443
FXUS63 KILX 212343
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
643 PM CDT Thu Aug 21 2025

.KEY MESSAGES...

- Fog will develop across central and southeast Illinois late
  tonight...with a low probability (10-20% chance) of visibilities
  dropping below 1 mile along and north of the I-74 corridor.

- A cold front will push through the region on Saturday,
  triggering isolated showers and thunderstorms.

- Much cooler weather will prevail next week with highs in the 70s
  and lows in the 50s. There is a 20-40% chance that lows will dip
  into the upper 40s both Monday night and Tuesday night.

&&

.SHORT TERM...(Tonight through Friday)
Issued at 140 PM CDT Thu Aug 21 2025

SCT-BKN diurnal Cu will dissipate toward sunset, followed by
mostly clear skies tonight. As high pressure currently over the
Great Lakes builds southward and winds become nearly calm, the
stage will be set for fog development overnight. While the
boundary layer has dried slightly over the past couple of days,
surface dewpoints will hover in the lower to middle 60s. This will
provide adequate moisture for fog...especially in river valleys
and low-lying spots. As has been the case for the past few days,
the CAMs are generally under-forecasting the fog...with only the
GFSLAMP showing widespread visby reductions. Following a persistence
forecast, think fog will indeed develop across the board late tonight
into Friday morning. The main question will be how thick/widespread
the fog will become and if a Dense Fog Advisory will eventually be
needed across parts of the area. The GFSLAMP focuses the lowest
visbys at or below 1/4 mile along/north of a Peoria to Bloomington
line. Meanwhile the 12z NBM indicates a 10-20% chance for visbys
below 1 mile along and north of I-74. Given poor agreement among
the high-res models, am not confident enough to issue an advisory
at this time. Will however forecast fog everywhere, highlighting
the northern half of the KILX CWA with areas of fog. After the
early morning fog dissipates, it appears a SCT-BKN Cu-field will
once again form by mid-morning Friday. The NAM Cu-rule has handled
the cloud cover quite nicely recently, and it becomes highly
negative by midday Friday...especially everywhere east of the
Illinois River. Have therefore gone with a partly sunny forecast
with seasonable afternoon highs in the lower to middle 80s.

Barnes

.LONG TERM...(Saturday through Next Thursday)
Issued at 140 PM CDT Thu Aug 21 2025

A cold front is progged to approach from the northwest and swing
through central Illinois on Saturday. Southwesterly winds ahead of
the front will boost highs into the middle 80s, while dewpoints
increase into the upper 60s to around 70 degrees. This will
support SBCAPEs of 1500-2000J/kg. Given modest instability and a
forcing mechanism in place during peak heating, think its prudent
to include 20 PoPs for showers/thunder despite the NBM suggesting
a dry FROPA. Once the front passes, Canadian high pressure will
build southward and provide an extended period of cool/dry weather
next week. High temperatures will only be in the 70s and
overnight lows will drop into the lower to middle 50s Sunday
through Thursday. The coolest nights will be both Monday night and
Tuesday night when the NBM shows a low probability (20-40% chance)
of readings dipping below 50 degrees.

Barnes

&&

.AVIATION...  (For the 00z TAFs through 00z Friday Evening)
Issued at 643 PM CDT Thu Aug 21 2025

High pressure settling southward from the upper Midwest tonight
will bring light winds by a few hours after sunset this evening,
along with clearing skies. Given afternoon dewpoints in the lower
60s and efficient radiational cooling in this environment bringing
lows to around 60, areas of fog are likely to develop overnight,
especially north of I-72 where the approaching high will be in
closer proximity. Predominant statistical guidance concurs with
this scenario, although exact visibility remains in question,
while most physical models still disagree. Will probably need to
refine timing and visibilities, but have incorporated a decline
to IFR vsby overnight, and kept a period of MVFR cigs from when
fog dissipates in the mid to late morning until mid afternoon.
Winds NE 4-8 kts this evening, becoming light and variable by
02Z-04Z.

37

&&

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

$$