245
FXUS63 KILX 301054
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
554 AM CDT Wed Sep 30 2026

.KEY MESSAGES...

- Scattered Showers Today: Rain showers focused north of
  Interstate 72 will exit the area by midday. While rain totals
  will mostly stay under 0.75", localized amounts exceeding 1"
  are possible northwest of the Illinois River Valley. Highs
  south of I-72 will reach the mid to upper 80s.

- Heavy Rainfall Thursday-Friday: Multiple rounds of widespread
  rain return Thursday afternoon through Friday morning, with
  0.25" to 1.25" anticipated across most areas east of the
  Illinois River. Narrow, heavy rain bands could produce
  localized totals of 2" to 3".

- Autumn Turn This Weekend: Cooler, drier air builds in Friday
  night, bringing crisp fall weather. Weekend highs will drop
  into the upper 60s with overnight lows in the 40s.

&&

.DISCUSSION...
Issued at 301 AM CDT Wed Sep 30 2026

.SHORT TERM... (Today through Thursday Morning)

Early morning water vapor imagery reveals a prominent closed
upper low centered over Arizona, which has successfully absorbed
the remnants of Tropical Depression Polo. Downstream of this
main feature, a series of compact mid-level shortwaves continues
to ripple across the Plains and Upper/Mid-Mississippi Valley,
generating sufficient synoptic forcing for scattered shower
activity across western and central Illinois. Concurrently, an
elongated low-level jet (LLJ) axis spanning from the southern
Plains to the Great Lakes is aiding in warm air advection and
deep isentropic ascent, contributing to top-down column
saturation.

Surface observations show showers remaining predominantly high-
based across western Illinois early this morning. As evidenced
by the 00z KILX sounding, a stubborn near-surface dry layer has
delayed ground-level saturation. High-resolution CAM guidance
indicates that shower activity will stay largely focused north
of the Interstate 72 corridor through the morning hours before
lifting north and exiting the CWA by midday. This timing aligns
with the low-level jet veering and decaying as its core
translates northeast of the forecast area.

Quantitative precipitation forecasts (QPF) via REFS LPMM
guidance yields light totals with this initial wave, generally
0.25" to 0.75" north of a Macomb to Pontiac line. Points south
and east will experience sharply lower totals due to persistent
dry air in the lowest 1 km. While convective instability remains
negligible today, efficient warm-rain processes--facilitated by
a deep warm-cloud layer and precipitable water (PWAT) values
exceeding the 99th percentile of late-September
climatology--could yield localized 1" + totals northwest of the
Illinois River Valley before precipitation wanes.

South of I-72, broken cloud cover and negligible precipitation
will allow strong surface heating, driving afternoon high
temperatures into the mid- to-upper 80s.

A lull in precip will take hold from this afternoon through
Thursday morning as mid-level heights build across the Mid-
Mississippi Valley in advance of the approaching western trough.
Dry and warm conditions will persist through early Thursday
morning.


.LONG TERM... (Thursday Afternoon through Mid-Next Week)

The upper-level trough over the southwest U.S. is projected to
open and eject northeastward across the Mid-Mississippi Valley
by midday Thursday, aligning parallel to a slow-moving, surface
frontal boundary. The coincidence of strong upper dynamics, a
rich tropical moisture plume, and favorable frontal forcing will
trigger multiple rounds of widespread rainfall from Thursday
afternoon through midday Friday.

Latest REFS probabilistic guidance places 10th-to-90th
percentile QPF totals between 0.25" and 1.25" across most areas
east of the Illinois River Valley into southeast Illinois.
However, high moisture flux and frontogenetical forcing along
the slow-moving boundary favor the development of narrow,
mesoscale rain bands. Where these bands develop, localized
totals of 2" to 3" remain well within the realm of possibility
across the CWA.

Conditions will dry out rapidly from northwest to southeast
Friday evening as the frontal zone clears the area and an
expansive Continental Polar (cP) high pressure system builds
eastward across the Midwest and Great Lakes. Cold air advection
in the wake of the front will usher in a distinct airmass
transformation, replacing early-week unseasonable warmth with a
classic autumn pattern. High temperatures Saturday through early
next week will drop into the upper 60s, with overnight lows
dipping into the 40s. Ensemble guidance maintains high
confidence that this broad Canadian high will remain anchored
over the region through at least midweek, yielding an extended
stretch of quiet, crisp fall weather.

&&

.AVIATION /12Z TAFS THROUGH 12Z THURSDAY/...
Issued at 551 AM CDT Wed Sep 30 2026

Warm-air advection driven showers and possibly a few storms
will lift across west-central Illinois this morning, but should
shift to northern Illinois by early to mid afternoon. Showers
are expected to be light and high- based initially with little
impact to visibility.

&&

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

&&

$$

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