227
FXUS63 KLOT 050213
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
913 PM CDT Thu Sep 4 2025

.KEY MESSAGES...

- A series of storm systems moving through the northern Great
  Lakes will reinforce fall-like conditions locally through the
  weekend.

- A period of gusty southwest to west winds is expected toward
  and after daybreak Friday.

- A gradual warming trend is expected next week.

&&

.UPDATE...
Issued at 913 PM CDT Thu Sep 4 2025

The going forecast is in good shape; no significant changes are
needed.

An impressive, decidedly fall-like system, is quickly shifting
east across the Minnesota Arrowhead region and into northern
Wisconsin. Intensifying divergence aloft along the northern
flanks of a 130+ jet streak has encouraged the development of a
notable mid-990s surface low just north of the Twin Cities.
While the bulk of the large scale forcing tied to this system
will remain well north of our forecast area tonight, ascent
will nonetheless facilitate enough mid-level saturation to
squeeze out some showers and sprinkles late this evening and
overnight, with coverage increasing north of the IL/WI state
line.

The low-level mass response in the vicinity of this system is
particularly robust: the ARX, MPX, and DMX VWPs all reveal
about 50-55 knots of flow at 850 mb, which aligns well with
latest RAP analyses and short term forecasts. The system-
preceding low-level jet is actually forecast to intensify a bit
further tonight as it sweeps eastward, with a narrow ribbon of
nearly 60 knots of 850 mb southwesterly flow advertised cutting
across the northern third of our forecast area overnight. Near-
surface static stability will hold much of this stronger flow
off the deck, although intermittent gusts around 35 mph appear
plausible after about 1 AM, particularly over the urban Chicago
corridor with somewhat "deeper" mixing typically occurs, and
can`t rule out a few gusts nearing 40 mph. GFS BUFKIT output
looks a bit more concerning from a stronger-than-advertised
wind scenario, but based on upstream observations in central
Iowa, it looks like it`s running a few degrees too warm at the
surface which is leading to near-surface lapse rates that are a
smidge steeper than reality.

Continue to expect a secondary, but brief, pop of winds Friday
morning during the 7-11 AM timeframe (and really mainly confined
to 8-10 AM) as deepening mixing starts to work into the base of
the departing strong flow aloft. Have boosted wind gusts a
little during this period, particularly across northeast
Illinois and near the lake.

Carlaw

&&

.DISCUSSION...
Issued at 154 PM CDT Thu Sep 4 2025

Through next Thursday:

Broad upper-level cyclonic flow encompasses much of the
northern US at press time thanks to a seasonably strong, and
large, low pressure system centered in central Ontario. A
vigorous upper-level short-wavelength trough is readily evident
in satellite imagery over the Dakotas on the western side of
the upper- level low. Ensemble model guidance remains in great
agreement that the upper-level trough will scream southeastward
this evening and through tomorrow morning, with the associated
surface low tracking from near Fargo, North Dakota this
afternoon to Escanaba, Michigan overnight to near Sault Ste
Marie, Ontario by tomorrow morning.

While the surface low will pass decidedly north of our area
tonight, impressive pressure falls (approaching 10mb/6hr in the
UP of Michigan) will induce the development of a strong
eastward- translating southwesterly low-level jet (950mb-850mb
flow of 45-50kt) across the Lower Great Lakes. As a result,
would expect any WAA-driven showers across Minnesota this
evening to survive into southern Wisconsin and far northern IL
with sprinkles reaching as far south as the I-80 corridor. With
a mostly clear start to the night and relatively large
temperature/dewpoint depressions, do anticipate the development
of a stout inversion layer by the time the low-level jet
develops which should tend to limit the ability for downward
mixing of winds at least through the nighttime hours. It may be
close, though, and will have to watch for a few southwesterly
gusts upward of 30 to 35 mph particularly after midnight and
especially on high-platform sites.

After daybreak, the cold front will race across the area
anchored by 3-hour pressure rises of 2-4mb and strong CAA (850mb
temperature drop of about 1C/hr). As a result, do have concern
in a quick "pop" of winds right around daybreak as downward
momentum (via CAA) and the "upward" effects of diurnal heating
collectively force mixing into the strong low-level wind field
along the backside of the low. At this point, we favor wind
gusts of 35 to 40 mph from daybreak through mid-morning, highest
near the Wisconsin state line. However, if mixing is
particularly efficient, it would not be surprising at all to see
a few locations hit 50 mph wind gust tomorrow morning. Even
with strong CAA, filtered sunshine (and a still reasonably
strong sun angle) should help highs tomorrow get into the 60s.

