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255
FXUS63 KLOT 202340
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
640 PM CDT Fri Sep 20 2024

.KEY MESSAGES...

- Isolated to widely scattered showers and thunderstorms
  possible through early evening, mainly along and east of I-57

- Unseasonable warmth will continue through Saturday, with one
  last 90 degree day of 2024 possible

- Periods of showers and some thunderstorms late Saturday night
  possibly continuing into Monday

&&

.DISCUSSION...
Issued at 325 PM CDT Fri Sep 20 2024

Through Saturday:

Isolated to widely scattered showers and thunderstorms will
continually try to develop through early evening ahead of a cold
front for areas along and east of I-57. While forecast
soundings continue to show around 30 kts of effective shear, the
modest instability looks like it will limit the intensity of
any storms that develop so the threat for severe weather appears
to be decreasing. Therefore, expect any storms to remain sub-
severe with the main threat being lightning and perhaps a
localized wind gust of 30-40 mph.

Showers and storms will taper by 7 PM this evening as the
aforementioned cold front exits into north-central IN. As a
result, rain-free conditions are expected overnight as much
drier air moves in. Though, there is the potential for some
patchy fog to develop late tonight into Saturday morning due to
the combination of strong radiational cooling and light winds.
While dew points are forecast to diminish overnight, it looks as
if some residual moisture will linger near the aforementioned
front in northwest IN and adjacent areas of eastern IL. Since
forecast soundings show the moisture being rather shallow
confidence is lower on coverage and intensity of fog especially
with westward extent. Nevertheless, felt the potential was
sufficient to warrant the introduction of a formal patchy fog
mention to the forecast mainly in northwest IN and far eastern
IL.

Any fog that does develop tonight will erode by mid-morning on
Saturday. So expect another partly sunny and unseasonably warm
afternoon with highs once again forecast to top out in the upper
80s to around 90 degrees. The exception, however; will be for
areas along the northern IL lakeshore which will see highs in
the upper 70s to lower 80s due to onshore winds and a lake
breeze.

Heading into Saturday night, another weather system is expected
to develop across the central Plains as a broad upper trough
ejects out of the southwest CONUS. This weather system will then
begin to move into northern IL (and eventually northwest IN)
late Saturday night into Sunday resulting in another period of
showers and thunderstorms with beneficial rainfall for our
ongoing drought.

Yack


Saturday Night through Friday:

Late Saturday into Sunday a closed mid-upper low will shift east-
northeastward from the Desert Southwest out across the central
Plains. As this occurs, a lead impulse emanating from this low
is expected to foster shower and thunderstorm development across
much of the Corn Belt late Saturday into Saturday night as it
ejects out across the Mid-Missouri Valley in tandem with a
southeastward shifting cold front. We are likely to see this
activity shifting/developing into our local area Saturday night
into Sunday. Favorable deep moisture pooling along this frontal
boundary should favor some beneficial rainfall for the area,
though possibly enough to be an overall drought breaker. The
threat for showers and thunderstorms will continue Sunday night
into Monday, though the highest chances by Monday look to be
sagging south of I-80 with the surface frontal boundary.

Following the departure of this early week system, mean long wave
upper troughing and northerly lower-level winds across the Great
Lakes will foster a much cooler, more seasonable type pattern
across our area. Accordingly, expect daily high temperatures to
primarily be in the middle 70s following early morning lows in the
50s.

KJB

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
-- Changed Discussion --
Issued at 640 PM CDT Fri Sep 20 2024

Key Messages:

- Potential for fog south/southeast of Chicago overnight.

- Winds become light/variable later this evening, then SSE
  Saturday.

- Rain moves into the area toward the very end of the ORD/MDW 30
  hour TAFs midnight Saturday night.

The cold front which moved across the terminals earlier in the
day has shifted well east and southeast of the area as of early
evening. A weak, slow-moving lake breeze had pushed through MDW
and GYY earlier this afternoon, and was about to ORD`s eastern
boundary as of 630 pm. This lake breeze should push slowly west
across ORD early this evening, before winds across the area
become light and variable as weak surface high pressure builds
in from the west. Winds will remain light/variable Saturday
morning before the high drifts off to the east, allowing light
south to south-southeast winds to develop by midday/afternoon.

While drier air has spread in behind the aforementioned cold
front, lingering low-level moisture (dew points in the upper 60s
to around 70F) remain from central IL into northern IN along
and just behind the slowly-departing cold front. Combined with
the light winds and otherwise clear skies, this will likely set
up the development of fog and low stratus overnight which may
impact south/southeast parts of the Chicago metro including GYY.
Guidance visibility output suggests the potential for
LIFR/VLIFR conditions especially east/south of GYY, and have
indicated a period IFR conditions at the airport overnight/early
Saturday. Have also included a tempo MVFR vis at MDW toward
morning, along the expected northwestern periphery of the denser
fog deck. Other terminals to the northwest are expected to be
fog-free. Otherwise, showers are expected to move into the area
later Saturday night, and have included a prob30 mention for
this at the end of the ORD/MDW 30 hour forecasts.

Ratzer
-- End Changed Discussion --

&&

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

&&

$$

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