839
FXUS63 KLOT 042332
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
632 PM CDT Wed Sep 4 2024

.KEY MESSAGES...

- Following a warm day Thursday, a cold frontal passage Thursday
  night will bring scattered showers and low thunderstorm
  chances (20%).

- Lake effect rain showers could persist Friday into Saturday,
  particularly into northwest Indiana.

- A big cooldown arrives heading into the weekend with highs in
  the 60s before warming back up early next week.

&&

.DISCUSSION...
Issued at 234 PM CDT Wed Sep 4 2024

A Great Lakes surface high continues to slide east into New
England this afternoon. While our area remains within a weaker
surface gradient along the western periphery of this surface
high, the corresponding lower-level gradient will support a
return to southerly (and offshore) winds along with an
associated notable upswing in temperatures during the day
Thursday. The influx of a warmer airmass into the area will
support high temperatures into the mid to upper 80s on Thursday.

A rather stout mid/upper-level impulse is progged to dig
southeastward into the western Great Lakes region Friday into
Saturday. This feature will drive a notable surface cold front
southeastward across the region Thursday night. A short (1 to 3
hour period) of scattered showers, and perhaps a few non-severe
thunderstorms, continue to be favorable with this frontal passage
into Friday morning. However, the main weather story following
this front will be the turn towards a much cooler airmass for the
weekend. In fact, high temperatures on Saturday still look to
remain in the low to mid 60s in most locations, while overnight
lows fall into the low to mid 40s Saturday night outside of
Chicago!

This cool airmass will also overspread the lower 70 degree waters
of Lake Michigan Friday night into Saturday. This will foster
increasingly supportive thermodynamics across the lake supportive
of lake effect showers (and possibly even a few thunderstorms),
which could move onshore into parts of northwestern IN into
Saturday. The possibility also exists for waterspouts on the
lake during this period. In addition to this, swimming and
boating conditions will become dangerous (if not life
threatening) late Friday through Saturday as northerly surface
winds increase on the lake and result in building waves above 6
feet.

Beyond Saturday, quiet and dry weather is expected through at
least the first half of next week as surface high pressure remains
dominant across the Ohio Valley and southern Great Lakes region.
Temperatures will also be moderating back into the 70s for Sunday
and Monday, then likely back into the 80s by Tuesday.

KJB

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 632 PM CDT Wed Sep 4 2024

Forecast concerns include...

Scattered showers, chance of thunderstorms Thursday evening.
Wind directions this evening and again Thursday evening.

A lake breeze boundary has moved just west of MDW and has
largely stalled over ORD. Its unclear how much more this
boundary may move before it dissipates after sunset. Wind
directions are likely to favor east of south this evening and
then slowly become more southerly and then eventually southwest
overnight through daybreak Thursday morning. Speeds are
expected to be under 10kts tonight. Southwest winds will
increase by mid/late morning, into the 10-12kt range with gusts
into the 15-20kt range, especially Thursday afternoon. Winds
will turn more westerly by Thursday evening and then there is
quite a bit of uncertainty for wind directions mid/late Thursday
evening, which may turn more northerly by the end of the 30
hour ORD/MDW period. Confidence is too low for any mention of
northerly winds with this forecast.

Scattered showers are expected to move across the terminals
Thursday evening and there is a chance for thunderstorms,
especially across southern WI and far northern IL, earlier in
the evening. While an isolated thunderstorm is possible for the
Chicago terminals, confidence is too low for mention with this
forecast. Low mvfr cigs will be possible late Thursday evening,
which may persist through Friday morning. cms

&&

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

&&

$$

Visit us at weather.gov/chicago