Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
853 PM EDT Tue Aug 18 2026
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 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt