419
FOUS30 KWBC 182025
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
425 PM EDT Tue Aug 18 2026

Day 1
Valid 16Z Tue Aug 18 2026 - 12Z Wed Aug 19 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MISSOURI VALLEY...

...16Z Update...

The main change this update was to reduce the coverage of the
inherited Slight Risk to focus on the Missouri Valley while
maintaining a Marginal Risk into the Midwest/Great Lakes region.
The latest 12Z deterministic and probabilistic guidance from the
CAMs is less aggressive with rainfall amounts into the Midwest/Great
Lakes with storms firing along the cold front later today. There
is also less agreement in a favored corridor of heavier rainfall to
support a Slight Risk, with some of the CAMs shifting further
north over southern Wisconsin and portions of Michigan compared to
the inherited Slight Risk over northern Illinois, and depicting a
relative minimum in precipitation to the west-southwest over Iowa.
Meanwhile, the latest guidance is well aligned favoring a corridor
of heavy rainfall through the Missouri Valley from northeastern
Kansas/southeastern Nebraska southeast through central Missouri.
Expect storm coverage to ramp up into the overnight hours as an LLJ
develops with clustering/potentially backbuilding storms
progressing southeastward along a similarly oriented frontal
boundary. Both the HREF and RRFS show moderate probabilities
(40-60%) for rainfall exceeding 2 inches with deterministic
guidance supportive of locally heavy amounts in the 3-4" range. The
Marginal Risk was also extended a bit further to the west into
southwestern Nebraska/northeastern Colorado where the latest
guidance shows the potential for some heavier downpours with
afternoon convection that could bring an isolated flash flood
threat.

Putnam

...Previous Discussion...

...Midwest to Missouri Valley...

Current WV satellite imagery depicts the arrival of our first
shortwave migrating southeast around the broad trough to the north
and over the northern fringes of the ridge dominating the Southern
CONUS. Pathway of least resistance will maintain a consistent
approach of northwest to southeast trajectory of the next series of
shortwaves wandering through the mean flow between the two key
upper features over the next 72 hrs. The first disturbance will be
the precursor to a more potent shortwave ejecting out of the
Northern Rockies the second half of the period. Anticipating the
current shortwave analyzed over the Wyoming Front Range through the
Sandhills to propagate east-southeast with the main energy pivoting
through the Corn Belt and Upper Mississippi Valley before kicking
east towards the southern portion of the Great Lakes by later this
evening and continuing its progression eastward. This disturbance
will be the headline feature for areas of the Midwest between
MN/WI/IL with the latest CAMs maintaining some variability in how
they want to handle the convective scheme during the afternoon and
evening period today. Current prospects are generally 50/50 on
whether or not we see any significant thunderstorm genesis across
the aforementioned area as relevant positive vorticity advection
and the introduction of a cold front should manifest at least some
scattered convective activity across the Upper Midwest towards the
western Great Lake areas spanning I-94/41 between Chicago and
Milwaukee.

Deterministic outputs around the Great Lakes maintain at least
some moderate totals with a few more aggressive CAMs noting more
substantial rainfall prospects as the setup yields a better
organized convective cluster, thus tipping the scales to a more
enhanced rainfall prospect by the late-afternoon and early evening
time frame prior to the arrival of the cold front from the
northwest. Latest 00z HREF neighborhood probs for >2" is hovering
between 20-35% across northeastern IL into southern WI with the
primary focus lying between I-94 and I-80 in the above region.
Considering the antecedent soil moisture anomalies remaining
elevated, especially the top layer soils which have taken a beating
the past week from incessant convective complexes and the like, as
well as elevated stream flows lingering from said impacts, the
prospect for localized flash flooding is a bit higher than normal.
As such, with a signal still present in some of the CAMs and the
local Chicago WFO messaging the threat for locally enhanced flood
concerns with the disturbance, there was no reason to deviate from
the previous SLGT risk in place as the environmental conditions and
synoptic pattern do favorable the potential for later today before
the cold front finally ushers in some much needed drier air in its
wake.

