948
FXUS63 KILX 082339
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
639 PM CDT Tue Sep 8 2026

.KEY MESSAGES...

- Heat indices will top out around 100 degrees Wednesday
  afternoon south of I-72 due to high temperatures in the 90s
  and rising humidity levels.

- Scattered thunderstorms are forecast Wednesday afternoon,
  becoming more widespread Wednesday night along a stalled
  front; localized heavy rainfall of 1 to 3 inches could cause
  localized flooding, particularly south of I-70. There is a
  marginal risk (level 1 of 5) for severe thunderstorms for
  much of central and southeast Illinois.

&&

.DISCUSSION...
Issued at 1256 PM CDT Tue Sep 8 2026

Short Term (Today through Wednesday Night)...

This afternoon, a potent 500mb low is currently lifting
northeastward across Manitoba, with an associated surface low
also over Manitoba and a cold front trailing southward across
the Middle Missouri Valley. Ahead of this system, Central
Illinois resides within a well mixed southwesterly flow regime.
This is facilitating the advection of an 850mb thermal ridge
(with temperatures of 22-24C) across the region. Consequently,
temperatures are warming into the low to mid-90s this afternoon.
Dew points remain in check--in the low to mid-60s--due to deep
boundary layer mixing, keeping peak heat indices near the
ambient temperatures in the low to mid-90s.

On Wednesday, low level flow will continue to strengthen,
transporting a more humid air mass into the region, with surface
dew points rising into the low to mid-70s. The combination of
high ambient temperatures in the 90s and this rich moisture
profile will drive afternoon heat indices to around or just past
100F, particularly for areas south of the I-72 corridor. The
cold front is expected to slowly push into central Illinois by
mid-to-late Wednesday morning. Given its arrival near the
diurnal instability minimum, initial convective activity is
projected to be in a decaying phase. However, as the front sags
south, it is forecast to slow and ultimately stall across the
I-72 and I-70 corridors during the afternoon and evening.
Scattered storm development remains possible along this stalled
boundary. Instability is progged to remain modest Wednesday
afternoon with MLCAPE values progged to peak around 500-750
J/kg. Deep layer shear will be seasonably strong, however,
around 35-40 kt which may support a few strong to severe storms.
Damaging wind gusts will be the primary hazard.

Overnight, convective potential increases significantly as a
modest low-level jet to our west interacts with the stalled
frontal zone, triggering a widespread round of storms that will
track eastward. Given that the steering flow (850-300mb) is
oriented nearly parallel to the surface boundary, training of
storms is a concern. Total rainfall through Wednesday night will
average under 0.50 inches north of I-72, but further south,
widespread amounts of 0.50 to 1.25 inches are expected, with
localized swaths of 1 to 3 inches possible (particularly south
of I-70) as highlighted by the HREF LPMM.

Long Term (Thursday through Next Tuesday)...
As the front sags south of the area on Thursday, high pressure
will build across the Upper Midwest, gradually shifting eastward
toward the Great Lakes by Friday. This will usher in a much
drier and more seasonable air mass. Expect quiet, pleasant
conditions across central Illinois by Friday with high
temperatures returning to seasonal norms in the 80s. Heading
into the weekend and early next week, the synoptic-scale pattern
is projected to flatten into a low-amplitude, zonal flow
regime. A series of weak, progressive waves will traverse this
flow, introducing periodic chances for showers and
thunderstorms. Due to persistent model discrepancies and low
predictability inherent in these low-amplitude systems, a
broad-brushed 20-40% Probability of Precipitation (PoP) has been
maintained from Friday night through next Tuesday by the NBM.

&&

.AVIATION /00Z TAFS THROUGH 00Z THURSDAY/...
Issued at 639 PM CDT Tue Sep 8 2026

A cold front is in place from northern Minnesota into the central
Plains states and will approach the area later in the period on
Wednesday. A broken line of showers and storms will approach west-
central Illinois Wednesday morning. Confidence is highest in
activity impacting KPIA with lower chances at remaining airfields.
South winds will veer to the west-northwest by Wednesday afternoon
with speeds remaining below 12 kts. Outside of shower and storm
chances, VFR conditions are expected.

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
None.

&&

$$

KEY MESSAGES...Deubelbeiss
DISCUSSION...Deubelbeiss
AVIATION...NMA