534
FXUS63 KLOT 170810
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
310 AM CDT Thu Sep 17 2026

.KEY MESSAGES...

- Showers and storms will move across the region this 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 310 AM CDT Thu Sep 17 2026

Isentropic upglide associated with an intensifying
southwesterly low-level jet overrunning a quasi-stationary
surface boundary over central Illinois continues to facilitate
blossoming convection from eastern Iowa eastward into portions
of northern Illinois early this morning. While storm coverage
south of I-80 remains scattered as of this writing, we continue
to monitor expanding showers and thunderstorms over eastern
Iowa. This activity is expected to spread eastward into northern
Illinois through the predawn hours as the low-level jet veers.

The environment in which these storms are developing is
anomalously moist (precipitable water value near ~2" per the 00z
DVN RAOB) with very high freezing levels (just under 15,000 ft)
supportive of deep warm cloud depths. Accordingly, very heavy
rain rates (1+" per hour) will likely accompany these showers
and thunderstorms this morning, potentially leading to a few
bands of 2"+ amounts (locally higher), particularly where
multiple storms train over the same area in northern IL. While
rainfall of this magnitude could result in localized flash
flooding, recent dry weather (30-day precipitation deficits of
10-20% of normal) and low soil moisture may limit this threat to
some degree. Nevertheless, we will continue to monitor radar
trends for potential hydro issues this morning, particularly in
the more urban areas around Rockford and Chicago. Otherwise,
the shower and thunderstorm threat is expected to shift eastward
out of the area by late morning.

Aside from a low (~20-30%) chance of a few afternoon and
evening storms near the surface frontal boundary across central
Illinois and Indiana, a period of rain-free weather is expected
this afternoon through at least the first half of Friday. We
should see another fairly large variation in temperatures across
the area, particularly this afternoon. Low clouds are likely to
persist across northern areas (especially along and north of
I-80), holding temperatures steady in the lower to mid-70s
through the afternoon. Conversely, areas farther south into
central portions of Illinois and Indiana will experience another
very warm and humid afternoon, with temperatures and heat
indices topping out in the mid-to-upper 80s (low 90s).

Attention then turns to another wet period across the region
this weekend into early next week. Ensemble guidance remains in
agreement that the upper-level wave over the western CONUS will
eject a couple of mid/upper-level shortwaves northeastward into
the Midwest Friday into the weekend. Increasing upper-level jet
diffluence and low-level convergence along the nose of an
intensifying low-level jet should lead to widespread showers and
storms in the Upper Mississippi River Valley Friday into
Saturday morning. As the parent Pacific wave drifts eastward and
a surface high-pressure system moves into southern Canada this
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. We will maintain broad-brushed PoPs from Saturday
afternoon onward, along with a temperature gradient (coolest
north, warmest south) similar to current conditions.

KJB

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1252 AM CDT Thu Sep 17 2026

Main Concerns:

- Occasional SHRA and TS through early to mid morning today

- Low flight categories this morning, temporarily down to LIFR
  CIGs and IFR VSBY in heavier rain and/or BR, improving to MVFR
  CIGs by early afternoon (except at GYY).

At the start of the period, TS are confined to south of the C90
airspace. While the highest TS coverage will likely continue to
focus primarily south of C90 through mid morning, expecting
scattered TS on the northern edge to support TEMPO mention in
the TAFs, adjusted slightly later than previous issuances.
Widely scattered SHRA (no TS anticipated) may then persist until
midday/early afternoon, followed by primarily dry conditions
through early Friday morning.

A mix of higher end IFR to lower MVFR CIGs are expected to build
down to prevailing lower IFR (500-600 ft) at all TAF sites, with
a period of LIFR still favored. During heavier rain and/or BR,
1-2SM VSBY appears likely at times. CIGs will lift to MVFR by
early this afternoon, and likely remain MVFR into Friday. GYY
has the best chance to hold onto IFR CIGs through this evening.

Light north to northeast and VRB winds this morning will become
steady northeasterly this afternoon.

Castro

&&

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

&&

$$

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