FOUS30 KWBC 171858
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
258 PM EDT Mon Aug 17 2026

Day 1
Valid 16Z Mon Aug 17 2026 - 12Z Tue Aug 18 2026

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

...OH/TN valley into the Appalachians...
Regional radar mosaic this morning shows convection already
blossoming across the Ohio Valley and into Pennsylvania, as well as
draped as far southwest as Arkansas along the slowly advancing
cold front. Weak impulses moving within broad eastern CONUS
troughing will help enhance ascent through this evening, and the
12Z CAMs indicate scattered to widespread thunderstorms will
develop through peak heating. Modest bulk shear of 20-25 kts within
a plume of 1.75 to 2.0 inch PWs will support clusters of
thunderstorms with rainfall rates of 1-2"/hr, briefly even heavier
than that, with the most concentrated heaviest rates focused from
the Ohio Valley into the Central/Southern Appalachians. This area
is also where the FFG is significantly compromised due to recent
heavy rain, reaching generally 0.75-1.5"/3hrs. Fortunately, mean
0-6km winds of 10-15 kts with similarly paced propagation vectors
aligned perpendicular to the boundary should prevent significant
training, although repeating rounds of heavy rain are possible in
some areas. The inherited SLGT risk encompasses the most vulnerable
areas today, and was tailored only cosmetically from inherited to
match the highest FFG exceedance probabilities from both the REFS
and the HREF.

...Southwest...
Expansive west-to-east oriented mid-level ridge draped from the Low
Deserts of California through the Southern Plains will maintain a
favorable pattern for at least some monsoonal convection today.
While airmass convection will be the primary mode, a potent
shortwave noted in GOES-E WV imagery lifting out of West Texas will
pivot W/SW through the day, providing at least locally enhanced
ascent through southern NM and southeast AZ. Subtly enhanced inflow
will help draw PWs to as high as 1-1.25", lesser farther north
into the Four Corners, to support rainfall rates for which the
HREF, REFS, and UA WRF suggest will reach 0.5-1"/hr. While the CAMs
are not extremely aggressive with coverage, mean 0-6km winds of
just around 5 kts indicate slow movement for any storms that
develop today, posing a flash flood risk over vulnerable terrain
features or urban areas. For this reason the MRGL risk remains
supported, with a subtle extension south from inherited to capture
the advancing shortwave.

...Central Plains to Upper Midwest...
Amplifying mid-level trough over Northern Manitoba will shed pieces
of energy through its base and into the Northern Plains today. One
of these impulses is progged to be potent as it drops NW to SE from
the Dakotas into Iowa this evening. This impulse will interact with
a surface front, leading to locally accelerated and backed inflow
within a modest warm sector to the south. This should result in
increasing coverage of thunderstorms from South Dakota into
Nebraska this evening, with organization into one or multiple MCS
likely overnight within a region of 25-35 kts of bulk shear. As the
LLJ backs and amplifies to 25-30 kts, it will draw PWs above 1.5
inches northward, overlapping with a plume of 1000+ J/kg MUCAPE to
produce thermodynamics capable of supporting 1-2"/hr rainfall rates
(20-40% chance of 1+"/hr from both HREF and REFS guidance).
While storm motions may be progressive on 20 kts of 0-6km winds,
Corfidi vectors aligned to both this mean wind and the elevated
front could support training from west to east, enhancing the
potential for excessive rainfall. The CAMs continue to be highly
variable in their footprint for tonight, but there appears to be a
focus from the Sandhills of NE into western IA, as well as a
secondary maxima across the Coteau des Prairies of SD/MN. Any
development farther southeast would impinge into lower FFG, but
otherwise the exceedance probabilities within the most likely
footprint tonight suggests the MRGL risk remains supported, and
only minor cosmetic adjustments were made with this issuance.

