916
FXUS63 KILX 181043
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
543 AM CDT Fri Sep 18 2026

.KEY MESSAGES...

- The heart of the hot airmass will shift south of central
  Illinois today: however, heat index readings will still climb
  to near 100 degrees along and south of a Jacksonville...to
  Effingham...to Robinson line this afternoon.

- There is a Marginal Risk (level 1 of 5) for severe weather
  both Saturday and Sunday as a nearly stationary frontal
  boundary wavers across the region.

&&

.DISCUSSION...
Issued at 314 AM CDT Fri Sep 18 2026

06z/1am surface analysis shows a stationary front extending from
just north of Saint Louis to near Cincinnati. Winds have shifted
to northeasterly across much of the KILX CWA: however, drier
air is only slowly filtering into the area...as surface
dewpoints are still in the 70s except across the far north
around Galesburg and Lacon where readings have dropped into the
upper 60s.

There will be two primary forcing mechanisms in the region
today...the frontal boundary to the south and a short-wave
trough tracking from the Dakotas into the Great Lakes. With
central Illinois between these two features, it appears rain
chances will remain low. Several CAMs indicate a few
showers/thunderstorms associated with the short-wave may graze
the northern half of the CWA along/north of I-72 this morning
through early this afternoon. Meanwhile models such as the
HRRR/RRFS have consistently shown isolated convection firing
along the front from central/southern Missouri into southeastern
Illinois this afternoon. Overall areal coverage remains low on
all models, so have only mentioned 20 PoPs in most areas. The
main weather story today will be the cooler conditions. A large
north-south temperature gradient will exist...with highs ranging
from the upper 70s far north around Galesburg to the middle 90s
far south near Flora and Lawrenceville.

Another short-wave trough tracking along the northern periphery
of the prevailing upper ridge centered over the Southern Plains
will trigger renewed clusters of convection across Iowa into
southern Wisconsin/northern Illinois late tonight into Saturday
morning. While most of this activity will stay north of central
Illinois, a few showers/thunderstorms may spread as far south
as I-72. Will need to keep an eye on the severe weather
potential for Saturday afternoon/evening as convection tries to
re-develop along existing outflow from the morning activity.
While surface dewpoints in the 70s will support moderate
instability, deep layer shear will remain weak. The 00z HREF
focuses the highest probability (40-60% chance) of SBCAPEs
greater than 1000J/kg and 0-6km shear of more than 30kt along
the I-74 corridor. While SPC indicates a Marginal Risk (level 1
of 5) for severe weather across all of central/southeast
Illinois on Saturday, think the I-74 corridor will see the
highest risk for a few storms with marginally severe wind gusts
during peak heating.

With the primary baroclinic zone wavering across the region and
additional short-wave troughs gradually flattening the upper
ridge, rain chances will increase further south across all of
central Illinois Sunday into Monday. Instability will be less on
Sunday: however, a severe weather risk may materialize during
the afternoon/evening mainly south of the I-72 corridor due to
an uptick in shear as the NAM shows 0-6km values reaching
30-40kt. The front will sink well south of central Illinois by
Monday, but additional short-wave energy will keep showers in
the forecast. After that, high pressure will build in from the
Northern Plains and bring cooler and less humid conditions
through the end of next week. The coolest period will be Tuesday
and Wednesday night when lows drop into the lower to middle 50s.
The 00z NBM even hints at cooler readings...with a 30-50% chance
of lows below 50 degrees everywhere north of I-70.

&&

.AVIATION /12Z TAFS THROUGH 12Z SATURDAY/...
Issued at 543 AM CDT Fri Sep 18 2026

IFR ceilings prevail at the central Illinois terminals early this
morning. While the overcast has scattered upstream across
northeastern Illinois, satellite trends strongly suggest the
clouds will hold firm for the next several hours. Both NAM and
HRRR forecast soundings show cloud bases gradually improving to
MVFR by mid to late morning, then to VFR this afternoon. Based
on HRRR trends...have gone back to VFR at KPIA/KSPI/KDEC by 17z, at
KCMI by 19z, and finally at KBMI by 20z. After that, skies will
remain partly to mostly cloudy and there is some indication
that the low overcast may return at the I-74 terminals
overnight. Confidence in this remains low, so have only lowered
ceilings to MVFR at KPIA/KBMI after 08z. Winds will initially be
NE at 5-10kt, then will veer to E/SE this afternoon...then to
S/SE by late tonight.

&&

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

&&

$$

KEY MESSAGES...Barnes
DISCUSSION...Barnes
AVIATION...Barnes