589
FXUS63 KILX 210745
AFDILX

Area Forecast Discussion
National Weather Service Lincoln IL
245 AM CDT Thu Aug 21 2025

.KEY MESSAGES...

- Patchy fog, some of which could be dense, is possible this
  morning (20-30% chance) and Friday morning (40-50% chance).

- A cold frontal passage on Saturday will drop high temperatures
  into the 70s for Sunday into next week. Aside from a low chance
  (15%) of rain on Saturday, precipitation is not expected over
  the next week.

&&

.DISCUSSION...
Issued at 241 AM CDT Thu Aug 21 2025

No major changes to the ongoing synoptic pattern this morning,
with upper ridging out west and Hurricane Erin off the mid-
Atlantic coast. An expansive 1022-mb sfc high pressure was located
over the upper Midwest. The only notable concern during what will
otherwise be a benign 48-hours across the ILX CWA is potential
for patchy fog (some dense) this morning and again Friday morning.

*** FOG POTENTIAL ***

NT Microphysics satellite shows an expansive stratus deck in place
(between 3-4 kft AGL) across much of IN and OH at 2am/07z Thurs,
and this cloud deck is gradually shifting west in time due to
northeasterly low-level flow. RAP 850mb RH analyses do a pretty
good job of capturing the current placement of this cloud deck,
while showing drier 850mb air in place across central IL (850mb
dewpoints of 6-8 degC over ILX CWA as opposed to 10-12 degC over
IN). This leaves some question as to how far west this stratus
deck will push through the morning hours. The cloud deck will
stymie radiational cooling, keeping temps warmer and lowering the
chances of fog compared to areas that stay clear through the
morning (temps beneath the cloud deck are in the mid/upper 60s as
opposed to low/mid 60s in areas of clear skies). The 21.00z HREF
generally has just a 20-30% chance for vis below 1 mile this
morning, and based on the stratus placement think areas from
Galesburg/Macomb to Decatur have the best chance of patchy fog
this morning.

Into the day, the RAP 850mb RH field shows the highest values
advecting into areas near/south of I-72/Danville. This is well
aligned with other guidance sources in showing mostly cloudy skies
across those areas today, with partly cloudy skies (scattered
diurnal Cu) elsewhere. Into tonight, as the diurnal Cu fade,
mostly clear skies combined with the sfc high sagging south (light
winds) should lead to favorable radiational cooling Thurs
night/Fri AM. The 21.00z HREF already encompasses much of the area
in a 40-50% chance of vis below 1 mile. Despite the low
confidence nature of fog, the combination of a favorable synoptic
pattern and a broad model signal this far in advance lead to
concern that dense fog is a possibility for the Friday AM
commute.

*** THE REST OF THE FORECAST ***

Temperatures underperformed the NBM forecast for today by a few
degrees, only reaching the low 80s in most locations. The latest
iterations of the NBM did seem to adjust to this though, lowering
the forecast highs for today through Saturday into the 80-85
degree range.

A cold front remains on track to sweep across the ILX CWA during
the day Sat. Global deterministic models have been remarkably
consistent in the timing of this front. The NBM continues to
deliver a dry forecast for Sat (PoPs below 10%), but we continue
to re-introduce a mention of a slight chance of showers and
thunderstorms with that frontal passage. Fcst soundings do not
inspire much confidence, barely reaching 200 J/kg of MUCAPE and
never showing a clear signal for saturation. Left a thunder
mention in for now due to the favorable diurnal timing and the
time of year, but thunder potential looks quite low based on the
fcst soundings. Weak mid-level forcing is likely to limit the
coverage of any light rain that does occur. All in all, any rain
from this front is likely to be sparse, light, and have minimal
impacts. The Grand Ensemble has a 10-20% chance of measurable
precip.

An early taste of fall remains on track following the weekend
frontal passage, with highs in the 70s, lows near 50, and
dewpoints in the upper 40s to mid 50s. Sfc high pressure will
slowly sink towards the area by mid-week. Precip is not expected
Sun through at least mid-week (chances below 15% each day).

Erwin

&&

.AVIATION...  (For the 06z TAFs through 06z Thursday Night)
Issued at 1220 AM CDT Thu Aug 21 2025

Some residual moisture remains in place late tonight beneath a
subsidence inversion. This is illustrated by both recent
satellite imagery and regional obs, with a few pockets of 3-4 kft
ceilings evident. Overall, the stratus and fog potential should
be limited by drier air filtering in. Nevertheless, we will
continue to mention a brief TEMPO group for MVFR visibility into
Thursday morning, particularly at KBMI and KCMI. Otherwise, VFR
conditions will dominate much of the day under the influence of a
high-pressure ridge.

MJA

&&

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

$$