Weiss


Day 2
Valid 12Z Tue Aug 18 2026 - 12Z Wed Aug 19 2026

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

20Z Update...
The previous discussion does an excellent job highlighting the risk
across this region on Tuesday, and guidance has unfortunately not
come into much better agreement. There will be two periods of
convection impacting this area on Tuesday, one in the morning from
a slowly decaying overnight MCS, and another with renewed MCS
development late in the evening/overnight. The CAMs are split as to
where the morning impulse will track, with some pushing it east
towards Chicago, and the others bringing it south across Missouri.
The second round at night should be farther south, with much higher
probabilities for KS/MO later in the D2 period.

The inherited SLGT risk was maintained, but expanded both to the
northeast after coordination with WFO LOT for potential in the
morning, and to the SW to capture the latest probabilistic
guidance. Although this entire area may not see excessive rainfall,
there is enough spread in the HREF and REFS exceedance
probabilities to feature this broader SLGT risk on Tuesday.

Previous Discussion:
Longwave pattern across the Northern CONUS into Canada will lead to
continued trough amplification down into the northern tier with
multiple shortwaves migrating under the trough base. Tuesday will
see one of the more prominent waves eject out of the Northern
Rockies with an anticipated downstream amplification as it motions
into the Northern Plains. Mid and upper level height falls with an
attendant vigorous shortwave will translate eastward on Tuesday
with sights on the eastern Northern Plains with heavy convective
prospects extending from eastern NE down into the Missouri Valley
and adjacent Corn Belt (Iowa). A secondary cold front will
accompany the disturbance a strong surface ridge follows in wake of
the trough swinging through the area. A combination of diffluent
flow aloft and ample mid-level vorticity advection into a well-
positioned theta_E ridge will lead to convective initiation across
the Sandhills, once again but with a greater focus downstream given
the vigor of the shortwave and sharper cold front. Ample
instability and PWAT anomalies between +1 and +1.5 standard
deviations above normal will compliment the upper forcing necessary
to promote a period of heavy thunderstorm activity where
convergence is maximized near the surface and diffluent flow aloft
works in tandem. Add in a modest LLJ developing across the Central
Plains with a nose into the Missouri River Valley and we'll see a
period of enhanced convective impacts that will materialize Tuesday
evening, carrying through the overnight period across the
intersection of MO/IA/KS/NE.

ML outputs have been consistently favoring this area over the past
several model iterations which historically provides a great enough
consensus to warrant more confidence in the forecast. Local totals
between 2-4" are forecast in the more heavily impacted areas when
assessing the latest ensemble probs and deterministic forecasts
that extend out through the D2 period. The CAMs window ends 00z
Wednesday, a bit before the more appreciable impacts occur, so the
next forecast update for the threat should provide more detail on
the threat itself, including more up to date probabilities from the
hi-res suite. In any case, there was enough confidence to maintain
the previous SLGT risk forecast with some modifications on the
southwest flank of the risk to account for the latest trends in
available guidance, as well as matching some of the consensus
within the ML fields.

...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 (30-40% chance according to both
HREF and REFS). While in general these storms should move steadily
southward along the pre-frontal trough/beneath the shortwave,
Corfidi vectors become increasingly aligned anti-parallel to the
mean wind and collapse to around 5 kts, suggesting storms may
backbuild along the sea breeze leading to a longer duration of
heavy rain. This could create 3-5" of rain in isolated locations,
leading to an increased risk for flash flooding, especially along
the urban I-95 corridor of the eastern Florida Peninsula.

Kleebauer/Weiss


Day 3
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...

...Mid-Mississippi and Ohio Valley...
Mid-level trough will gradually sharpen across the eastern third of
the CONUS on Wednesday in response to a shortwave diving from the
Central Plains into the Mid-Mississippi Valley. This impulse will
help to amplify the longwave pattern in the east, leading to
increasingly impressive forcing for ascent through height falls and
overlapping RRQ diffluence as an upper jet streaks into New
England. The amplifying trough will help dig a cold front south
towards the Tennessee Valley by the end of D3, and the accompanying
ascent along this front may help spawn a wave of low pressure as
well to enhance lift.

