621
FXUS63 KILX 200522
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1222 AM CDT Sun Sep 20 2026

.KEY MESSAGES...

- Overnight Rain & Thunderstorms: Showers and thunderstorms
  will build across northern Illinois tonight, mainly along and
  north of Interstate 80. Heavy rainfall over 1 inch carries a
  low chance (20% or less) across central Illinois, with rain
  tapering off by Sunday morning.

- Sunday Afternoon Storms: A few stronger thunderstorms may
  redevelop south of Interstate 72 late Sunday afternoon.
  Localized damaging winds and marginal hail will be the primary
  threats.

- Cooler & Mostly Dry Next Week: High pressure will bring
  autumn- like weather Monday through Wednesday, with highs in
  the upper 60s to lower 70s. Widespread severe weather and
  flooding are not expected.

&&

.DISCUSSION...
Issued at 154 PM CDT Sat Sep 19 2026

Synopsis:
A slow-moving frontal boundary will waver across central
Illinois through Sunday, resulting in periodic showers and
thunderstorms. Cooler, fall-like conditions will then settle in
for the upcoming week. Although a drier weather pattern is
generally expected, persistent low clouds and sporadic sprinkles
are likely to continue throughout next week. The potential for
severe weather and flooding will stay low beyond Sunday.


Overnight through Sunday Morning:
The most widespread shower and thunderstorm coverage will take
shape overnight near the Interstate 80 corridor as robust
shortwave energy rides over the surface boundary. Uncertainty
remains regarding how far south of I-80 convection will reach,
but with the nose of the low-level jet positioned north of the
area through Sunday morning, precipitation is unlikely to extend
significantly south of a Macomb-to-Champaign line. Heavy
localized rain exceeding 1 inch carries low confidence (20% or
less). Shower activity will abate by midday Sunday as the low-
level jet weakens and mid-level subsidence overspreads the
region behind the departing wave.


Sunday Afternoon:
Another shortwave impulse traversing the sagging front will
likely trigger renewed scattered thunderstorm development south
of Interstate 72 on Sunday afternoon. Pooling dewpoints in the
lower 70s along the boundary will yield around 1500 J/kg MLCAPE,
coinciding with an intensifying westerly flow regime that will
boost bulk shear into the 30 kt range. Although mid-level lapse
rates remain unimpressive, the overlapping CAPE and shear
parameter space may support a few stronger updrafts capable of
localized marginally severe wind and hail.


Sunday Night through Midweek:
Ensemble and deterministic guidance remain in strong agreement
that a closed upper-level low will migrate eastward across the
Plains, entering the Mid-Mississippi Valley by Monday. This
disturbance will support additional rain chances through Monday
morning before it dismantles the dominant heat ridge while
driving the frontal boundary south and east of our area. In its
wake, expansive surface high pressure will establish itself
beneath a blocking split- flow regime, ushering in significantly
cooler and drier conditions. Daytime highs Monday through
Wednesday are forecast to settle into the upper 60s to lower
70s, and minor shortwave perturbations rotating around the
persistent low overhead could maintain brief, isolated
sprinkles.

&&

.AVIATION /06Z TAFS THROUGH 06Z MONDAY/...
Issued at 1221 AM CDT Sun Sep 20 2026

Unsettled conditions are expected across central Illinois with
a frontal boundary lingering over central Illinois and showers
and thunderstorms expected in its vicinity much of the period.
There are two windows of more favored activity on station: the
first associated with decaying activity from the area of storms
currently over northern Missouri as it moves into central
Illinois Sunday morning; the second with redevelopment over the
area during peak heating Sunday afternoon and early evening.
Numerous convective outflow boundaries lend to low confidence in
the wind forecast, but outside the influence of these
boundaries, winds should generally favor a northeasterly
direction. Finally, low ceilings currently in place over
northern Illinois are expected to spread south across central
Illinois late tonight into Sunday and linger over the region the
remainder of the period.

&&

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

&&

$$

KEY MESSAGES...MJA
DISCUSSION...MJA
AVIATION...Deubelbeiss