053
FXUS63 KLOT 250755
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
255 AM CDT Wed Sep 25 2024

.KEY MESSAGES...

- Patchy dense ground fog over rural areas of interior northern
  IL will dissipate shortly after sunrise

&&

.DISCUSSION...
Issued at 255 AM CDT Wed Sep 25 2024

Through Thursday:

Probably jumped the gun a bit with dense fog advisory issuance
for our northwestern CWA earlier when multiple METAR were down
to 1/4SM and spotter reports of near zero visibility came in.
Given recent rainfall, calm/nearly calm winds, and longer
nighttime was concerned dense fog was going to become
widespread. Since that time, VSBYs have come up considerably in
METARs and it is pretty hard to find much fog in area webcams.

A couple of fortuitously timed ACARS soundings out of RFD just
after 06z showed a negative hydrolapse right off the surface.
Guidance indicates continued dry air advection just off the
above the sfc early this morning, so that may help keep fog in
check. Even though there are serious doubts about how much dense
fog there will be, course of least regret is to leave the dense
fog advisory in place through sunrise, as there is still a
chance for a "sunrise surprise" and an expansion of dense fog
development. In fact, just in the past 10 minutes, dense fog
has rolled in here at the WFO, so there is some dense fog
threat still early this morning. Given the shallow nature of
the fog and dry air just off the deck, expect any fog to
rapidly dissipate within an hour or so after sunrise.

Otherwise, lake effect showers should remain mostly east of our
CWA early this morning, possibly just nicking northeast Porter
County at times. Lake effect should shut down later this
morning.

Otherwise, expecting a couple days of sunshine and seasonably
mild temperatures today and tomorrow. Could be some patchy
shallow ground fog again late tonight in the typical rural low
lying areas, but boundary layer will only dry further today, so
fog should be less prominent tonight than early this morning.

- Izzi

Thursday Night through Tuesday:

(discussion forthcoming soon)


&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 109 AM CDT Wed Sep 25 2024

Key Messages:

- Lingering MVFR ceilings at GYY and near ORD and MDW prior to
  daybreak.

- Fog this morning; worst conditions (LIFR/VLIFR) expected at
  RFD and possibly DPA.

- Lake breeze to cause easterly/northeasterly wind shift at
  ORD, MDW, and GYY this afternoon.


An MVFR stratus deck extends inland from southern Lake Michigan
at the beginning of this TAF period and is affecting GYY. ORD
and MDW are close enough to this deck where MVFR conditions may
be briefly be attained over the next couple of hours, but with
time, this deck is expected to lift and progress southeastward
away from the terminals. Can`t entirely rule out a lake effect
shower passing over GYY before mid-morning, but the probability
of this occurring appears to be too low to warrant advertising
this potential in the GYY TAF.

Wherever skies clear out prior to sunrise, fog is likely to
develop. RFD has already observed VLIFR conditions, and see no
reason why conditions would improve there much prior to sunrise
with still several hours of radiational cooling to go. DPA will
likely observe sub-VFR visibilities later on in the night,
though just how low they`ll get there is somewhat uncertain.
Clouds will likely limit the overall likelihood of meaningful
visibility reductions occurring at ORD, MDW, and GYY, but it`s
possible that some light mist could still develop or that
residual stratus debris from the fog to the west advects in
later this morning.

Lastly, winds should generally be out of the northwest quadrant
and under 10 kts through much of the TAF period. However, a lake
breeze passage should introduce winds with an easterly component
to ORD, MDW, and GYY at some point this afternoon.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Dense Fog Advisory until 9 AM CDT this morning for ILZ003-
     ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-ILZ012.

IN...None.
LM...None.

&&

$$

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