While there is still spread among the models as to how widespread
activity will be on Wednesday (both in space and time due to
multiple possible MCS/convective clusters), any storms will be
influenced by impressive thermodynamics to support rainfall rates
of 1-2"/hr. Bulk shear forecast to be 35-45 kts will help organize
this convection, and with both Corfidi vectors and mean 0-6km winds
aligned to the front, expect there will be at least short-term
training to lengthen the duration of these intense rates. Where
this occurs, 1-3+" of rain is possible, some of this falling atop
primed soils from prior rainfall. The adjusted SLGT risk was
modified to match the greatest probabilities from the GEFS/ECENS,
with some extension in the MRGL risk to capture potentially lower
rainfall, but atop very vulnerable soils into the Ohio Valley from
recent heavy rain.

...Southwest into the Central High Plains...
Upper ridge centered over the Four Corners will re-center and
amplify as 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 D3, 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.

Weiss


Day 4 and Day 5

Valid 12Z Thu Aug 20 2026 - 12Z Sat Aug 22 2026

...THERE ARE MARGINAL RISKS OF EXCESSIVE RAINFALL ACROSS PORTIONS
OF THE SOUTHWEST, OHIO VALLEY, AND MID-ATLANTIC...

2030Z Update...
Except for a bit of a southward expansion in the eastern US on
each day due to spaghetti plots of QPF...mainly driven by EC
ensembles...there were few changes needed to the previous Day 4/Day
5 ERO graphics.

Bann

Previous Discussion...

...Ohio Valley into Mid-Atlantic...
Large scale mid to upper level troughing moving into the eastern
U.S. will drive an active weather pattern across portions of the
OH Valley and Mid-Atlantic Thursday into Friday. On Thursday (day
4), an embedded shortwave should track across the OH Valley into
the Mid-Atlantic, initiating convection from portions of AR and MO
northeastward into WV. While some recovery is expected by this
time, recent heavy rainfall has left soils saturated over portions
of this corridor, and thus hydrologic sensitivity will remain
heightened and an isolated flash flood threat should exist.

Further east, a stronger model QPF signal exists across the
eastern Mid-Atlantic, spanning from VA into NJ. In addition to a
surface frontal boundary acting as a low level focus, upper level
jet dynamics within the favorable right entrance region will
enhance large scale lift. PW values exceeding 2" overlapping
strong instability will support heavy rainfall rates Thu into Thu
night. An embedded Slight risk may eventually be required somewhere
from eastern VA into southeast NJ...however lingering uncertainty
regarding mesoscale positioning and the climatologically higher FFG
across the coastal plain favor keeping the risk at Marginal for
now.

By Friday (day 5), the mid/upper trough will deepen over the East,
resulting in a stalling surface front. With anomalously high PWs
and moderate to strong instability remaining in place, the
environmental setup will remain conducive for additional heavy
rainfall and possible flash flooding. Model spread increases
regarding the timing and strength of individual shortwaves
traversing the deepening trough which will impact where the better
ingredient overlap will be. The GFS and ECMWF are in relatively
good agreement bringing a heavy rainfall threat from the Carolinas
into MD, whereas the AIFS and AIGFS shunt more of the deeper
moisture offshore, keeping the heaviest rain either offshore or
along coastal areas. While a Slight risk could eventually be
needed, will hold at Marginal until model guidance better converges
on the exact spatial overlap of forcing and moisture.

...Southwest...
A Marginal risk remains in place across portions of the Southwest
for both Thursday and Friday, with the risk expanding slightly
northwestward on Friday relative to Thursday. A strong upper level
ridge will remain anchored over the region, trapping embedded
shortwave energy within the light flow. Plentiful monsoonal
moisture and instability will remain in play, making localized
flash flooding likely both days. Because convective forcing will be
primarily terrain induced, overall storm coverage should stay
below Slight risk thresholds. However trends will continue to be
monitored, as any increase in convective coverage/organization
within the favorable CAPE/PW footprint could support a more
concentrated flash flood risk.

Chenard


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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