019
FXUS63 KILX 302338
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
638 PM CDT Thu Jul 30 2026

.KEY MESSAGES...

- Periods of heavy rain will spread across the area between
  Friday and Saturday. Slow-moving thunderstorms Friday night
  into Saturday morning will increase the risk for localized
  flash flooding over saturated ground. Total rainfall will
  average 1 to 2 inches across central Illinois, though
  localized spots could see 3 to 4+ inches where heavy rain
  bands repeat.

- Pleasant, dry conditions return Monday and Tuesday with high
  temperatures rebounding into the mid-80s.

&&

.DISCUSSION...
Issued at 257 PM CDT Thu Jul 30 2026

.SHORT TERM... (Tonight through Sunday)

Quiet summertime weather has persisted today as large-scale
subsidence remains intact beneath an upper-level ridge axis
centered directly over Illinois. However, this tranquility will
be short- lived as the synoptic pattern undergoes a substantial
transition beginning late tonight. A pair of shortwave troughs,
including a convectively-augmented feature cresting the ridge
axis, will consolidate and dig sharply across the Mid-
Mississippi Valley over the next 24 to 36 hours.

A moisture-laden air mass will wrap rapidly into this synoptic
system, driven by robust Gulf moisture return and enhanced by
localized crop evapotranspiration. Forecast soundings portray
an environment highly conducive to efficient warm-rain
processes, characterized by anomalously high PWATs in the
1.8-2.2 inch range (near the 97th percentile for late July),
long-skinny CAPE profiles, and deep warm-cloud layers exceeding
12,000 feet. Unmitigated moisture transport will favor high
rainfall rates; however, overall convective intensity will be
somewhat tempered by poor mid-level lapse rates and a diurnally
unfavorable initial frontal timing.

At the surface, an occluding low (~1004 mb) and its trailing
cold front will slowly cross Illinois between Friday and Sunday
as the system becomes vertically stacked. Rain is forecast to
overspread the forecast area late Friday morning within a broad
wing of warm advection. Rain will become heaviest Friday
afternoon along and north of the I-74 corridor, focused along a
strong zone of mid-level frontogenesis on the northern flank of
the surface low. Latest HREF guidance targets areas between I-80
and I-74 with the highest probabilities for heavy downpours
through late afternoon.

By Friday evening, a well-defined dry slot will punch across
central Illinois, working to temporarily diminish or shut off
widespread stratiform rain. However, as the center of the
surface low tracks across the area late Friday night into
Saturday morning, scattered thunderstorms are expected to
redevelop near the triple point. While severe weather risk
remains low due to muted instability, there is a growing
concern for localized flash flooding. Weak flow beneath the
upper low will result in slow cell motion, raising the
potential for training convective bands over saturated soils.

Extensive cloud cover and widespread precipitation will keep
daytime temperatures constrained to the mid-to-upper 70s both
Friday and Saturday. As the deformation zone and primary upper
trough axis slowly slide east toward the Wabash River Valley,
low stratus and lingering light showers will persist into
Sunday morning across eastern Illinois before dry air advection
takes over by Sunday afternoon. Total rainfall from mulit-model
guidance (NBM) generally range from 1.75 to 2.25 inches near
and north of a Quincy to Terre Haute line, with 1.00 to 1.50
inches further south. High-resolution HREF LPMM guidance
continues to highlight localized heavy rain cores of 3 to 4+
inches where convective training occurs.


.LONG TERM... (Monday through Next Thursday)

Upper-level height rises and an amplifying western ridge will
build back into the Great Lakes early next week, bringing a
swift return to seasonably warm, dry, and pleasant summer
conditions for Monday and Tuesday. Highs will rebound back into
the mid-80s under mostly sunny skies.

The next opportunity for showers and thunderstorms arrives mid-
week as a robust northern-stream shortwave drives a cold front
toward the Upper Midwest. However, model guidance displays
considerable spread regarding frontal penetration into central
Illinois as the primary upper-level dynamics rapidly displace
eastward into Canada. Will maintain lower-end rain chances (20%)
for Wednesday into Thursday to account for lingering timing and
boundary position uncertainties.

&&

.AVIATION /00Z TAFS THROUGH 00Z SATURDAY/...
Issued at 638 PM CDT Thu Jul 30 2026

VFR flight rules and light winds are expected through the overnight
hours as high pressure largely remains in control. Conditions are
expected to deteriorate on Friday as an approaching low pressure
system brings waves of showers and storms, low ceilings, and breezy
south-southeast winds. Confidence is higher that widespread
precipitation will lie north of I-74 closer to the center of the low
with more scattered activity south of there. Ceilings will fall to
at least MVFR category for most airfields, with a low chance (20-
40%) for IFR at KPIA and KBMI.

&&

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

&&

$$

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