639
FXUS63 KLOT 170006
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
706 PM CDT Wed Sep 16 2026

.KEY MESSAGES...

- Showers and storms will move across the region late tonight
  through tomorrow morning. Localized flooding may occur in
  typical prone locations.

- A brief break in the pattern is expected tomorrow afternoon
  through Friday before another period of wet weather
  materializes this weekend.

&&

.DISCUSSION...
Issued at 205 PM CDT Wed Sep 16 2026

Per a recent hand surface analysis, the Lower Great Lakes
reside in a broad low-level baroclinic zone along the southern
flank of an expansive high pressure system centered over Lake
Superior and an unusually strong (600dm!) upper-level ridge
centered over the southeastern United States. The broad region
of low-level confluence within the baroclinic zone is supporting
a pool of unusually high moisture content across portions of
Iowa, Missouri, Illinois, and Indiana with surface dew points in
the upper 60s to lower 70s and PWATs of 2.1 to 2.25" (near
record highs for the middle of September). A series of weakening
MCVs are parading within the baroclinic zone, leading to areas
of showers stretching from the IA/MO border through northern
Illinois and into northern Ohio.

Tonight, an upper-level wave propagating within an elongated
upper- level jet streak along the US/Canadian border will induce
modest low- level pressure falls in the central Great Lakes. As
a result, south to southwesterly low-level flow will increase
across the middle Mississippi River Valley (25 to 30kt at 850mb)
facilitating a gradual increase in isentropic ascent and an
increase in shower and thunderstorm coverage within the
baroclinic zone. Coverage of storms should first be highest
across central Iowa near the nose of the low- level jet before
expanding eastward into northern Illinois toward daybreak as the
direction of the low-level jet veers westerly. Storms should
gradually weaken in intensity and coverage through the morning
hours as the low-level jet weakens in magnitude.

CAM-based guidance matches such a conceptual model with a
somewhat narrow band of storms parading within the baroclinic
zone tonight into tomorrow morning, but will note the usual
differences in exact storm placement owing to disparity in the
exact location of best low-level forcing. Tend to give a bit
more preference to model outcomes that depict a more
southeasterly trajectory in line with conglomerating outflow,
leading to a favored corridor more or less near or just south of
the I-80 corridor. Accordingly, will feature the highest PoPs
(>80%) near and south of I-80 tonight through tomorrow morning.
With at least some hint of upper-level jet forcing in the
general vicinity to facilitate weak frontogenesis, can also
envision additional showers and storms north of the primary
axis near the IL/WI state line, as well. So, will hold onto
likely PoPs as far north as Rockford and Evanston through the
morning hours.

With the aforementioned anomalously moist environment (PWATs
perhaps ticking upward toward 2.3 to 2.4"), rain rates beneath
thunderstorms tonight into tomorrow morning will be solidly in
the "torrential" category. Conceptually, would expect there to
be a stripe or two of 12-hour rainfall amounts in the 2 to
locally 5 inch range tonight into tomorrow morning stretching
from central Iowa through northern Illinois. Did give serious
consideration to hoisting a Flood Watch across the area, though
was compelled to hold off by 30-day precipitation deficits of a
mere 10 to 20% of normal and soil moisture values well below the
seasonal values. With that said, can certainly envision areas
of ponding water in low-lying areas where downpours are most
persistent as well as a typical urban flash flood threat.

After a wet morning, current expectations are for the latter
half of Thursday and into Friday to be dry as a surface high
pressure system builds into the Great Lakes and shunts the low-
level baroclinic zone southward. Friday may offer the best shot
of seeing more than a few peeks of the sun over the next several
days. Persistent onshore flow on the southern side of the
surface high should lead to a modest lake shadow with highs
generally ranging from around 80 inland to the mid 70s lakeside.

Attention then turns to what will be another wet period across
the region this weekend. Ensemble guidance remains in excellent
agreement that the upper-level wave moving across California
will stall by midweek as the stubborn upper-level ridge across
the southeastern US migrates toward the central Plains. As this
occurs, an upper-level wave (or two) originating from northern
Mexico will propagate northward across the western US and shift
eastward toward the Midwest by Friday. Increasing upper-level
jet diffluence and low-level convergence along the noses of an
intensifying low-level jets should lead to hefty regions of
showers and storms in the Upper Mississippi River Valley to our
north and west Friday into Saturday morning. As the parent
Pacific wave drifts eastward and a surface high pressure system
moves into southern Canada this the weekend, a low-level frontal
zone will shift southward across the Lower Great Lakes and
serve as a focus for several rounds of showers and storms
Saturday afternoon through the middle of next week. Will hence
advertise nearly continuous broad-brushed PoPs Saturday
afternoon onward with a gradient in temperatures (coolest north,
warmest south), not unlike what we`re dealing with now.

Borchardt

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 706 PM CDT Wed Sep 16 2026

Key Messages:

- Showers are likely with a chance for thunderstorms at the
  terminals overnight into early tomorrow morning.

- IFR ceilings are expected tonight into tomorrow morning with a
  chance for LIFR ceilings as well.


Showers and thunderstorms are likely to increase in coverage to
the west of the terminals after midnight before propagating
eastward or east-southeastward towards the terminals. The
greatest coverage of storms may tend to focus near and south of
the I-80 corridor, where a greater degree of instability will
reside, but isolated to scattered convective cells may
nevertheless develop farther to the north and pose a threat for
lightning at the terminals towards and after daybreak/sunrise.
Gave strong consideration to converting the inherited PROB30
groups for TSRA to TEMPO groups, but opted to maintain the
PROB30 groups for the 00Z TAF issuance given the lingering
uncertainties. The overnight/early morning shower/storm
activity should clear the area by midday tomorrow.

Widespread MVFR and patchy IFR ceilings are ongoing at press
time. Further lowering of these ceilings is expected later
tonight with widespread IFR conditions likely prior to daybreak
and at least patchy LIFR ceilings appearing to be a distinct
possibility as well. These low ceilings will likely be slow to
improve after daybreak tomorrow, but a return to MVFR conditions
is favored by the early-mid afternoon. Sub-VFR visibilities may
also occur at times into tomorrow morning as a result of the
showers, possible patches of drizzle, and/or lingering fog/mist.

Otherwise, winds will generally remain near or under 10 kts
throughout the TAF period.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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