749
FXUS63 KLOT 301123
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
623 AM CDT Tue Sep 30 2025

.KEY MESSAGES...

- Dry and unseasonably warm daytime conditions through next
  weekend.

&&

.DISCUSSION...
Issued at 138 AM CDT Tue Sep 30 2025

Through early this morning: patchy/shallow dense fog will be
possible, mainly across parts of northeast and interior northern
Illinois outside of Chicago, and in/near river valleys. For
later tonight into Wednesday AM: with an increased potential
for dry advection from the east along with somewhat increased
high-level cloud cover, have left additional fog mentions out of
the forecast for now. Dewpoints will remain highest across
northeast Illinois, and particularly Lake/McHenry counties where
some spotty BR development remains possible, but chances are
too low to introduce anything in the grids at this time.

High pressure will continue to remain solidly in control of our
weather and will be pivoting directly overhead today into
Wednesday. While a shortwave will eventually impinge on the
region tomorrow night and into Thursday--eventually shunting the
upper high eastward--this will only result in a temporary
increase in cirrus cloud cover above 20 kft.

A hint of column cooling and more persistent northeasterly flow
(with winds turning off the lake through mid-morning) will hold
temperatures within about 20 miles of the lake in the mid 70s to
near 80, while inland locales will still manage to warm back
through the mid to even upper 80s. A wedge of very dry air (850
mb dewpoints around -10 to -15 C at 850 mb) will push southwest
out of lower Michigan today, and this will likely result in
some widely-varying surface dewpoint trends today, the lowest of
which will likely set up south of I-80 and away from the
immediate influence of the lake where afternoon RH values are
expected to fall into the 25 to 35 percent range. Similarly dry
conditions are forecast on Wednesday. East to northeasterly
breezes, with gusts generally under 15-20 mph will limit the
fire weather threat, but with fine fuels continuing to dry and
cure (Midewin`s 10-hour fuel moisture dropped to 6 percent
yesterday), the potential for field fires will increase this
week.

As the surface ridge shuttles off to the east on Thursday and
Friday, southerly return flow will once again start to push high
temperatures back into the mid and upper 80s across most of the
area. The lone exception on Thursday will be across the
immediate Illinois shore where southeasterly onshore flow will
continue. By Saturday, medium range guidance suggests we`ll
finally start to see an increase in gradient flow, at least
across our I-39 locales where we may need to keep an eye on a
threat for a somewhat elevated fire danger.

Largely dry/precip-free conditions are expected over the
weekend. At some point, a cold front will press through the
region bringing with it perhaps a chance for some showers, but
the latest multi-model consensus suggests this may not occur
until early next week.

Carlaw

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 623 AM CDT Tue Sep 30 2025

There are no major weather concerns for the 12Z TAF period.

Some light mist is floating around early this morning dropping
visibilities to around 5 to 8SM at a number of sites. Expect low
VFR, and possibly intermittent 5SM, vsbys to persist into the
12Z hour.

NE winds will build to around 10 kt by late morning and subside
for tonight. Expect easterlies to around 10 kt during the day
on Wednesday.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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