458
FXUS63 KILX 180055
AFDILX

Area Forecast Discussion...Updated
National Weather Service Lincoln IL
755 PM CDT Sun Aug 17 2025

.KEY MESSAGES...

- Hot and humid conditions will continue into Monday, with heat
  indices peaking around 100 to 105 degrees. Heat and humidity
  will persist for portions of the area mainly south of I-72
  Tuesday before more seasonable conditions return midweek.

- There will be occasional chances (20-50%) for showers and storms
  through late Tuesday. At this time, the severe threat appears
  low, but locally heavy rain cannot be ruled out.

&&

.UPDATE...
Issued at 755 PM CDT Sun Aug 17 2025

Thunderstorms have been percolating the last few hours, along a
boundary roughly extending from Cedar Rapids to Danville. These
mostly have been just north of our forecast area. WoFS ensembles
suggest this staying outside of our area, though latest HRRR runs
bring them down to about I-74 this evening. Some tweaks to the
forecast PoP`s have been made to focus rain chances in this
corridor.

Regarding heat headlines, the advisory has been extended into
early Monday evening for areas south of I-74. Along and north of
I-74, there is some question as to how much the evening and
overnight convection will play a role in Monday`s heat index
values. Earlier runs of the HREF had only about a 30-40% chance
of reaching 100 degree heat index Monday afternoon in this area.

Geelhart

&&

.DISCUSSION...
Issued at 201 PM CDT Sun Aug 17 2025

A cold pool from overnight and morning convection surged across
central Illinois this morning putting a damper on the hottest
temperatures that would have otherwise occurred this afternoon.
Still, we were able to recover enough for heat indices to push to
near 100 degrees as of 2pm obs. With additional clearing and
recovery, we may marginally reach triple digit heat indices this
afternoon, thus will let the advisory ride as-is through this
evening despite the possibility that some spots may not quite
reach criteria. Note: local single day Heat Advisory criteria is
105 degrees: however, there is a secondary criteria of prolonged
heat with 4 days of 100+ degree heat index. Meanwhile, the
aforementioned cold pool has pushed to near the Mississippi River
this afternoon and latest runs of the HRRR and NAM Nest suggesting
this will be the focus for isolated to scattered thunderstorms
this afternoon and evening. Storms will push east or southeast off
of this boundary into portions of central Illinois.

Strong upper ridge will remain over the mid Mississippi Valley
Monday and should allow heat indices to peak in the 100-105 degree
range area-wide. There remains some uncertainty around convective
trends that could impact temps Monday, but unstable, albeit
weakly forced, conditions will keep an isolated PoP mention
(20-25%) in the forecast for much of the day. LREF probs for
exceeding 100 degree Heat Index runs around 70-80 percent west of
a Peoria to Springfield line Monday but quickly tapers off further
east. Given the uncertainty, will wait to see how convection
through the rest of today into tonight plays out to get a better
feel for where headlines may be needed Monday.

Tuesday and Wednesday, a series of shortwaves will help to break
down the upper ridge locally as it starts to retrograde back
towards the Desert Southwest. As northwest flow aloft overspreads
central Illinois, the heat and humidity will begin to be shunted
south across the area, replaced by more seasonable conditions with
highs in the 80s. Precip chances will ramp up Tue/Wed as a slow
moving surface cold front sags south across the region. Ahead of
the front, moderately strong instability should remain in place
across central Illinois, though deep layer shear will be on the
weaker side precluding a more organized severe weather risk. Still
wouldn`t rule out some pulse severe thunderstorms with localized
downburst winds the primary concern.

High pressure builds back across the Upper Midwest and will
provide fair weather conditions across central Illinois through
the end of the work week and the upcoming weekend. Thursday
through Saturday are favored to remain dry with temps topping out
in the low to mid 80s. Slightly lower humidity levels will also be
in place due to the northerly flow.

Deubelbeiss

&&

.AVIATION...  (For the 00z TAFs through 00z Monday Evening)
Issued at 625 PM CDT Sun Aug 17 2025

Storms are scattered across the northern third of Illinois early
this evening, and the threat may drift southward to KPIA/KBMI for
a time mid to late evening. Will include PROB30 mentions for TSRA
at both sites. VFR conditions otherwise prevail. Late in the
period, potential for thunderstorms increases as a cold front
approaches from the northwest. Can`t rule out something developing
by 00z, but enough uncertainty to preclude a mention in the TAF`s
for this cycle.

Geelhart

&&

.ILX WATCHES/WARNINGS/ADVISORIES...
Heat Advisory until 8 PM CDT Monday for ILZ036-040>042-047>054-
056-061>063-066>068-071>073.

&&

$$