Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
846 PM EDT Fri Aug 7 2026

Day 1
Valid 01Z Sat Aug 08 2026 - 12Z Sat Aug 08 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL FOR PARTS OF THE
UPPER MIDWEST AND ALSO FROM THE SOUTHWEST U.S. EASTWARD TO THE OHIO
AND TENNESSEE VALLEYS AND THE NORTHEAST...

The Slight Risk area that was in effect across portions of the
northern Mid-Atlantic has been dropped for the 01Z update as the
much of the stronger convection from earlier this evening as moved
off to the east and weakened. However, there are still some
clusters of convection impacting the interior of the Mid-Atlantic
and also the central Appalachians as there continues to be some
modest small-scale shortwave energy and remnant outflow boundaries
in place that are driving some additional focus for convective
initiation. Much of this is occurring where there is still some
leftover daytime instability which is still coinciding with a
regionally moist airmass with higher PWs locally into the 1.75 to
2+ inch range. All of these areas are maintained under a Marginal
Risk given the environment and at least a small scale threat of
additional heavy showers and thunderstorms.

The Marginal Risk extends well back to the southwest through the
OH/TN Valleys and the Lower MS Valley where multiple MCVs and/or
shortwave impulses are noted in late-day IR/WV satellite imagery.
This includes portions of MO/AR and OK where MCVs are currently
facilitating some loosely organized areas of convection. Some
models from the 18Z HREF suite suggest potential for convection to
take on some nocturnal support overnight with a modest increase in
moist southwest low-level flow interacting while interacting with
the vort energy and remnant outflow boundary activity. Locally
some 2 to 4+ inch rainfall amounts will be possible, but the
antecedent conditions are also quite dry with very high FFGs, so
any flash flood threat from near the Ozarks on down into the Lower
MS Valley will most likely be urban in nature. Some consideration
was given to a Slight Risk, but given the model spread and the dry
antecedent conditions, a Marginal Risk was maintained for this
update.

Stronger upper-level jet support is noted across the Upper Midwest
in association with an upper trough crossing the region along with
an attendant cold front. This energy interacting with the nose of
moisture and instability ahead of it, including stronger shear
profiles will maintain a threat for clusters of convection into
portions of the overnight period. This activity will tend to be a
bit more progressive, but locally high rainfall rates with the
stronger storms will pose an isolated threat for mainly urban
runoff problems. A Marginal Risk covers much of far northern MO,
eastern IA, the northern half of IL and up through WI and the U.P.
of MI to address the convective threat overnight.

Finally, monsoonal moisture is in place across a sizable area of
AZ/NM and extending northwestward into central and southern NV and
southwest UT. Orographically enhanced convection with lingering
instability should persist through the evening hours, and this will
continue to support an isolated threat for flash flooding. Therefore,
the Marginal Risk area extends well into the Southwest U.S.

Orrison


Day 2
Valid 12Z Sat Aug 08 2026 - 12Z Sun Aug 09 2026

...THERE IS A SLIGHT RISK ALONG THE SOUTHEAST COAST OF SOUTH
CAROLINA...

20Z Update..

In coordination with CHS, a Slight Risk was introduced for the
southeast coast of South Carolina. An inverted trough will move
northward from the east coast of FL, eventually reaching the SC
coast. Moisture from the Atlantic/Gulf Stream will push inland
beginning in the morning. 850mb flow will be nearly orthogonal to
the coast, and a persistent influx of moisture should aid in
sustaining showers and thunderstorms along the coast. HREF and RRFS
indicate medium-to-high chances (65% and 70% respectively) of rates
exceeding 1"/hr, and even medium (~45%) chances of 2"/hr during
the morning and early afternoon. SREF Spaghetti Plots also indicate
a large concentration of at least 1" along the coast. The biggest
limiting factor is expected to be the dryness of the soils.
Antecedent conditions suggest that soils will be able to handle a
lot of rain, however, persistent moisture flow, favorable rain
rates, and the urban centers of Charleston and Savannah being prone
to flash flooding, ultimately resulted in the addition of the
Slight Risk area.

Elsewhere, only minor adjustments were needed for the remaining
MRGL Risk areas. Global models were in a surprising amount of
disagreement over the Iowa and southern Minnesota, particularly
between the GFS and ECMWF regarding the QPF core. AI/ML guidance
(EC-AIFS & AIGFS) have honed in on an area in northern Iowa, almost
along the border. Totals also seem to be trending down with this
latest run. Given this and overall lack of a consensus among the
global suite, an upgrade was not considered. There is, however, an
agreement that northern Illinois no longer seems to be in the
picture for this setup, and as such, the MRGL Risk area was reduced
to remove that area.

Blanco-Alcala

Previous Discussion...

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 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...

20Z Update..

Minimal changes were needed for this update, other than cosmetic
changes to match with the latest run of global models and
ensembles. There was a surpring level of agreement with the global
models, even more so than for Day 2. The inherited Slight remains
to be in good shape for this update, and further refinements may be
possible as later model runs come in and CAMs guidance become
available.

Blanco-Alcala

Previous Discussion...

...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 4 and Day 5

Valid 12Z Mon Aug 10 2026 - 12Z Wed Aug 12 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
OHIO VALLEY AND THE CENTRAL APPALACHIANS...

2030Z Update...

A rather active pattern is expected for the medium range period as
a series of shortwave troughs digging down across the Midwest
advance into the OH Valley and the central Appalachians. This
energy coupled with proximity of a frontal zone will facilitate
multiple rounds of heavy showers and thunderstorms. High PWs along
the front that may exceed 1.75 inches along with the pooling of
moderate to strong instability should favor convection with high
rainfall rates capable of reaching 2.5 inches/hour with the
stronger and more organized cells. Generally a couple of organized
MCS rounds are expected along with concerns for cell-training
across portions of central/northern IL through sizable areas of
IN/OH and also down south of the OH River. There is some guidance
suggesting that the central Appalachians may see impacts as well.
The Slight Risk areas for day 4 and 5 have been locally expanded
and also dropped farther south across these areas to account for
the consensus of the latest global model guidance and the AI
solutions (EC-AIFS and AIGFS).

The Marginal Risk area has been expanded locally across areas of
the Midwest and the northern Plains where additional shortwave
energy dropping east-southeastward will interact with sufficient
moisture and instability for additional convection with locally
heavy rainfall rates. Also, the Marginal Risk for the Southwest
U.S. has been expanded given the expanding footprint of anomalous
monsoonal moisture into areas of southern CA, including parts of
the central and southern Sierra Nevada and adjacent areas of the
Intermountain West. Generally pulse convection capable of being
terrain- anchored is expected, with some development likely through
differential heating processes. Locally heavy showers and
thunderstorms with high sub-hourly rates are expected which may
reach as high as 1 to 1.5 inches in 30 minutes, and thus it is
likely there will be isolated to scattered areas of flash flooding
concerns which will include burn scar, slot-canyon and arroyo
impacts.

Orrison

Previous discussion...

...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


Day 1 threat area: www.wpc.ncep.noaa.gov/qpf/94epoints.txt
Day 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt
Day 3 threat area: www.wpc.ncep.noaa.gov/qpf/99epoints.txt