Further to the west and southwest, the threat is a bit less
variable in its presentation within the latest CAMs suite as the
second, more prolific shortwave to eject out of the Northern
Rockies is all but certain to provide to a more enhanced convective
episode across areas of eastern NE down through the quad-state area
of NE/MO/KS/IA as the shortwave amplifies and digs southeast out of
the Sandhills. Fairly textbook setup of regional diffluence ahead
of the trough as it migrates through the High Plains into the
Missouri River Valley, entering an environment that will yield much
more favorable low to mid-level shear and elevated PWAT anomalies
as flow becomes backed for a time with the troughs approach.
Convective initiation across NE will ensue late this afternoon,
carrying through the evening as the mean flow will take any
activity and press it southeast with sights on northeastern KS and
northern MO as we move into the evening. From there, a modest
nocturnal LLJ will nose up through KS allowing for a localized
convergence signature and a narrow corridor of low-level shear
enhancement to maintain better mesocyclone structures that will
wander southeast through northeast KS into MO. Heavy rainfall will
maintain a southeastward heading with a pretty solid consensus of
1-2", locally higher amounts of rainfall extending from the KS/NE
border down through central MO. This is highlighted by modest >2"
probs off the latest HREF with the axis aligned perfectly with the
expected frontal positioning by the end of the forecast. These are
the more likely locations to see flash flooding given the more
favorable forcing, moisture presence, and potential for repeating
leading to a maintenance of the SLGT risk from the previous
forecast(s).

Kleebauer

...Eastern Florida Peninsula...

Broad trough expanding across the east will dig slightly into the
Southeast Tuesday, pushing a cold front and pre-frontal trough
towards the Gulf Coast. This pre-frontal trough is progged to push
down the Florida Peninsula in response to a push of a modest
shortwave impulse embedded within the flow. As this
trough/shortwave dive southward, the overlapping ascent will merge
with the pinned sea breeze into robust thermodynamics characterized
by PWs of 2-2.25 inches and MUCAPE above 3000 J/kg. This will
support scattered to widespread thunderstorm development, with
rainfall rates eclipsing 2"/hr. While this setup is favorable for
locally enhanced rainfall and totals between 3-5" in the hardest
hit locations, antecedent soil moisture remains low and probs for
FFG exceedance maintain a meager signal as FFG`s remain upwards of
3.5-4"/hr across all of northeastern FL into the FL Panhandle.
Considering the nature of the precip being scattered in nature and
the signals within the latest CAMs a little less robust than the
previous forecast, decided to remove the previous MRGL risk, but
maintain a posture for a non-zero flash flood threat with the
greatest potential likely between Jacksonville to Daytona Beach.

Kleebauer/Weiss


Day 2
Valid 12Z Wed Aug 19 2026 - 12Z Thu Aug 20 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE MID-MISSISSIPPI AND OHIO VALLEYS...

...2030 UTC Update...

Minimal areal adjustments were made to the inherited Slight Risk
across portions of the Middle Mississippi and Lower Ohio Valleys
based on the now available CAM guidance through the full day 2
period. Despite some expected spatial disparity at this time
range, all of the CAM guidance is generally focused on this region
with locally heavy rainfall amounts upwards of 3-5" supported by
the synoptic setup outlined below. Both the HREF and RRFS also
highlight this region with moderate to high probabilities of
amounts greater than 2". The only notable adjustment was an
extension of the broader surrounding Marginal Risk northwestward
back into the Middle Missouri Valley. A passing upper-level
shortwave and pooling moisture ahead of a frontal boundary will
lead to renewed thunderstorm development into the early afternoon
hours. Most of the now available CAM guidance supports some
locally heavier downpours of 2"+ which could lead to some isolated
flash flooding concerns. The forecast for the southwestern U.S.
also remains on track as greater instability and embedded
shortwaves within the overall flow through the upper-ridge lead to
increased Monsoon-related activity compared to day 1 supportive of
an isolated flash flood threat.

