815
FXUS63 KILX 201059
AFDILX

Area Forecast Discussion...Updated Aviation
National Weather Service Lincoln IL
559 AM CDT Wed Aug 20 2025

.KEY MESSAGES...

- Patchy fog is possible early Wednesday morning, primarily west
  of I-55.

- Isolated showers (20% chance) west of the Illinois River today,
  then there is a low chance (20%) of rain with a cold front on
  Saturday. Otherwise, dry conditions are expected over the next
  seven days.

- Seasonable temperatures are expected through Saturday, with
  highs in the 80s, then well-below normal temperatures arrive
  Sunday into early next week, with highs in the 70s.

&&

.DISCUSSION...
Issued at 256 AM CDT Wed Aug 20 2025

*** TODAY ***

After an unsettled stretch, a quieter pattern lies ahead. Early
Wed AM, an amplified upper level ridge (597 dam) was centered over
the Four Corners region and Hurricane Erin was centered several
hundred miles southeast of NC. In between these features, a weak
upper trough was shifting towards the Appalachians, with a subtle
sfc low near Lake Erie. The associated cold front has now pushed
completely south of the ILX CWA, bringing an end to the intense
heat and humidity that gripped the region for much of the last
week.

In the immediate short term, the only forecast concern is
potential for fog (some dense) through 14z/9am. Current satellite
imagery shows broken high clouds to the east of a Jacksonville-
Bloomington line as of 07z/2am, with some mid-clouds also present
east of I-55. Radiational cooling will be most effective to the
north of the cloud shield, and this lines up with where CAMs have
focused the fog development. Locations across northern IL,
southeast IA, northeast MO and Galesburg have reported vis between
0.5 and 2 miles. 20.00z HREF depicts a 30-50% chance of vis below
1 mile in areas north of the current cloud shield. No plans for
dense fog advisories at this time, but will monitor conditions
closely through the morning and issue if needed.

Into the day today, northeasterly flow will prevail along with
seasonable temps (highs in the mid 80s). Dewpoints trend a bit
lower, but stay in the upper 60s/low 70s. NAM Cu-rule suggest
scattered to broken diurnal cumulus decks today, a notion which is
supported by fcst soundings and well-captured by guidance. CAMs
produce a few showers this afternoon, mainly northwest of the IL
River. Fcst soundings look pretty paltry in terms of instability
with weak lapse rates in the H7-H5 layer, but there could be
enough instability above the cloud base to squeeze out some light
rain. Lower dewpoint air will continue to advect in during the
evening, and lows are forecast to drop into the low 60s tonight.
These will be the coldest temps for our area since an earlier cold
spell during the first week of August.

*** THURSDAY THROUGH EARLY NEXT WEEK ***

There is not much in the way of hazardous weather during the
extended forecast period, and there should be at least a few days
of truly fantastic weather. For the remainder of the work week,
sfc high pressure over the Great Lakes will gradually sag
southward, resulting in dry conditions and continued seasonable
temps (highs in the low/mid 80s, lows in the low/mid 60s).

Into the weekend, global models have been consistent in showing a
fairly robust upper wave tracking towards the northern Great
Lakes, sending a cold front across IL on Sat. Despite limited mid-
level forcing this far south, continue to think that at least some
precip mention is warranted with this FROPA, so kept 15-20%
chance of rain in the forecast. Even if rain does occur, limited
instability and moisture is expected to result in minimal rainfall
amounts (less than a tenth). Depending on how close the upper
wave is on Sunday and to what extend shortwaves dive towards our
area, cooler temps aloft and the resultant steep lapse rates could
support a few sprinkles on Sun in the post-frontal cyclonic flow
regime. Fcst soundings for the ILX CWA on Sun show an inversion
above the cloud layer and high cloud bases, which suggests any
precip is unlikely. The better signal for sprinkles is further
north across northern IL. Precip chances on Sun are less than 10%
in our area, this is merely something to monitor in an otherwise
benign forecast pattern.

Following the cold frontal passage on Saturday, a period of
seasonably cool temps and low humidity is expected for Sun into
early next week, with highs in the 70s (including the low 70s
north of I-74) and lows ranging from the upper 40s to mid 50s.
Daytime dewpoints are forecast to be in the upper 40s to mid 50s
as well, which should feel wonderful compared to the upper 70s
dewpoints of the past week. It doesn`t get much better than this
for August weather in central Illinois.

Erwin

&&

.AVIATION...  (For the 12z TAFs through 12z Thursday Morning)
Issued at 556 AM CDT Wed Aug 20 2025

A swath of dense fog and LIFR ceilings is present over BMI at the
start of the period, and appears likely to persist through 14z.
Other sites could experience visby reductions, but based on
current satellite do not think the visbys impacts will be as bad
at the other terminals as it is at BMI. Elsewhere, a MVFR cloud
deck is on track to reach CMI during the first few hours of the
period, but should rise about 3kft by 15z. For the rest of the
period, VFR conditions should be predominant, with scattered
diurnal Cu around 3-5 kft. Guidance is hinting at another round of
fog development Thurs AM, so that will be something to monitor for
future TAF updates.

Erwin

&&

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

$$