106
FXUS63 KILX 291712
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
1212 PM CDT Tue Sep 29 2026

.KEY MESSAGES...

- Warm Midweek Temperatures: Highs will reach the low to mid-80s
  today and Wednesday, running 5 to 10 degrees above typical
  late-September averages.

- Initial Rain Chances Wednesday: Rain chances increase late
  tonight into Wednesday, particularly near the Illinois River
  Valley, though confidence in widespread rain remains low
  before Thursday.

- Heavy Rain Potential Thursday to Friday: Widespread heavy rain
  is expected Thursday through Friday. Areas north of Interstate
  70 have a 50-80% chance of exceeding 1", with a 40-60% chance
  of over 2" north of a Jacksonville to Rantoul line.

- Cool and Dry Weekend: Drier, autumn-like conditions move in
  late Friday into Saturday, dropping weekend highs into the
  upper 60s and lows into the 40s.

&&

.DISCUSSION...
Issued at 247 AM CDT Tue Sep 29 2026

.Today through Wednesday...

A rapid transformation in the synoptic pattern unfolds today
across the Central CONUS as an expansive western trough absorbs
the remnants of Tropical Depression Polo. Downstream shortwave
energy ejecting out of the Plains into the Mid and Upper-
Mississippi Valleys will flatten the amplified ridge overhead,
simultaneously accelerating the departure of an East Coast upper
low into the Atlantic basin. Associated return flow and robust
thermal/moisture advection will send afternoon highs into the
low to mid 80s both today and Wednesday--nearly 5 to 10 degrees
above late-September climatology.

Recent model guidance continues to display significant
variability regarding the precise timing and magnitude of
incoming precipitation, maintaining a fair degree of uncertainty
along the heavy rainfall axis. However, notable trends in the
00z model suite--particularly across the ECMWF and RRFS-
MPAS--have shifted the initial push of rain farther south for
Wednesday. This reflects a distinct shift from yesterday`s
global consensus, which maintained the predecessor rain event
well to our northwest across Iowa and Wisconsin.

In light of these trends, PoPs have been nudged upward from
late tonight through midday Wednesday to capture the robust
shortwave energy traversing the Illinois River Valley in tandem
with forced WAA and isentropic lift. Forecast soundings
illustrate that low-level saturation will occur gradually via
top-down moistening. Consequently, confidence remains low
regarding deterministic model depictions of widespread 1-inch
rainfall totals prior to Thursday, owing to the predominantly
non-convective nature of the rain and weak frontogenetical
forcing during this early phase. Even so, an anomalously rich
moisture plume--with PWATs exceeding the 99th percentile for
late September (>1.75 inches)--underscores the potential for
highly efficient rainfall during this initial round should it
fully materialize on Wednesday.


.Thursday through Monday...

The highest confidence for widespread, heavy rainfall remains
centered in the Thursday through Friday timeframe. During this
period, the main western shortwave opens up and tracks eastward
along a slow-moving surface cold front. High confidence exists
in widespread measurable QPF across Central and Eastern
Illinois; however, exact storm-total accumulations will hinge
heavily on the frontal speed and the sharpness of the baroclinic
zone. Blended NBM guidance currently highlights a 50-80%
probability of rainfall exceeding 1" north of Interstate 70
through Friday night. The potential for heavy rain exceeding 2"
is highest (40-60% chance) north of a Jacksonville to Rantoul
line, where localized mesoscale banding and training echoes
could readily yield higher swaths.

While low-end precipitation chances (15-25%) linger through
midday Saturday to account for a potential delay in the northern
stream shortwave, conditions will dry out rapidly from
northwest to southeast as a broad Continental Polar (cP) high
expands across the Midwest and Great Lakes. This airmass
transformation will usher in a classic autumn cool-down relative
to our midweek warmth, holding daily highs in the upper 60s and
lows in the 40s to close out the week. Extended ensemble
guidance suggests this expansive Canadian high will remain
entrenched across the region through at least midweek,
delivering a prolonged stretch of dry, ideal fall weather.

&&

.AVIATION /18Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 1212 PM CDT Tue Sep 29 2026

VFR conditions are expected through the TAF period. Southerly
surface winds will back to the southeast this evening before
returning to the south Wednesday morning. Clouds will lower
overnight ahead of an approaching system, with scattered rain
showers expected at KPIA, KBMI, and possibly KSPI by morning.

&&

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

&&

$$

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