Excessive Rainfall Discussion
NWS Weather Prediction Center College Park MD
835 PM EDT Mon Aug 17 2026
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 2 threat area: www.wpc.ncep.noaa.gov/qpf/98epoints.txt