Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
825 PM EDT Thu Aug 6 2026
Day 1
Valid 01Z Fri Aug 07 2026 - 12Z Fri Aug 07 2026
...THERE IS A BROAD MARGINAL RISK OF EXCESSIVE RAINFALL FROM THE
SOUTHWEST THROUGH THE CENTRAL/SOUTHERN PLAINS, MIDWEST, AND EAST...
...Central Plains into the Lower Missouri Valley and Midwest..
0100Z Update: Any additional organized convection overnight along
and south of the quasi-stationary front is expected across
southeastern KS into northeast OK given the latest CAM trends,
including the 18Z HREF QPF exceedance probabilities. This given the
nocturnal uptick in the 925-850 mb flow (increase in 0-6km Bulk
shear), healthy deep-layer instability (MUCAPEs 2000-3000+ J/Kg),
and PWs approaching 1.75". However, given the degree of low-mid
level shear along/near this quasi-stationary surface front
(especially directional), strong northerly MBE or Corfidi Vectors
will allow for a fairly swift southerly motion of any organized
convective segments exhibiting upscale growth, thus limiting the
risk for cell training overnight. The latest HRRR simulated
reflectivity guidance supports this.
...Eastern U.S....
0100Z Update: A slow-moving frontal boundary expected to persist
as shortwave energy track east along the southern edge of the
Westerlies, helping to accentuate lift in the high PW axis in the
vicinity of this front. Moderate to locally heavy rain totals
expected with hourly rain amounts up to 2.5" possible, supporting
at least isolated runoff issues in urban areas and places with some
degree of soil saturation. The loss of peak heating however will
substantially reduce the risk (i.e. to "sub Marginal") after
03-05Z, given the weak 0-6km bulk shear and weak mid-level lapse
rates (~6 C/Km).
...Southwest/Southern High Plains...
Monsoonal convection will continue through this period thus the
Marginal Risk was maintained for much of the Southwest and Southern
Rockies. While there is continued low confidence with details,
locally heavy rains with hourly amounts up to 2" could lead to
isolated runoff issues.
Hurley
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
337 AM EDT Fri Aug 7 2026
Day 1
Valid 12Z Fri Aug 07 2026 - 12Z Sat Aug 08 2026
...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR THE SOUTHWEST
THROUGH MUCH OF THE EAST, MIDWEST, AND SOUTH...
Marginal Risks for excessive rainfall cover large portions of the
country from the Southwest through much of the eastern half of the
country. The Marginal was expanded to include much of Oklahoma and
north Texas, as well as expanded through Louisiana and portions of
the central Gulf Coast, as well as much of Florida.
Abundant deep tropical moisture is in place across much of the
Southeast and up the Eastern Seaboard. Forcings remain mesoscale,
as the synoptic pattern with a trough generally along the northern
tier of states generally is not overlapping most of the deepest
moisture. Thus, where there is enough forcing, there's a lack of
moisture, and in large swaths of the country where there's plenty
of moisture, a lack of a defining synoptic feature to provide
forcing will likely mean any storms will be reliant on smaller
mesoscale features such as local terrain, cold pools, sea breezes
etc. to get them going.
Due to a lack of defining features overall, large Marginals remain
the most prudent forecast. However, given the aforementioned deep
tropical moisture over much of the country, it's quite feasible
that there will be Slight Risk levels of impacts wherever the
storms can interact with each other in a positive way. This appears
most likely on Friday across portions of Arkansas, where the deep
tropical moisture will run into the terrain of the Ozarks, and the
base of the distant upper level trough could provide a bit of
organization to the convection. That said, the very dry soils from
the lack of rainfall across the state in recent days will make
Slight level impacts difficult to attain.
From eastern Ohio through much of Pennsylvania and southern New
York, there too will have some minimal amount of forcing from weak
upper level shortwaves running over the abundant summertime
moisture in place across much of the east. Here too there is a
higher risk of Slight Risk level impacts, in part due to low FFGs.
Convection across this region looks to be quite progressive though,
so it will likely take topographic enhancement in localized areas
to produce enough rain to produce widely scattered instances of
flash flooding. These should be few and far between enough to not
need a Slight there for now.
Wegman
Day 1 threat area:
www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
337 AM EDT Fri Aug 7 2026
Day 2
Valid 12Z Sat Aug 08 2026 - 12Z Sun Aug 09 2026
...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL FOR THE
SOUTHWEST, PORTIONS OF THE MIDWEST, AND MUCH OF THE EASTERN
SEABOARD...
Monsoonal moisture across the Southwest will continue on Saturday,
especially across Arizona. As usual, the greatest rainfall will be
along the Mexican border where the Gulf of California's influence
is greatest, as well as along the Mogollon Rim. There appears to be
little rain in between in the populated areas. Lack of signal for
more organized heavy rain should preclude a Slight Risk upgrade.
Across the Midwest, specifically Iowa, an overnight Saturday night
MCS is expected to impact much of Iowa and small portions of the
neighboring states. While guidance has come into better agreement
on the areas likely to be affected, there remains a broad variety
of solutions as to how much rain will fall across much of central
and eastern Iowa. Thus, for now the area remains in a Marginal, but
will continue to be monitored for a Slight risk upgrade with future
updates.