Also for tomorrow, opted to introduce low (20%) chances for
showers tomorrow morning and afternoon along the US-24 corridor
as trailing upper-level shortwaves excite mid-level
frontogenesis along the west-to-eastward stalled baroclinic
zone draped across the mid-Mississippi River Valley. While the
low-level airmass in the wake of the cold front will be quite
parched (850mb RH dropping toward 30% for a time), forecast
cross sections depict hints of CSI within the mid-level
frontogenetical circulation, which may allow for surprisingly
strong precipitation echoes and rates to develop and survive the
trip to the ground. These types of scenarios are more dangerous
in the cold season as they can support "surprise" snow bands
(and indeed all lift will be centered in the DGZ tomorrow),
though the low-level thermal profile will be plenty warm enough
to melt the snowflakes well before they reach the ground
(provided the dry air doesn`t just chew them up).

Upper-level cyclonic flow and the associated pool of seasonably
cool air will continue across the Great Lakes region through
the weekend. Embedded shortwaves may provide enough oomph to
kick off a few sprinkles from time to time, though the pattern
does not appear supportive of meaningful precipitation. Highs
will top off in the 60s and overnight lows will fall into the
40s (save for around 50 lakeside and in Chicago).

After the weekend, ensemble model guidance is advertising the
development of a cut-off low pressure system in the Pacific
Northwest. Broad and persistent mid-level warm-air advection
east of the cut-off low should encourage the subsequent
flattening of the upper-level pattern across the northern US,
affording a gradual warming trend locally next week. Ensemble
multi-run meteograms of high temperatures demonstrate relatively
high confidence in a return of highs in the upper 70s to lower
80s by midweek. In addition, ensemble meteograms of QPF suggest
a return of precipitation chances by midweek as well, though
admittedly there is no clear signal favoring one period of time.
Nevertheless, a transition toward more seasonable conditions
appears on track to arrive by the middle of next week and, if
extended GEFS/EPS runs were to be taken at face value, continue
through the end of the month.

Borchardt

&&

.AVIATION /00Z TAFS THROUGH 06Z FRIDAY/...
Issued at 629 PM CDT Thu Sep 4 2025

Forecast concerns for the terminals...

- Increasing westerly winds after midnight with gusts of 25-30
  kts expected. Period of higher gusts possible Friday morning.

- Couple hour period of showers/sprinkles late this evening.


A 998 mb surface low continues to organize over the Dakotas this
evening and will be tracking into the northern Great Lakes
overnight. As it does so, winds will increase after midnight
with gusts in the 25-30 kt range expected overnight and through
Friday morning. While forecast soundings do show a very weak
stable layer developing just above the surface tonight, the
strength of the low-level jet (50-55 kts) should allow at least
some gusts to reach the surface. That said, if gusts do become
less frequent then a threat for LLWS could materialize as the
core of the aforementioned jet will be centered around 2000-3000
ft AGL. Furthermore, confidence is growing in a period of
higher gusts (in excess of 30 kts) developing after daybreak
Friday morning as the atmosphere mixes into the base of the jet.
Therefore, have introduced a gust 30 kt mention in the TAFs
between 12-16z though suspect the duration may be only 2-3 hours
at most. Regardless, winds will gradually diminish Friday
afternoon as the surface low moves into the eastern Great Lakes
with gusts expected to subside Friday evening.

Outside of the winds, there is also a chance for a few showers
and/or sprinkles around midnight at the terminals as the
system`s cold front sweeps through. While moisture depths are
not overly impressive, the front and low-level jet should
provide sufficient forcing to squeeze out some noticeable rain
at the terminals for a couple hour period. Thus TEMPO groups
have been added for this potential. Otherwise, expect VFR
conditions to prevail through the forecast period.

Yack

&&

.MARINE...
Issued at 154 PM CDT Thu Sep 4 2025

This evening and into tomorrow, a seasonably strong low
pressure system will track from near Fargo, North Dakota to
Escanaba, Michigan to Sault Ste Marie, Ontario while deepening
to nearly 990 mb. Rapid drops in surface pressure ahead of the
system in the Upper Peninsula of Michigan will force the
development of a strong southwesterly low-level jet overnight.
With relatively warm lake water temperatures and seasonably cool
low-level ambient temperatures, the low-level jet is expected
to mix downward to the water surface leading to a period of 35
to 40 kt southwesterly gale force winds tonight into Friday
morning. Locally higher storm-force gusts in excess of 50 kt are
possible on elevated observation stations such as the top of
the Chicago Crib, as well. A Gale Warning is in effect from 1 AM
through 10 AM Friday.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...Beach Hazards Statement from 7 AM CDT Friday through Friday
     afternoon for INZ002.

LM...Gale Warning from 1 AM to 10 AM CDT Friday for the IL and IN
     nearshore waters.

&&

$$

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