484
FXUS63 KLOT 250839
AFDLOT

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

.KEY MESSAGES...

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

- Chances for showers return Friday night into early next week,
  but confidence in specifics remains low.

&&

.DISCUSSION...
Issued at 339 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:

The synoptic weather pattern from the end of this week through
early next week continues to look complex with a high-amplitude
ridge over the Great Lakes, a cut-off upper-level low settling
in to our south, and then Tropical Storm (and soon-to-be
Hurricane) Helene jumping into the fray. However, some
noticeable trends have continued to emerge in the latest model
runs and ensemble means, allowing us to refine our forecast a
little bit.

After making landfall, Helene will encounter the upper low and
likely engage in a Fujiwhara-like interaction that will see the
tropical cyclone rapidly get torn apart with its remnants then
getting slung anticyclonically around the low and eventually
absorbed into the low itself. It looks increasingly likely that
this interaction will play out too late in the game for it to
rain pretty much anywhere in our forecast area during the
daytime on Friday, especially with the Great Lakes ridge still
trying to suppress everything southward, but did still keep some
sub-30% PoPs going south of the Kankakee River in the odd event
that the increasingly diminutive ensemble support for a
faster/farther north progression to Helene ends up verifying.
Regardless of exactly how quickly Helene arrives and interacts
with the upper low, high temperatures on Friday should top out
in the mid-upper 70s, while tightening pressure fields in the
vicinity of these two interacting systems will yield a period of
breezy northeasterly winds Friday into Saturday that will
induce high wave action on Lake Michigan, leading to dangerous
conditions along the lakeshore.

The latest trends in forecast guidance also suggest that the
time period when we`d be most likely to see rain would be
Friday night into Saturday as Helene`s remnants get whipped
around the northern periphery of the conjoining low from east to
west. There is still a fair amount of spread amongst the
various ensemble suites in how far northwest the QPF footprint
will get, with some solutions still keeping it entirely south
and southeast of our forecast area, but there appears to be
enough support to carry high-end chance PoPs across the
southeast third or so of our CWA with lower chance and slight
chance PoPs farther to the northwest, where the northern periphery
of the precipitation shield will be less likely to reach.

Whether we`ll see rain Saturday night into early next week will
depend on where the center of the merged low ends up wobbling
to after the merger of the two cyclones has concluded. If it
remains in close enough proximity to our CWA, then we could see
periodic showers continue all the way into Monday. On the other
hand, if it meanders too far to the east or the aforementioned
ridge keeps it bottled up farther to the south, then our
forecast area would be favored to remain dry into early next
week. The general trend in forecast guidance has been towards
the latter scenario, and the latest NBM PoPs have decreased
accordingly, with slight chance and low-end chance percentages
now appearing to appropriately characterize the likelihood of
precipitation on Sunday and on Monday based on this trend and
the existing spread of forecast solutions in the latest
ensemble suites. A fairly strong cold front currently pegged to
arrive sometime Monday night into Tuesday should then shunt
whatever is left of the upper low eastward and bring in more
autumn-like temperatures for the middle of next week.

Ogorek

&&

.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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