905
FOUS30 KWBC 040822
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
422 AM EDT Mon Aug 4 2025

Day 1
Valid 12Z Mon Aug 04 2025 - 12Z Tue Aug 05 2025

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

...Southeast...

Wet and unsettled conditions to persist across the Southeast as a
mid-level trough axis lingers to the west allowing for moisture to
stream northeast from the Gulf. The trough draped from the Great
Lakes to the Arklatex will weaken slightly as ridging from the
Atlantic expands northwestward. Heavy rain will spread across the
region as PWs remain 1.75 to 2.25 inches, highest within a narrow
channel from the FL Panhandle into coastal South Carolina,
overlapping MUCAPE that is progged to rise above 1000 J/kg. The
presence of the strong upper level jet will enhance forcing for
ascent over the region while the surface front remains draped
across the region, and a shortwave ejects northeast from the Gulf.
This environment should support expanding showers and
thunderstorms, with just a slight shift Northwest expected in the
axis of heaviest rain due to the expansion of the mid-level ridging
from the east.

Antecedent moisture has lowered FFGs and increased soil sensitivity,
especially for portions of Alabama and Georgia. The Slight Risk
areas was expanded a bit further west into Alabama and
northeastward across Georgia and far western South Carolina. With
rainfall rates likely eclipsing 2 inches/hour at times, any of this
heavy rainfall falling atop these vulnerable soils could quickly
become runoff leading to instances of flash flooding.

...Northern Plains...

Mid-level impulses lifting slowly northeast from the Northern
Plains into the Upper Midwest will weaken Monday. However, forcing
for ascent will continue, despite being in a weaker state, as PVA
from the accompanying vorticity maxima pivots across the area and
interacts with persistent WAA on 20-25 kts of 850mb southerly flow
demarcating the LLJ. This will push PWs to above 1.25 inches, or
around the 90th percentile according to NAEFS. MUCAPE will be
somewhat limited, but the same WAA could drive CAPE to above 500
J/kg, supporting HREF neighborhood probabilities for rainfall rates
above 1"/hr to 10-20%.

In general, cells should be somewhat progressive to the northeast
on 0-6km mean winds of 10-15 kts. However, a region of enhanced
bulk shear nearing 30 kts close to the MN/SD border will also be
within an area of deformation which could result in regenerating
and backbuilding convection. This will lead to slower net motion
and repeating heavy rainfall, which is likely to be the focus of
heaviest rainfall accumulation reflected by HREF 3"/24hr
neighborhood probabilities of 20% (displaced south of the REFS but
seemingly more likely in position). This could occur atop soils
that become primed by rainfall on Sunday, but the excessive rain
risk still appears MRGL so no upgrades appear needed at this time.


...Lower Ohio Valley...

An inverted surface trough is expected to lift northeast today into
the lower Ohio Valley. Low- level convergence along this boundary
will combine with broad thickness diffluence and a shortwave
pivoting northeast to drive pronounced lift into a region of high
PWs (above 1.5 inches) and a collocated plume of MUCAPE above 1000
J/kg. Very weak flow across this area suggests that as convection
blossoms, cells will move slowly, generally from south to north,
with repeating rounds of 1-2"/hour rainfall rates leading to
pockets of 3+ inches of rainfall. The Marginal Risk area was
maintained.

...Northern Rockies...

During this period a shortwave trough will emerge from the Pacific
Northwest and track into the Northern Rockies. Impressive ascent,
primarily through height falls and increasing mid-level divergence,
helping to spawn a wave of low pressure tracking into the High
Plains by Tuesday morning. Showers and thunderstorms are expected
to increase in coverage as a response, and with easterly low-level
flow feeding in the higher PW values, rainfall rates of 1
inch/hour or greater will be possible.


Campbell/Weiss


Day 2
Valid 12Z Tue Aug 05 2025 - 12Z Wed Aug 06 2025
...THERE IS A SLIGHT RISK FOR EXCESSIVE RAINFALL IN THE SOUTHEAST...

...Southeast...

With the ridge extending into the Atlantic across Florida and the
trough positioned into the lower Mississippi Valley., much of the
environment described during the Day 2 period has not changed.
Between these two features, moisture will continue to surge
northeast beneath shortwave impulses that are being squeezed by
this synoptic pattern. The result of this will be waves of
convection lifting northeast, with rainfall rates of 1-2
inches/hour continuing to be supported despite a modest forecast
reduction in PWs.

THe latest guidance and WPC QPF indicates 2 maxima, one over the
higher terrain of western Carolina/northeast Georgia and the other
over central Georgia. Both locations will have areal average of
roughly 2 inches. Portions of the region have had multi-day
accumulations and will be vulnerable to additional rainfall. The
inherited Slight Risk was adjusted on the northern bounds to the
northeast, covering western parts North Carolina and South
Carolina. AS such, the Marginal Risk area was expanded into
southern Virginia.

...Northern Plains and Minnesota...

Mid-level impulse closing off over Saskatchewan will shed vorticity
downstream to drive height falls and PVA from Montana through North
Dakota and into Minnesota. This mid-level ascent will help deepen a
surface low moving across southern Canada, with triple-point
development/occlusion pushing a complex frontal structure
southeastward. Ascent across this area will impinge upon favorable
thermodynamics surging northward, especially in the vicinity of the
warm front, leading to widespread showers and thunderstorms
pivoting northeast through the day. PW values of greater than 1.5
inches (between the 90th and 97th percentile according to NAEFS)
and MUCAPE reaching 1000-2000 J/kg, convection should support
rainfall rates of 1-2 inches/hour. There continues to be a fair
amount of spread on the QPF accumulations and overall footprint.
Backbuilding into the higher instability will allow for some
training along the frontal boundaries. Areal averages of 1 to 2
inches possible. The inherited Marginal Risk area was expanded
further south across eastern South Dakota and southwest Minnesota
to encompass the southern end of the convection.

Campbell/Weiss

Day 3
Valid 12Z Wed Aug 06 2025 - 12Z Thu Aug 07 2025

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

...Upper Mississippi Valley...

The features described in the Day 2 period will propagate across
across the northern tier states shifting the areas of precipitation
to the east. Two distinctions cluster of higher QPF is expected
during this period. The first over southern Minnesota and the other
across northern Minnesota/International border where areal averages
of 1-2 inches is possible. A Marginal Risk was maintained for
eastern portions of the Dakotas, most of western/central Minnesota.

...Carolinas and Mid-Atlantic...

By this time, the surface low will be moving offshore taking the
frontal boundary with it and the showers and thunderstorms will
decrease from west to east, lingering over coastal areas. The best
concentration of storms and QPF totals will be over coastal Georgia
and southern/central South Carolina. A Marginal Risk spans from
far northeast Florida to central Virginia.

Campbell


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