004
FXUS63 KLOT 061122
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
622 AM CDT Sun Jul 6 2025

.KEY MESSAGES...

- Scattered showers and thunderstorms are expected today (50 to
  60% chance), ending from north to south this evening. The main
  concern will be torrential rain with localized flooding
  possible. Some areas may stay completely dry.

- Dangerous swimming conditions at Lake Michigan beaches this
  afternoon through midday Monday.

- A period of dry and slightly cooler weather is on track to
  start the upcoming week, though isolated showers and storms
  cannot be ruled out in the Tuesday night through Thursday
  timeframe (20-40% chance or so each day).

&&

.DISCUSSION...
Issued at 218 AM CDT Sun Jul 6 2025

Through Monday:

A cold front will be moving south across the area later today
into this evening with the models attempting to develop a
secondary, weaker area of low pressure along the front near the
southern end of Lake Michigan this afternoon. This should allow
for scattered thunderstorm development from late morning through
late afternoon, perhaps with the best coverage across northeast
IL and northwest IN. Overall coverage remains uncertain but
given the pattern, maintained likely pops across the south, but
increased pops to likely for much of the Chicago metro area and
its possible pops may need to be increased further as trends
emerge later today. Main threat from these storms will be very
heavy rain with precipitable water values approaching 2 inches.
While much of the area is in drought conditions, should this
heavy rain materialize over urban areas, there will a localized
flooding potential. Likewise, some areas may remain completely
dry today as well. Though the marginal/level 1 severe risk is
just east of the local area, a few stronger storms are
possible, with damaging wind gusts the main threat. Model trends
are looking good for this activity steadily dissipating and
shifting south through the evening hours. Maintained low chance
pops in the far southeast overnight, but even during this time
period, precip may be over. Monday is also looking dry for the
area and removed pops from the southern cwa.

Northerly winds will increase behind the cold front this
afternoon, possibly gusting into the 30 mph range over southern
Lake Michigan, especially if another surface low develops. This
will allow waves to quickly build into the 4-6 foot range,
creating dangerous swimming conditions at Lake Michigan beaches.
These northerly winds will continue into Monday afternoon when
they finally begin to diminish and depending on how fast that
happens, the beach hazard statement may need to be extended.

High temps today are somewhat dependent on cloud cover/precip
timing, but with much of the area likely being dry until late
morning and then the expected scattered nature to the precip,
highs in the mid 80s are still possible. Then as the cold front
moves south, temps will likely fall into the 70s across
northeast IL by late afternoon and possibly even into the upper
60s near the lake by evening. Cloud cover may persist south of
Lake Michigan into Monday morning, but with clearing skies west
of the lake, low temps may drop into the lower 60s, even a few
upper 50s Monday morning, across the western cwa. Highs look to
return into the 80s for most areas Monday, cooler near Lake
Michigan. cms


Monday Night through Saturday:

Monday night through Wednesday, the overarching upper-level
pattern from the northern Plains to the Great Lakes is expected
to be characterized by broad/disorganized northwesterly flow.
Ensemble model guidance suggests a shortwave moving along the
US/Canadian border, perhaps augmented by upstream convection in
the Plains, will move into the Great Lakes region sometime in
the Tuesday night to early Wednesday timeframe. If such timing
were to hold (definitely no guarantees), any convection that
develops Tuesday afternoon across Iowa would be prone to
weakening by the time it reaches our area Tuesday night. Tuesday
otherwise looks like a nice day with highs in the mid to upper
80s

Wednesday and Thursday, upper-level ridging will build
northward toward Hudson Bay atop weak troughing across the Gulf
Coast. Such a regime will lead to mid-level heights "spreading
out" across the Mississippi River Valley, altogether leading to
somewhat benign weather. Healthy cumulus fields each day may
sprout an isolated storm or two, particularly near daily lake
breezes where localized convergence will be maximized. With
little to no flow through the meager convective layer, any storm
that manages to develop would be fairly short-lived with
threats for occasional lightning strikes and downpours. Will
maintain the NBM PoPs of 20 to 40% through the period, keeping
in mind they are likely overdone for many areas. Highs both days
should be in the mid to upper 80s, with cooler readings in the
upper 70s in the wake of the aforementioned lake breezes.

Friday into the weekend, the upper-level low currently stalled
in the Gulf of Alaska is expected to swing southeastward and
track along the US/Canadian border. At the same time, the upper-
level low meandering off the California coast is expected to
swing east-northeastward atop a strengthening ridge over the
southwestern US, placing it in line to interact with the
approaching trough from the northwest. If and how both interact
will be something to watch as we get closer. Regardless, the
general idea is that chances for showers and storms should
return to the general region sometime next weekend.

Borchardt

&&

.AVIATION /12Z TAFS THROUGH 18Z MONDAY/...
Issued at 622 AM CDT Sun Jul 6 2025

A recent surface analysis revealed an eastward-moving MCV
located along the IL/WI state border east of a frontal boundary
near I-39 and south of a secondary cold front stretching from
northern Lake Michigan through central Wisconsin. A bank of IFR
to MVFR cigs is present across northwestern IL, and showers
continue to percolate on the southern side of the MCV in far
northeastern Illinois.

Over the coming hours, the MCV and associated showers will move
over Lake Michgian and the MVFR cigs will spread eastward.
Improvement in cigs appears possible by late morning as the
boundary layer warms, though trends will need to be monitored
closely. Southerly winds will also become decidedly
southwesterly.

Early this afternoon, showers and storms are expected to develop
along the southward-moving secondary front near the IL/WI state
line and close to Lake Michigan. The showers and storms should
then shift east-southeastward through the afternoon. With very
high PWATs (nearing 2.25") this afternoon, any storm may produce
torrential downpours(rain rates of 1-2"/hr) with very low
visibility (1 mile or less). With growing confidence that
storms will impact the Chicago terminals, elected to upgrade the
PROB30 groups into TEMPO groups. Meanwhile, at RFD, will
cautiously advertise no precipitation owing to forecast
soundings depicting a somewhat deep layer of capping.

Winds will quickly shift north to northeasterly in the wake of
the storms and secondary cold front. Initial gust to 20 kt will
slowly ease through the remainder of the TAF period. Another
bank of MVFR cigs should develop over Lake Michgian behind the
secondary cold front as well, which may impact ORD/MDW/GYY
through the night.

Borchardt

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from 3 PM CDT this afternoon through
     Monday afternoon for ILZ006-ILZ103-ILZ104.

IN...Beach Hazards Statement from 6 PM CDT this evening through
     Monday afternoon for INZ001-INZ002.

LM...Small Craft Advisory from 3 PM this afternoon to 9 AM CDT
     Monday for the IL nearshore waters.

     Small Craft Advisory from 6 PM this evening to 9 AM CDT Monday
     for the IN nearshore waters.

&&

$$

Visit us at weather.gov/chicago