747
FXUS63 KLOT 010748
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
248 AM CDT Sun Sep 1 2024

.KEY MESSAGES...

- Large waves and strong currents will make swimming dangerous
  and potentially life threatening today into Monday at all
  southern Lake Michigan beaches.

- Cool temps to start the week then a gradual warm-up until the
  next cold front passage Thursday, followed by a noteworthy
  cool down and breezy northerly winds to end the week.

&&

.DISCUSSION...
Issued at 248 AM CDT Sun Sep 1 2024

Through Monday:

Conditions are nice and quiet out there beneath clear skies
early this morning. A cold front is on its way here from the
north and is currently (2AM) draped east-west across central
Wisconsin. It should begin to approach the IL/WI state line not
long after dawn and continue to work southward across our CWA
through late morning. A dry atmosphere should keep any precip at
bay with this frontal passage, but there could be enough
moisture there to possibly stir up some lower hanging clouds
this morning with some help from the lake. Additionally, visible
satellite data from yesterday evening as well as guidance from
the HRRR and RAP suggest that a smoke plume originating from
High Plains wildfires is pooled on the equatorward side of the
front. This may provide a slight milky haze to skies during the
morning.

The primary concern with this front is the hazardous lake
conditions that are developing in its wake. Marine obs behind
the front, while few and far between, have been consistently
gusting between 20 and 25 kt. Similar gusts are expected to
quickly develop over southern Lake Michigan behind the boundary
through the morning. The gusty northerly flow will promptly
generate high waves and dangerous rip currents along the
shoreline resulting in outright life-threatening swimming
conditions on IL and IN Lake Michigan beaches. Swimming in the
lake must be avoided today and Monday as conditions will remain
dangerous through the Labor Day holiday.

Inland areas are also in for a breezy day today, just not quite
as much as near and over the lake. Highs today will vary from
the lower 70s near the IL/WI stateline to the upper 70s south
of I-80. Labor day tomorrow will be a tad cooler with highs
forecast in the lower and middle 70s. Some high-res guidance,
namely the HRRR, NAM Nest, and ARW, tuck a little saturation
beneath a low level inversion late tonight into Monday morning
and produce some sprinkles or very light rain near and downwind
of the lake. Didn`t think this was quite worthy of a slight
chance mention at this time, but did opt to throw in some 10%
"silent" PoPs across the metro to illustrate the low potential.

Doom


Monday Night through Saturday:

Another crisp fall-esque night and start to the shortened
workweek is in store for early Tuesday morning with
temperatures dipping into the 40s away from the lake and outside
of the urban core of the Chicago metro. Still expecting
temperatures in the upper 50s to near 60 downtown.

In addition to continued dry conditions, temperatures gradually
warm back to near to above normal for early September through
midweek as the upper ridge expands across the region in
response to the departing upper trough. Thursday will be the
warmest day of the week with highs in the mid 80s.

A cold front is still forecast to move across the area during
the late Thursday into Friday timeframe bringing with it the
potential for isolated to widely scattered showers. The thunder
potential continues to look low, though a few lightning strikes
can`t be fully ruled out Thursday evening. This frontal passage
will usher in yet another below normal stretch of temperatures
and comfortable humidity to end the week. While there is medium-
high confidence in this renewed cool stretch, the degree of
cooling and its longevity remains less certain given continued
ensemble variability in the strength of the upper trough and how
quickly it departs to the east (or if it stalls).

While we still have a period of high waves to get through over
the holiday weekend, another period of dangerous waves and
currents could develop behind the aforementioned cold front for
Friday into Saturday.

Petr

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1226 AM CDT Sun Sep 1 2024

Key Messages:

- Gusty N to NNE wind shift behind a cold front this morning
  (~12-14Z)

Light southwest winds will very gradually veer westerly ahead
of an approaching cold front currently diving across central
Wisconsin. Expect the front to reach northern Illinois toward
daybreak, turning winds northerly with quickly increasing gusts
into the low to mid 20kt range. Expect directions to trend ENE
at ORD/MDW/GYY by 13-15Z if not already with the initial wind
shift.

Can`t rule out SCT to BKN 2-3000ft status in the wake of the
front, with perhaps a locally better chance into northwest
Indiana (GYY).

Wind gusts gradually ease toward this evening remaining
northeasterly through early Monday morning.

Petr

&&

.MARINE...
Issued at 235 PM CDT Sat Aug 31 2024

Illinois and Indiana Nearshore and Beaches:

A backdoor cold front passage mid Sunday morning will bring a
wind shift to several hours of brisk north to northeast winds
through Sunday evening. Gusts will top out in the 25-30 mph
range, with sporadic gusts up to 35 mph possible from the mid
morning through the early. Given the nearly full lake fetch and
cool air causing unstable conditions, expecting waves to
quickly build and become hazardous for boaters and swimmers. A
Beach Hazards Statement and Small Craft Advisory are in effect
starting 7 AM CDT Sunday. This is a slightly earlier start time
to align with a couple hour earlier forecast cold frontal passage.

Winds will ease and shift easterly Sunday night into Monday,
though lingering swell will prolong the hazardous conditions,
especially for swimmers at Illinois beaches. The Beach Hazards
Statement is in effect until 7pm Monday for the Illinois shore
and 12pm Monday for Lake and Porter IN, while the Small Craft
Advisory is currently set to expire at 10am Monday.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from 7 AM CDT this morning through
     Monday evening for ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement from 7 AM CDT this morning through
     Monday morning for INZ001-INZ002.

LM...Small Craft Advisory from 7 AM this morning to 10 AM CDT
     Monday for the IL and IN nearshore waters.

&&

$$

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