FOUS30 KWBC 190811
QPFERD

Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
411 AM EDT Wed Aug 19 2026

Day 1 Valid 12Z Wed Aug 19 2026 - 12Z Thu Aug 20 2026

...THERE IS A SLIGHT RISK OF EXCESSIVE RAINFALL ACROSS PORTIONS OF
THE CENTRAL PLAINS, MID-MISSISSIPPI AND OHIO VALLEYS, AS WELL AS
SOUTHERN ARIZONA...

...Central Plains to Ohio Valley..

Current satellite and radar composite depicts the initial stages of
our shortwave ejection from the High Plains making headway into the
Missouri Valley as we move through the evening. Expectation is for
this maturing area of convection to propagate southeastward into
the Mid-Mississipi regions of MO, leading to the first round of
convection being the remnants of the overnight MCS with an
expected decay of the complex later this morning as we lose the
nocturnal LLJ and we approach the diurnal instability minimum
across the western Ohio Valley. A cold front currently analyzed
over the Northern Plains extending east into the Great Lakes will
also continue its progression south and southeastward through the
morning hours with sights on 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 another potent shortwave into the center of the CONUS leading
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 current 00z CAMs suite maintaining this particular zone as one
of the more favorable areas of organized convection as step into
the second half of the forecast period (Wednesday evening into
early Thursday AM).

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.
The added bulk shear (30-40kts) and increasing upstream forcing
with the addition of the next shortwave is typically a signature
that correlates to a focal point for heavy convection with back-
building potential until the eventual frontal passage 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. Latest 00z HREF 1" EAS probs
depict the area of far southeastern MO through southern IL and
western KY as the primary region of focus coming through with
30-60% probabilities, a threshold typically reserved for more
appreciable organized convective setups leading to higher
confidence in this area seeing at least a scattered flash flood
threat when you couple with the current FFG indices. Overwhelming
consistency in the signal has allowed for a continuation of the
previous SLGT risk with the main area of interest lying 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.

The last few iterations of the CAMs have added another wrinkle to
the new D1 as the frontal progression and the preceding environment
have increased the overall threat of heavy rain across areas of the
Central Plains over into the Ohio Valley as thunderstorm genesis in
these areas are liable to cause scattered flash flood instances,
especially in those more prone areas across the Ohio Valley from
eastern IL to OH. Latest HREF neighborhood probabilities for >2" of
rain has increased significantly over the past 24 hrs. with the
newest set of probs firmly settled between 50-80% for the threshold
with even a "bullseye" of closer to 90% over portions of eastern NE
near Omaha. The upstream areas across NE and MO are a combination
of both the frontal ascent posed by the addition of the cold front,
as well as the added mid-level diffluence provided by the trailing
shortwave trough descending out of the Northern High Plains on the
backside of the mean trough. There has been a consistent signature
of locally heavy rainfall materializing between 15-18z today
upstream over northeastern NE, moving southeast towards the
Missouri River Basin by mid-afternoon to impact the Omaha area and
points southeast into northwestern MO. Rainfall rates in some of
these cells could eclipse 2"/hr at times, especially when you
couple the intra-hour rate variables that ultimately cause a
majority of the flash flooding incidents. Local totals of 2-4" are
certainly viable in this setup over the above locations leading to
an addition of a SLGT risk now across those area of eastern NE down
through northern MO and far southern IA.

Further east into the Ohio Valley areas stretching from eastern IL
through IN/OH, antecedent soil moisture anomalies remain into the
90-95th percentile over a large area leading to a lower threshold
necessary for any convective elements to cause trouble, especially
ones that linger for multiple hours at a time. We still have
hydrologic concerns aplenty for this region of the CONUS as
several Flood Warnings remain due to the swollen rivers and streams
in recovery from last weeks onslaught of heavy rainfall. The latest
HREF is keen on developing a long line of thunderstorms along the
advancing cold front with some of the activity likely to push rates
between 1-2"/hr by late-afternoon as we see a slow increase in
regional shear (25-35kts) as the upstream pattern evolves and
provides a better environment for sustaining mesocyclone
development. Mean flow will also run fairly parallel to the
boundary, a signature that typically acts as basis for repeated
convection for a time until the cold front finally trudges past the
area of interest. This signature is anticipated to extend from IL
through IN/OH, eventually with the eastern fringes possibly
reaching as far as western PA where the front will be located by
the evening hours. Considering the sensitivity of so many areas
bounded within this region, and in coordination with the local Ohio
Valley WFO's, a SLGT risk was added to encompass a broad area of
the Ohio Valley with emphasis on those sensitive areas spanning
eastern IL, central IN, and much of central/eastern OH.

Kleebauer

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

The previous forecast remains on track with the latest 00z CAMs
maintaining agreement on scattered thunderstorm activity likely to
transpire later this afternoon, carrying into the evening hours.
Areas of the Front Range and southern AZ are the most notable for
seeing more organized convection in the setup thanks to added
shear and regional ascent attributed to a shortwave proxy off the
Central Rockies to the north, and a deeper moisture layer and
instability maxima near the International border. The general
synoptic pattern has remain persistent in its presentation the
last several forecast cycles with the consensus unchanged on where
convection will occur.

The expected 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 D1, 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.

In coordination with the Tuscon WFO in southern AZ, a SLGT risk was
added over the areas along and west of the Huachuca Mtns through
the I-19 corridor into the eastern sections of Pima/Pinal counties
due to a more robust axis of convection expected across far
southern AZ by later today. Forecast is for a cluster of
thunderstorms maturing and migrating off the Huachucas, advancing
westward through Santa Cruz and southeastern Pima counties as we
head into the evening. HREF probs for >1" have ballooned upwards of
60-80% over the I-19 corridor in southern AZ, including over the
city of Tuscon lending a favorable potential for flash flooding,
even into the metro area. Rainfall rates between 0.5-1"/hr are
anticipated in the strongest cells, plenty aggressive enough to
exceed the already low FFG thresholds in place across the southern
AZ deserts. Expecting scattered flash flood potential given the
current signal, more than sufficient for a targeted SLGT risk upgrade.

...Florida into Far Southeast Georgia...

A small mid-level vorticity maxima will slowly organize over
central and northwestern FL with a proposed surface trough likely
situated very close to the northeastern coast of the area. The
combination of the two features and generally moist and unstable
airmass presence will allow for quite a convective burst to occur
by the afternoon hours, especially as you add the typically diurnal
components of the sea breeze as a focal point for convection. HREF probabilities are pretty aggressive for >3" locally (60-90%)
extending from Orlando up through just south of Jacksonville. We
saw some isolated flood instances crop up from storms earlier in
the previous period from an even less favorable setup compared to
what will transpire today. Hourly rates of the heaviest cells will
likely be between 3-4"/hr after assessing individual deterministic,
which makes sense considering PWATs easily pushing 2.1-2.4" over
the entire stretch of the east coast of FL and all of North FL into
the Panhandle. Given the signal, a MRGL risk was added across that
northeastern extent of FL into far southeastern GA, south of Brunswick.

Kleebauer

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