outlook update,
other than to remove a small portion of the western side of the
risk area.

PREVIOUS DISCUSSION: An inherited Marginal Risk was maintained and
adjusted slightly for a potential repeat of the pattern from
yesterday, just displaced slightly to the east over S IN, C KY, SW
OH. A slow moving mid-level wave is expected to continue to slowly
shift east, in the presence of relatively strong instability.
Therefore, scattered afternoon thunderstorms are expected. Although
moisture levels are not expected to be anomalously high, storm
motions should be very slow with mean winds in the lowest 8-km
around 5 kts or so. This could lead to localized flash flooding by
extending the duration of heavy rain with the most intense storms.
Rain rates should reach the 1-2"/hr range.

Churchill/Lamers


Day 2
Valid 12Z Wed Aug 06 2025 - 12Z Thu Aug 07 2025

...THERE ARE MARGINAL RISK AREAS FOR EXCESSIVE RAINFALL ACROSS
PORTIONS OF THE UPPER MISSISSIPPI VALLEY AND FOR THE COASTAL
SOUTHEAST AND MID-ATLANTIC REGION...

...Upper Mississippi Valley...

A region of anomalously high moisture, with PWs near the 90th
percentile, should shift east during the Day 2 period, and
generally continue to be superimposed with moderate to strong
instability on the periphery of a ridge. Confidence in the
convective details remain low at this time, but the overall
environment will remain supportive of organized convection with
high instantaneous rain rates. The pattern may still lead to a
forward-propagating MCS, which would mitigate flash flood risk
overall, but the overall environment is favorable enough to
continue to maintain a Marginal Risk across the region.

...Coastal Southeast and Mid-Atlantic...

2030Z Update: The 12Z guidance today came into better consensus
that in the anomalously high moisture plume, additional convection
is likely to develop late tonight into Wednesday across SC into
central NC. The activity is likely to be slow moving and highly
efficient rain producers, so some localized higher amounts are
possible as well as some training elements. Some of this could fall
over the more urban corridor from Charlotte to Raleigh. By the
afternoon/evening, the guidance is consistent that additional
storms are likely closer to the coast where some intense rain rates
are anticipated. 12Z HREF neighborhood probabilities for 3-5" for
the 24-hr period exceeds 50%. With all this in mind, a Slight Risk
was introduced for this part of the region.

Further south into southeast Alabama into southern Georgia, the
high moisture and proximity to the stalled frontal boundary will
likely create scattered to widespread thunderstorms. Given the high
PWs and general slower storm motions, some localized heavy rainfall
totals are possible and in coordination with the local WFOs, a
Slight Risk was extended further south into that area as well.

---previous discussion---

Confidence remains relatively low in the placement of the heaviest
rainfall on the Day 2 period, as the ECMWF/ECENS suite continues
to concentrate QPF farther north (mostly across North Carolina)
whereas the bulk of the other global models and ensembles are
focused south and east (mainly in SC and GA). Regardless of the
precise details, a plume of anomalously high precipitable water
values is likely to remain present across the coastal Southeast
and Mid-Atlantic region into the Day 2 period, and the flash flood
threat areas will likely come down to mesoscale details that will
come much more into focus in subsequent outlook periods.

Churchill/Lamers/Taylor


Day 3
Valid 12Z Thu Aug 07 2025 - 12Z Fri Aug 08 2025

...THERE ARE MARGINAL RISK AREAS FOR EXCESSIVE RAINFALL ACROSS
PORTIONS OF THE UPPER MISSISSIPPI VALLEY AND NORTHERN PLAINS, AS
WELL AS FOR THE COASTAL SOUTHEAST AND MID-ATLANTIC REGION...

...Upper Mississippi Valley and Northern Plains...

The Marginal Risk area was maintained and adjusted across North
Dakota into the Upper Midwest area, as models are in good
agreement in indicating a potent upper-level shortwave digging
southward into the CONUS by Day 3. The approach of the trough is
likely to result in another wave of low-level return flow into the
Northern High Plains, sending PWs to the 90th percentile or higher
once again. Moderate to strong instability is expected to build
once again with daytime heating on the periphery of a ridge, and
organized convection is likely to ensue in the late afternoon and
evening.

...Coastal Southeast and Mid-Atlantic...

Confidence remains low in the placement of the heaviest rainfall
going into the Day 3 period, with substantial differences still
between the global model suites. As anomalously high tropospheric
moisture is likely to remain near the coast, maintained the
existing Marginal Risk area across portions of the area most
likely to realize heavy precipitation with daytime heating.

Churchill/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