240
FXUS63 KILX 170858
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
358 AM CDT Tue Jun 17 2025

.KEY MESSAGES...

- Thunderstorm chances will increase this afternoon through
  Wednesday...with the highest probability for severe weather
  focusing Wednesday afternoon and evening.

- There is a strong signal for hotter weather by the weekend. High
  temperatures will rise into the lower to middle 90s both
  Saturday and Sunday, resulting in peak heat index readings above
  100 degrees.

&&

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

Areas of dense fog have developed early this morning, primarily in
areas 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 US 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. CAPE/shear vectors will support
discrete supercell development at the onset, particularly across
the northern half of Illinois where the shear vectors will be
oriented somewhat perpendicular to the line of forcing. The
hodographs look fairly straight, suggesting splitting supercells
capable of giant hail, but with one or numerous convective
outflows lurking, 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. Very hot temperatures then set in by Friday. 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 the weekend.

MJA

&&

.AVIATION...  (For the 06z TAFs through 06z Tuesday Night)
Issued at 1244 AM CDT Tue Jun 17 2025

Main concern in the short term is for fog and stratus development
over the area, particularly the I-72 corridor where it appears
mid-level cloud cover will dissipate earliest as an upper level
shortwave moves over the area tonight. Guidance points toward at
least brief periods of IFR or worse vsbys in and near the I-72
corridor. At this point, have kept KPIA out of MVFR forecast as it
appears mid-level cloud cover may linger long enough to avoid
this. Next concern is potential for thunderstorms encroaching on
the central IL terminals this afternoon and evening. Due to
considerable uncertainty with location of thunderstorm
development, have kept mention to PROB30 at this point, starting
as early as 22Z at KPIA to as late as 02Z at KDEC and KCMI.
Thunderstorms would likely create MVFR conditions as well as gusty
and erratic winds. General winds S 4-10 kts through the period.

37

&&

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

$$