523
FXUS63 KILX 171745
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
1245 PM CDT Tue Jun 17 2025

.KEY MESSAGES...

- Thunderstorm chances return later this afternoon and extend
through Wednesday.  There is currently a low (< 15%) probability for
severe weather today, then a medium (15-30%) probability for severe
weather on Wednesday.

- Heat stress returns today across the region with peak heat index
values near 90 F.  Then, even hotter conditions arrive this weekend
with peak heat index values above 100 F.

&&

.DISCUSSION...
Issued at 355 AM CDT Tue Jun 17 2025

Areas of dense fog have developed early this morning, primarily in
areas near and south of I-72 where a gentle south wind is
overrunning a weak sfc front. Short term guidance is in good
agreement that fog and low stratus will mix away prior to 10am, due
partly to a high summer sun angle, but also because the sfc front
departs further south and east by then.

Temperatures and humidity will both trend upward this afternoon as a
western Gulf airmass surges northward in between an expanding
subtropical high across the SE U.S. and a decaying ridge across the
Plains. Consensus is for afternoon temperatures to top out somewhere
between 85-91F, with dewpoints around 70F. Peak heat index values
will jump into the lower 90s for many. This is not out of the
ordinary for this time of year, but impactful nonetheless.

Scattered thunderstorms remain a possibility later this afternoon,
though not all the hi-res guidance supports this idea. The latest
HREF guidance projects a 30-50% chance in areas generally west of
a Taylorville-to-Bloomington line, while the all-inclusive NBM is
considerably more bullish at 50-80%. The conceptual model
certainly favors convection developing by late afternoon as a few
shortwave impulses lift along a weak baroclinic zone, which
extends from roughly Kansas City to Chicago. Conditions will
become moderately unstable south of this boundary, with RAP/HRRR
guidance supporting MLCAPE values near 2000 J/kg. Weak mid-level
flow and poor deep-layer shear should limit the severe potential
as skinny updrafts quickly collapse. But, with the favorable
instability in place, we will continue to hedge our bets for an
isolated 60 mph downburst or spot ping pong hail report.

Thunderstorm clusters may then fester through the overnight period
as improving kinematics/shear push across the Mississippi Valley
ahead of a well-defined mid-level shortwave trough and attendant
sfc low. The severe weather risk should theoretically continue
overnight, though by then the risks shift more toward damaging
winds and urban flooding as the parameter space favors good cold
pool generation/maintenance.

Both the probability and coverage of severe weather increases by
Wednesday, primarily in areas east of the Illinois River Valley,
as a compact shortwave trough lifts across the Mid-Mississippi
Valley and pushes a sfc cold front through central Illinois.
Convective initiation could be shunted further east if debris from
overnight convection lingers through midday, but the general trend
seen in hi-res guidance is for storms to develop near and east of I-
55 during the afternoon hours.

The CAPE/Shear profile will support discrete supercells at the
onset, particularly across the northern half of Illinois where the
shear vectors will be oriented somewhat perpendicular to the line of
forcing. A straight hodograph suggests very large hail could be the
primary hazard, as supercells will have a tendency to want to split.
But, with one or numerous convective outflows lurking ahead of the
cold front, the sig tor threat cannot be discounted.

As you venture more toward the southern half of Illinois, the shear
vectors are oriented more parallel to the line of forcing. This
favors quick upscale growth into a linear MCS with an increased risk
for bowing segments, embedded tornadoes, and to some extent, flash
flooding.

Drier conditions return Thursday behind the departing frontal
system. Hot temperatures then set in by the end of the week. By
then, a broad 588-mb ridge axis is modeled to push across the Corn
Belt, becoming amplified (594mb) by Saturday and Sunday as a deep
western trough emerges. Daily temperatures are forecast to warm into
upper 80s by Friday, then lower 90s Saturday through Monday amid low
70s dewpoints. This all adds up to triple-digit heat index values by
throughout the weekend.

MJA

&&

.AVIATION...  (For the 18z TAFs through 18z Wednesday Afternoon)
Issued at 1245 PM CDT Tue Jun 17 2025

Robust Cu-field is noted on 1730z/1230pm satellite imagery
along/southeast of a KCMI to KTAZ line. SCT to occasionally BKN
MVFR ceilings are anticipated at both KDEC and KCMI through 20z
before the clouds begin to scatter and/or shift further east.
Further west, FEW-SCT diurnal clouds will prevail for the
remainder of the afternoon. Most CAMs suggest thunderstorm
development west of the Mississippi River later this
afternoon...with the convection lifting E/NE across locations
along/west of the I-55 corridor this evening. Based on latest
RAP/HRRR, have opted to carry a TEMPO group for thunder at
KPIA/KSPI/KBMI from 01z through 03z/04z...before ending the
thunder mention for the night. Latest models indicate MCS activity
will remain well S/SW of central Illinois overnight, but an
outflow boundary from that convection will likely push across the
region Wednesday morning. Scattered convection will be possible
along the boundary, so have introduced a PROB30 group for all
sites between 13z and 18z. Winds will initially be SW at 8-12kt,
then will decrease to less than 10kt by sunset. SW winds will
resume on Wednesday and will become strong/gusty by midday through
the afternoon.

Barnes

&&

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

$$