Putnam

...Previous Discussion...

...Mid-Mississippi and Ohio Valley...

Amplifying shortwave trough from the previous period will continue
its progression southeastward with sights on the Mid-Mississippi
and western Ohio Valley`s as we step through Wednesday. Enough
trailing energy will feed into the backside of the trough allowing
for a bit more of a robust response compared to what transpires
upstream the prior evening. First round of convection will the
remnants of the overnight MCS with expected decay of the complex of
thunderstorms as we step through Wednesday morning. Cold front will
also continue its progression south and southeastward through the
Ohio and Missouri Valley during the period with a bit more
latitudinal loss further west compared to downstream. By Wednesday
afternoon, we`ll see the introduction of the shortwave into the
center of the CONUS lead to broader amplification of the longwave
trough positioned east of the Mississippi allowing for greater
large scale ascent focused over the western Ohio Valley back into
the confluent area of the Ohio and Mississippi River between the
MO/TN/IL area. This has been consistently depicted over the past
series of forecast runs with the end of the current 00z CAMs suite
also noting this particular zone as the more favorable area of
organized convection with multiple rounds anticipated.

After 00z, nocturnal LLJ will kick in across the Mississippi Valley
with the nose of the jet acting as a low-level convergent feature
and aiding in an increasing regional bulk shear to provide a
necessary element to organize and sustain convective development.
Forecast soundings via the NAMNest and ECMWF are consistent with
robust ascent within the mid-levels thanks to the amplifying
trough, as well as the added low-level convergence with the
arrival of the cold front and nose of the LLJ into the region. This
signature typically correlates to a focal point for heavy
convection with back-building potential until fropa is complete
leading to some heavier rainfall lingering for several hours before
the threat subsides. Considering PWATs between 2-2.25" centered
across the tri-state region outlined above, and the added ascent
factors in both the boundary layer and mid-levels (sfc-500mb),
there will likely be an area that sees a period of 1-2"/hr rainfall
for at least 3-6 hrs. leading to flash flood prospects carrying
overnight into early Thursday morning. Consistency in the signal
has allowed for a maintenance of the previous SLGT risk, however
some adjustment was made to convey more certainty across the area
between southeastern MO into southern IL and western KY/TN. Some
areas are liable to see 2-4" of rainfall with a max of up to 6"
plausible considering the environment at large.

Other areas of convection will line up along the front as it
progresses slowly southeast through the rest of the Ohio Valley,
entering areas that are still in recovery from the onslaught of
heavy rainfall the past 7 days. These storms are not anticipated to
be nearly as prolific as the past several days, and even not as
prolific as what will transpire upstream. That said, antecedent
soil moisture anomalies approaching the 95th percentile over a
large area, plus the threat of slow moving convection along the
front is more than sufficient for a MRGL risk carrying through
parts of KY/IN/OH with the eastern extent into portions of WV. This
whole region remain sensitive, so the forecast will be monitored
closely for any targeted upgrades if trends seem fit.

Kleebauer

...Southwest into the Central High Plains...

