FOUS30 KWBC 091548
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
1148 AM EDT Sun Aug 9 2026

Day 1
Valid 16Z Sun Aug 09 2026 - 12Z Mon Aug 10 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
THE UPPER MIDWEST...

...Upper Midwest...

16Z Update: Initial wave of convection is currently pushing through
IA/WI at this time with heavy rain focused over southern WI with a
primary convective cluster leading to bouts of heavy rainfall
between 1-2"/hr based on current obs. The forward propagation of
the storms should limit the threat for flash flooding, but some
isolated instances of flash flooding will plausible between Madison
to Milwaukee with emphasis on the area between I-94 and I-41 (For
more on this evolving setup, please see MPD #0984). This shortwave
associated with the current convection will migrate east-southeast
with a steady motion into the southern Great Lakes by the
afternoon, kicking out of the area by the time the evening rolls
around. Still some risk of thunderstorms across southern MI into
the northern Ohio Valley with the disturbance, but flash flood
threat will remain isolated overall.

The coming evening is when the next round of thunderstorms will
really cause issues as we see the approach of a warm front moving
up from the south with the aid of a budding nocturnal LLJ and some
backing flow centered over northern IL/IN up into southern WI and
MI. The backed flow along with increasing surface convergence
within the proximity of the warm front and any remnant cold pool(s)
generated via the storms early in the period will provide the
opportunity for a secondary pulse of convection in and around the
area focused along and east of I-39/55 in IL over into northwestern
IN and southwest MI after 00z. Hi-res suite continues to pin some
significant rainfall totals between 2-4", locally higher near or
within the Chicago metro and surround suburbs over into those areas
downstream around the shores of Lake Michigan across IN/MI. 12z
HREF neighborhood probabilities for >3" are actually a touch lower
than the previous forecasts, but still a respectable 40-60%
centered over the above area with the max positioned over
southwestern MI.

The urbanization factors along with rates between 1-2"/hr all
within the expected environment will be sufficient for at least
scattered flash flood instances with the highest prospects in those
heavily urbanized areas within the Chicago metro to along the I-94
corridor bordering the lake. The northern inflection of the threat
seems to be near Holland, MI with the primary core just south.
There is some prospects of the heavy rainfall axis focusing a bit
further south with more emphasis on areas between I-80 and I-74 in
IL into I-65 in IN. These outputs are generally more aggressive
with the cold pool positioning being further south than the
consensus leading to the convergence pattern displaced away from
the southern portion of Lake Michigan. Something to monitor when
assessing any potential deviations from the current forecast, but
still a lower probability comparatively. As a result, maintained a
broad SLGT risk across the Midwest into the southern portion of the
Great Lakes and northern Ohio Valley with the axis of the risk
genuinely encompassing the forecast positioning of the warm front
by tonight into early tomorrow morning.

Kleebauer

...Southeast...

16Z Update: Changes were minor from the inherited MRGL risk across
the Southeast and southern Mid Atlantic. The main area of concern
will continue to be the Cumberland Plateau and points east through
the southern Appalachians where slow cell motions and rainfall
rates between 1-2"/hr could spur some localized flash flood impacts
in the terrain over Tennessee into the Carolina mountains. Totals
between 2-4" in the hardest hit areas seems to the consensus
within the HREF prob fields indicating modest >3" probs between
30-50% across southwest NC into the Smokey Mountains. Heavy rain
prospects extend southwest into AL/MS/GA with local maxima >3"
plausible in the mountains of north GA and over the Central Gulf
coast in southern AL. Broad MRGL risk remains for all areas.

Kleebauer

..Previous Discussion..

Abundant deep tropical moisture will remain in place across much of
the Southeast, as it has been for several days already, through the
day today. Unlike further north, at these latitudes, a
comprehensive forcing mechanism, especially from the upper levels
will be lacking. Thus, with plentiful daytime heating, widespread
disorganized convection is likely once again this afternoon over a
large swath of country. The inherited Marginal was expanded down to
the Gulf coast, as storms in the vicinity of New Orleans and
Houston could cause localized and isolated flash flooding since any
storms in this environment will be capable of short periods of
multiple inch per hour rainfall rates given the abundance of
moisture for the storms to work with. If flash flooding were to
occur, it appears most likely in this region in the southern
Appalachian region of far north Georgia and western North Carolina,
where terrain effects could locally increase the duration of heavy
rain from any storms that form.