Signals for heavy rain across much of the Eastern Seaboard remain
very diffuse again on Saturday. Similar to Friday, there remains
plentiful moisture all up and down the Atlantic coast. Somewhat
better forcing from a negatively tilted trough north of the Great
Lakes should allow for more widespread storms, but they will remain
quite progressive. Further south, there will be more moisture, but
a lack of forcing. Most of the heaviest rains look to be from north
Florida through coastal South Carolina.
Wegman
Day 2 threat area:
www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
337 AM EDT Fri Aug 7 2026
Day 3
Valid 12Z Sun Aug 09 2026 - 12Z Mon Aug 10 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
UPPER MIDWEST AND UPPER MISSISSIPPI VALLEY...
...Upper Midwest...
Multiple rounds of storms are expected on Sunday across portions
of the upper Midwest, specifically from the southeast corner of
Minnesota southeast through northern Indiana. With plentiful
moisture from an LLJ originating from the Gulf being advected
northward into the storms, there is little reason to see why the
storms won't be able to backbuild and train both with the morning
convection with an MCS, and then secondarily with late afternoon
through the Sunday night convection across these same areas. Since
guidance has come into better agreement, even better than Days 1
and 2, a Slight Risk upgrade was introduced with this update.
...Elsewhere...
Elsewhere, the Southwest monsoon will continue to promote
afternoon and evening convective development into the 4 Corners
region, and southwesterly moisture advection into the southern
Appalachians could also provide a focus (due to terrain) for
afternoon and evening storms. As on prior days, widespread widely
scattered convection is likely all across the Southeast Sunday
afternoon and evening. Should storms merge over urban areas, then
flash flooding is quite possible outside of the drawn Marginal Risk
area. Thus, it's probable additional expansions and changes will
be needed over the coming days into the Southeast.
Wegman
Day 3 threat area:
www.wpc.ncep.noaa.gov/qpf/99epoints.txt
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
337 AM EDT Fri Aug 7 2026
Day 4 and Day 5
Valid 12Z Mon Aug 10 2026 - 12Z Wed Aug 12 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR
NORTHERN INDIANA...
...Great Lakes/Midwest...
Early to mid next week, broad cyclonic flow expected to persist
across the northern tier of the Lower 48 from the Northern
Plains/Upper MS Valley, eastward through the Midwest/Great Lakes
and into the Northeast, to the south of a closed mid-level low in
Hudson Bay. A frontal boundary will lie along the southern
periphery of this broad trough. Embedded shortwaves in this broad
cyclonic flow will accentuate lift in the vicinity of this front
where PW values are expected to rise to ~1.75" ahead of each set
of shortwaves and low-level confluence is expected. When looking
at the ingredients, organized convection looks most likely south of
the Great Lakes within eastern portions of the Corn Belt, where
hourly rain amounts to 2.5" are possible. Due to a bit better
agreement in the guidance, and the southward trends towards where
conceptual models would have placed a Slight Risk 24 hours ago,
went ahead and added a Slight Risk for areas in and around northern
IN both days. While there is still some possibility of further
southward trends, this trend may materialize more as a later
southward expansion as we get closer to the event.
..Southwest...
Broad Marginal Risk areas exist both days early to mid next week across
portions of the Southwest, Rockies, and Great Basin. While there
continues to be low confidence with the details, locally heavy
rains with the potential of 2" an hour amounts and isolated runoff
issues will continue in the axis of above average PW values
expected across this area. As usual, urban areas, arroyos/dry
washes, box canyons, and burn scars would be most at risk for flash
flooding.
Roth
Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
337 AM EDT Fri Aug 7 2026
Day 4 and Day 5
Valid 12Z Mon Aug 10 2026 - 12Z Wed Aug 12 2026
...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL IN AND NEAR
NORTHERN INDIANA...
...Great Lakes/Midwest...
Early to mid next week, broad cyclonic flow expected to persist
across the northern tier of the Lower 48 from the Northern
Plains/Upper MS Valley, eastward through the Midwest/Great Lakes
and into the Northeast, to the south of a closed mid-level low in
Hudson Bay. A frontal boundary will lie along the southern
periphery of this broad trough. Embedded shortwaves in this broad
cyclonic flow will accentuate lift in the vicinity of this front
where PW values are expected to rise to ~1.75" ahead of each set
of shortwaves and low-level confluence is expected. When looking
at the ingredients, organized convection looks most likely south of
the Great Lakes within eastern portions of the Corn Belt, where
hourly rain amounts to 2.5" are possible. Due to a bit better
agreement in the guidance, and the southward trends towards where
conceptual models would have placed a Slight Risk 24 hours ago,
went ahead and added a Slight Risk for areas in and around northern
IN both days. While there is still some possibility of further
southward trends, this trend may materialize more as a later
southward expansion as we get closer to the event.
..Southwest...
Broad Marginal Risk areas exist both days early to mid next week across
portions of the Southwest, Rockies, and Great Basin. While there
continues to be low confidence with the details, locally heavy
rains with the potential of 2" an hour amounts and isolated runoff
issues will continue in the axis of above average PW values
expected across this area. As usual, urban areas, arroyos/dry
washes, box canyons, and burn scars would be most at risk for flash
flooding.
Roth