The previous forecast remains on track with the global
deterministic and newest 00z CAMs suite in agreement on scattered
thunderstorm activity likely to transpire Wednesday afternoon,
carrying into the evening hours with areas of the Front Range the
most notable for seeing more organized convection thanks to added
shear and regional ascent attributed to a shortwave proxy off the
Central Rockies. Synoptically, the evolution of the setup across
the Southwest and Front Range begins with an upper ridge centered
over the Four Corners, re- centering and amplifying while an
inverted trough pushes west on Wednesday. This will promote a more
favorable setup for monsoonal convection as a surge of
thermodynamics (PWs above 1.25" overlapping MUCAPE of 1000-2000
J/kg) lifting northward behind the inverted trough. At the same
time, weak shortwaves embedded within the flow and rotating around
the ridge will produce areas of locally enhanced ascent, which into
the improving thermodynamics suggests more widespread convective
activity is likely on Wednesday. While a more substantial risk for
greater coverage and intensity exists just beyond D2, a large MRGL
risk for excessive rainfall appears supported from the Deserts of
Arizona northeast into the Central High Plains of CO/KS. With
rainfall rates of 0.5-1"/hr likely at times, and mean storm motions
of just 5-10 kts, (higher in CO but with stronger bulk shear for
organization) this will support at least isolated instances of
flash flooding, especially over vulnerable terrain, urban areas, or
recently saturated soils. A MRGL risk remains in place for much of
the Desert Southwest into portions of the Central Rockies through
the Colorado Front Range.

Weiss/Kleebauer


Day 3
Valid 12Z Thu Aug 20 2026 - 12Z Fri Aug 21 2026

...THERE IS A MARGINAL RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE OHIO VALLEY, MID ATLANTIC, AND SOUTHWESTERN U.S...

...2030Z Update...

Minimal adjustments were made to the inherited Marginal Risk areas
over the Ohio Valley to Mid-Atlantic and southwestern U.S. as the
prior forecast remains on track. The one area of potentially
greater intrigue/upgrade potential as mentioned in the prior
discussion is throughout portions of the coastal Mid-Atlantic. The
global guidance continues to maintain a more focused corridor of
heavier rainfall with storms developing along a quasi-stationary
frontal boundary expected in vicinity of the Virginia Tidewater
region northeast through the DelMarVa Peninsula and into New
Jersey. Forecast amounts remain similar in the latest 12Z guidance
with areal average QPF above 1" and localized heavier amounts in
the 2-3" range. However, there remain a few factors that lead to
enough uncertainty that a Marginal Risk will be maintained for now.
First, much of this region has relatively higher FFGs given the
proximity to Chesapeake Bay/the Atlantic coast outside of more
sensitive urban areas (like the Norfolk vicinity) and thus a more
limited flood risk. Second, the location of the front and thus
corridor of heavier rainfall remains uncertain, with guidance such
as the ECMWF focused on the Tidewater through DelMarVA and southern
New Jersey, and the GFS further offshore with the heavier rain
limited to the Tidewater and southward into northeastern North
Carolina. The CAM guidance may provide more confidence in higher
rainfall amounts and their location that would lead to a greater
risk for flooding despite the relatively high FFGs. Another
scenario for an upgrade would be if the front/corridor of rainfall
were also to set up a bit further northwest and begin to reach
portions of the I-95 corridor which wouldn`t require as much
confidence in higher amounts given the increased coverage of more
sensitive urban areas.

Putnam

...Previous Discussion...

...Ohio Valley to Mid Atlantic...

Troughing for areas east of the Mississippi will continue with
modest amplification occurring as shortwaves dive down the backside
of the mean trough out of Canada and allow for further height falls
across the Ohio Valley to Mid Atlantic. Cold front from the past
few periods will continue making progress to the east coast with
the frontal positioning a critical part in the convective posture
over much of the above areas. The consensus for frontal placement
and eventual convective development puts the best opportunity for
heavy rainfall across the Tennessee Valley and the northern reaches
of AL/MS up through the central Mid Atlantic with the core of
heaviest rains focused along the Delmarva and VA Tidewater. Latest
global deterministic and ensemble means note a more robust low-
level convergence pattern initiating across the coastal areas of
the Mid Atlantic as the timing of the front coupled with diurnal
instability maxima and low-level convergence along the western
periphery of a ridge centered in the Atlantic will allow for a
heavier axis of QPF to materialize on Thursday afternoon and
evening.