Wegman

...Southwest...

16Z Update: No changes to the previous MRGL risk across the
Southwestern CONUS as the forecast remains on track with solid
run-to-run continuity on the expected thunderstorm coverage this
afternoon into the early evening. One area of interest not within
any risk is across the Peninsular Range in Southern California
where thunderstorm genesis is anticipated after 18z today.
Environment doesn't appear to be as robust as the coming days with
more of a modest threat for locally heavy rainfall this afternoon.
Considering the deterministic outputs running below what we would
want to see for a more formidable flash flood setup, decided to
maintain the nil in the area, but will make mention of a non-zero
opportunity in that corridor of terrain for the period.

Kleebauer

..Previous Discussion..

Of the next 3 days, the monsoonal storms impacting the Southwest
will be at a minimum today, generally focused on the larger terrain
features such as the Mogollon Rim, and portions of Utah. The
greatest moisture, as usual, will be closest to the Gulf of
California in southwestern Arizona. So any storms that form along
the Mexican border will likely have a higher chance of producing
flash flooding compared to most other areas of the Southwest.
Regardless, with a lack of real forcing, the storms should still be
disorganized and therefore only produce isolated flash flooding in
their localized areas today.

Wegman


Day 2
Valid 12Z Mon Aug 10 2026 - 12Z Tue Aug 11 2026

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
MIDWEST AS WELL AS SOUTHERN ARIZONA...

...Midwest...

The MCS that will be moving across Ohio at the start of the day is
likely to persist as it traverses the state and moves into
southwestern Pennsylvania and West Virginia. By Monday morning,
the MCS should be fast enough moving that flash flooding resulting
from it, especially during this most hostile period of the day for
MCS sustenance. Nonetheless, the storms will prime the soils in PA
and WV for later rounds of rain. During peak heating in the late
afternoon, additional storms are likely to form in Indiana and
Ohio, which will continue southeastward into West Virginia during
the evening hours. Behind this complex of storms, the persistent
LLJ advecting additional moisture and instability into the large
collective cold pool will lay down a pseudo-cold front from
northeastern Indiana into western and southern Ohio.

The primary flooding threat will come Monday night as the LLJ
running into the cold pool results in numerous additional storms
forming along the slowly retreating cold pool boundary. The storms
could reform as far back/west as Chicago, but appears most likely
from northern Indiana and into western Ohio. The storms will track
southeastward across those states, but due to the cold pool setup,
the environment will be very favorable for training and
backbuilding. Considering the rain in some areas of northern
Illinois and northern Indiana could be hitting the same areas for
the second consecutive day, a Moderate Risk upgrade can't be ruled
out, as well as anywhere the storms could be most persistent into
Ohio with a more short-range update.

...Southern Arizona...

Monsoonal moisture will peak across southwestern Arizona on
Monday. Storms are expected to initiate along the Mogollon Rim, and
the numerous ranges of southeastern Arizona as normal during the
afternoon hours. Unlike prior days, enough easterly flow,
especially in the upper levels, will successfully push those
showers and storms off the mountains and towards the lower
elevations and valleys of southwestern Arizona. As the storms track
westward, they are expected to backbuild given the impressive
amounts of moisture and instability they will run into as they move
westward. Thus, the bigger cities such as Tucson and possibly
Phoenix may see periods of showers and storms that will persist
into the evening hours, since the ample moisture and instability in
southwestern Arizona will sustain the storms past peak heating.
The inherited Slight Risk is little changed, and the I-19 corridor
from Nogales to Tucson remains the primary area that could be
affected by the storms and their flash flooding.

