603
FXUS63 KLOT 110806
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
306 AM CDT Thu Sep 11 2025

.KEY MESSAGES...

- Dry weather expected the remainder of the work week with a
  continued warming trend.

- Unseasonably warm with a (less than 30 percent) chance of
  scattered showers and some thunderstorms late Friday night and
  again Saturday night.

- Seasonably warm and dry for much of next week.

&&

.DISCUSSION...
Issued at 306 AM CDT Thu Sep 11 2025

Through Friday:

A couple of relatively minor forecast concerns early this
morning, including some fading isolated showers/sprinkles over
the southern Chicago suburbs, and an area of fog and low stratus
pushing south along the lake and across parts of southeastern
WI. Beyond that, dry and warmer conditions (though with a bit of
a lake-cooling footprint today) are expected for the end of the
work week.

One last mid-level vort was dropping south across northeast IL
early this morning, within the western periphery of an eastward-
moving short wave trough axis. This had produced a little pocket
of isolated showers/sprinkles from some mid-level clouds
earlier in the night from RFD southeast into the
western/southern Chicago suburbs. GOES vapor imagery depicts
strong drying spreading south across the area in the wake of the
vort however, and these showers have faded over the past hour
or two.

In the lower levels, surface high pressure building across the
far northern Great Lakes into eastern Canada is producing modest
north- northeasterly low-level flow down Lake Michigan into
eastern WI/northeast IL. Advection of upper 50s to lower 60s dew
points over the lake has resulted in an area of low stratus and
fog along the western Lake Michigan shore overnight, which has
spread inland across the Milwaukee/Racine areas and over the
water off Lake county IL. Relatively light winds over land and
decreasing coverage of patchy mid-level cloud cover may support
further stratus and fog development into parts of northeast IL
through sunrise, and thus have added areas of fog to the
forecast generally northeast of I-90 for early this morning.
Visibilities are currently as low as 1/4SM north of the WI
border, so we`ll have to keep an eye on trends for potential
need for an advisory. Where fog does develop, visibilities
should improve within a few hours after sunrise.

Otherwise, subsidence will strengthen across the area later
this morning as the short wave trough continues to pull away to
the east, and upper level ridging bolds into the upper
Mississippi Valley and western Lakes region through Friday. This
should yield mostly sunny skies by midday/afternoon, with
temperatures warming into the 80s away from the lake. Low-level
thermal fields support highs from around 80 in our northeastern
counties, to the mid-80s farther to the southwest. Along the
lake, synoptic onshore flow should keep readings to the low-mid
70s. Can`t rule out some patchy fog or stratus redeveloping
tonight especially across far northern IL west and north of the
core of the metro, as temps dip into the 50s away from Chicago.

While some high clouds may spill over the upper ridge into the
area Friday, partly to mostly sunny conditions are expected to
support still warmer temps as low level flow turns more
southerly once again. An east-southeast lake breeze will result
in a smaller lake cooling footprint, allowing afternoon highs to
range from the upper 70s along the immediate IL shore to the
mid-upper 80s inland. Could even see a 90 degree high across our
warmer west/southwest cwa counties, however relatively low dew
points (50s) should keep heat indices about the same as our air
temps.

Ratzer


Friday Night through Wednesday:

A Rex block over the northern Rockies and central Canada will
transition into a more prominent omega block across central
North America this weekend before breaking down and shifting
eastward early to midweek. As a strong ridge develops over the
Canadian prairie provinces, a weak wave emanating from Colorado
today is progged to track across the northern Great Plains
within a weaker southward extension of the ridge. The wave will
track southeastward over the western Great Lakes and brush the
forecast area to the northeast late Friday night into Saturday
morning. Modest mid-level ascent within a pocket of Pacific
moisture should yield an area of elevated showers during this
time. Lapse rates above an estimated LPL of 600-700 hPa will be
marginally favorable for embedded thunderstorms as well.
Coverage this far southwest does remain less unclear, so have
continued to limit precip coverage to areas east of a Rockford
to Rensselaer line.

A cold front trailing from the above system will then drift
southwestward and possibly stall across the forecast area
Saturday night into Sunday. Low-level WAA overrunning the
backdoor front below a highly elongated upper-level wave
crossing the area Saturday night would typically be a concern
for slow-moving or backbuilding convection during the warm
season. In fact, propagation vectors turn decidedly northeast
late Saturday night. However, the deeper profile over the region
suggests there may be insufficient moisture for initiation of
convection. Will maintain a slight chance for thunderstorms over
northeast Illinois and northwest Indiana since even meager
moisture can still support isolated development.

Temps this weekend look to be warm, with highs well into the
80s and possibly some low 90s closer to the low-level thermal
ridge to the southwest. The timing of the backdoor front will
dictate the degree of warmer conditions over at least northeast
Illinois on Sunday, with the latest setup favoring an early
arrival of the front and modestly cooler conditions closer to
the lake.

Some indications of modest troughing over the northern Great
Plains could bring periods of decaying showers toward the area
late Monday through much of the week, but consensus guidance
maintains no appreciable rain throughout the week amid
seasonably warm conditions.

Kluber

&&

.AVIATION /06Z TAFS THROUGH 12Z FRIDAY/...
Issued at 1231 AM CDT Thu Sep 11 2025

While VFR conditions are favored through the period, developing
shallow fog and/or LIFR stratus in and around the Milwaukee
area will need to be monitored for a SSW expansion into
northeast Illinois toward daybreak. Lower dew point depressions
and several patches of clearing of mid-level clouds are likely
to support at least some BR at DPA and ORD, with a <30% chance
for the stratus to reach both airfields. Have included TEMPO VFR
BR at ORD and MVFR BR at DPA while also including a mention of
IFR clouds for a couple hours on either side of sunrise. There
is also a lower chance that the similar setup repeats late
tonight into Friday morning, but confidence remains too low to
include in the TAF this far out.

Otherwise, winds will remain generally NE 5-10 knots through
tonight, with a lake-enhanced increase to 10-15 knots during the
afternoon.

Kluber

&&

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

&&

$$

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