Proximity of the front will act as a focal point for the setup
with the projected low-level flow to run parallel to the front over
areas of eastern VA up through the Delmarva Peninsula with the
northern fringes lining through the southern half of NJ. Ensemble
mean QPF already shows a solid 1-1.5" areal QPF average with some
of the deterministic forecasting between 2-3" over the time frame,
a testament to a relatively steady signal with a relatively
pronounced range of 1-3" potential. NBM 90th percentile is also
solidly within that 2-3" range, albeit a much larger footprint of
higher totals with the heavy precipitation output even down into
northern NC. NBM probs for >2" are between 20-35% across all of the
Delmarva, extending south into the northern reaches of the VA
Tidewater. This signal has been consistent the past 4 cycles of the
NBM lending credence to the threat as the mean hasn`t really
wavered much the past few days. In this case, the MRGL risk
inherited was maintained and will hold off on any upgrade until we
get closer to CAMs range heavier QPF signals may present themselves
if the synoptic evolution remains fixed to the area.

...Southwest U.S. into the Southern Rockies...

Relatively textbook monsoonal pattern across the west will lead to
scattered thunderstorm development across much of the Desert
Southwest extending north into parts of the Great Basin and through
the Southern Rockies. As of now, the signal is mainly seasonal with
regards to the PWAT anomalies and general expectation as the most
notable areas will likely be in those terrain focused zones across
AZ (Mogollon Rim), NM (Sangre de Cristos and Sacramento Mtns.), and
UT (Zion National Park). Low probs for >1" on the latest
ECENS/GEFS suites and 01z NBM allow for a standard MRGL risk
issuance for the area as the setup is genuinely favored for pulse
convection and slow cell motions tied to the terrain as a broad
597dm ridge centers over the Four Corners and makes little
progression in the D3 window. The previous MRGL was extended into
southern UT to account for the increasing signal in the area of
Zion National Park where slot canyons provide an inherent danger to
any slow moving thunderstorm activity.

Kleebauer


Day 4 and Day 5
Valid 12Z Fri Aug 21 2026 - 12Z Sun Aug 23 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL FOR PORTIONS OF
VIRGINIA AND THE CAROLINAS ON FRIDAY...

A deepening negatively tilted upper level trough extending
southeastward from over the Great Lakes towards the Mid-Atlantic
will be a primary driver of heavy rain over the region on both
Friday and Saturday. As the trough digs, a stalled out surface
front will support large scale lift over Virginia and the Carolinas
both Friday and Saturday afternoons. Plentiful tropical moisture
has been drawing north up the Eastern Seaboard ahead of the front.
PWATs on Friday will still be around 2 inches. Unidirectional
southwesterly flow through all levels of the atmosphere will
support backbuilding and training storms. The front will uplift
that moisture-rich and sufficiently unstable air to allow for
numerous rounds of showers and thunderstorms. The front may be
stalled along the Appalachians, which will support those storms
forming over areas where the terrain will enhance the severity and
quicken the formation of any downstream flash flooding. While total
rainfall amounts into western NC and VA will likely be lower than
further east along the coastal plain, the terrain effects also
bring that area into the Slight Risk category.

The heaviest rains will be along the coastal plain, where sea
breeze convergence, cold pools from prior storms, and neighboring
storms themselves will all provide additional localized uplift,
supporting new storm formation. Thus, the higher confidence at
Slight Risk level impacts on Friday is from the Hampton Roads area
of Virginia south and west, roughly following I-95, to northeastern
South Carolina.

Elsewhere, the monsoon will continue to support isolated to widely
scattered showers and thunderstorms over the Four Corners region.
Storms forming along the lake breezes and the terrain of the
eastern U.P. may also allow for some isolated instances of flash
flooding on Friday, where a new Marginal Risk has been issued, as
well as further south and east into portions of OH, PA, and western
NY on Saturday. Terrain impacts and low FFGs also warranted a
Marginal Risk issuance for those areas.

Wegman


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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