Wegman


Day 3
Valid 12Z Tue Aug 11 2026 - 12Z Wed Aug 12 2026

...THERE ARE SLIGHT RISKS OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
MIDWEST AS WELL AS FOR SOUTHERN ARIZONA...

...Midwest...

A rinse-and-repeat pattern will unfold across the Midwest on
Tuesday. Any morning MCSs from Monday night will continue towards
the southeast into West Virginia from Ohio while the storms
generally weaken Tuesday morning. Additional rounds of storms will
likely follow in the form of fast moving lines, but still tracking
over many of the same areas from prior days, from northern Indiana
across much of Ohio, and weakening as they move into West Virginia
late in the day. Just as on prior days, the storms will establish a
cold pool/pseudo-front across Ohio that will act as a train track
for additional storms to form along and track along by Tuesday
night. That precise scenario will play out again Tuesday night, as
storms form either immediately in the wake of any afternoon lines
of storms or along their cold pool. Westerly flow of deep moisture
will support the individual storms rather fast southeasterly motion
from Indiana across Ohio and eventually into West Virginia.
However, training and backbuilding will be common, and along a
narrow corridor that could stretch from as far north as Pittsburgh
south through the southern tip of West Virginia.

It appears this training and repeating scenario will ramp up as
compared to previous days on Tuesday, resulting in more common,
stronger, and more persistent lines of storms developing and moving
southeastward along the front. Given the likelihood that prior
days' rains over these same areas will have saturated the soils by
Day 3, and the general very flashy nature of the rivers of West
Virginia, it appears a Moderate Risk upgrade will be probable in
this region in the coming days.

...Southern Arizona...

The strongest storms associated with the monsoon across southern
Arizona on Tuesday are expected to form closer to the Mexican
border as compared to prior days. Given the likelihood of flooding
rains in this same area on Monday, despite the relative lack of
rainfall expected on Tuesday as compared with Monday, a small
Slight Risk upgrade for along the Mexican border was introduced
with this update. Otherwise, expect a return to a "base state" as
far as storm coverage, notably a significant decrease as compared
to Monday, but similar to today/Sunday. This should allow some
areas to dry out due to lack of activity on Tuesday.

Wegman


Day 4 and Day 5

Valid 12Z Wed Aug 12 2026 - 12Z Fri Aug 14 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF THE
OHIO VALLEY, MIDWEST, & CENTRAL HIGH PLAINS...

...Portions of the Ohio Valley/Midwest/Corn Belt...
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
and near Quebec. 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 ~2" ahead of each set of
shortwaves and low- level confluence is expected. When looking at
the ingredients, organized convection looks most likely near
portions of the Ohio Valley and the Corn Belt Wednesday into
Thursday night, where the better 1000-500 hPa thickness difluence
is forecast. Hourly rain amounts to 3" are possible. The Slight
Risk on Wednesday/Wednesday night was broadened somewhat from
continuity, with another Slight Risk introduced for
Thursday/Thursday night near the IA/MO/IL state junction where
guidance spread on the heavy rain signal is smaller than average.

..The West/Central & Northern Plains...
A broadening upper level trough is forecast across portions of the
West and Great Basin Wednesday into Thursday night. Broad Marginal
Risk areas exist near and ahead of this upper level trough for portions
of the Southwest, Intermountain West, and Northern/Central Plains.
Increased upper level troughing opens the door to strengthening
low-level level flow and broader convective development/coverage
across the West and Plains as the mid-level cap further erodes.
Above average precipitable water values slowly shift northward
through the region, while AZ's return back towards seasonal
averages Thursday night/early Friday. While there continues to be
low confidence with the details, locally heavy rains with the
potential of 2.5" an hour amounts could be problematic. A Slight
Risk was introduced for areas near the CO/WY/NE irregular border
intersection where there was a fairly cohesive signal for heavy
rainfall within a region of probable antiparallel flow between the
low-level upslope flow and organized thunderstorm propagation
moving against the low level wind. Urban areas, arroyos/dry washes,
box/slot canyons, and burn scars would be most at risk for flash
flooding.

Roth


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



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