852
FXUS63 KLOT 292349
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
649 PM CDT Sun Jun 29 2025

.KEY MESSAGES...

- Isolated gusty thunderstorms will persist through tonight,
  with gusty winds possible with any thunderstorm through this
  evening.

- At least scattered thunderstorms are likely on Monday ahead of
  a cold front, with the strongest storms capable of producing
  localized damaging winds.

- Heat and humidity will return for the 4th of July holiday
  weekend, along with thunderstorm chances, particularly on
  Saturday-Sunday.

&&

.DISCUSSION...
Issued at 332 PM CDT Sun Jun 29 2025

Through Tonight:

Isolated showers and widely isolated thunderstorms continue to
bubble across most of the area this afternoon, particularly on
the periphery of Lake Michigan, east of I-55, and south of the
Kankakee River Valley. The environment remains uncapped, but
little mid to upper-level forcing exists. Where surface forcing
via an outflow-reinforced lake breeze as well as weak outflow
boundaries are present, isolated convection has been persistent.
This is especially true as slightly more-favroable low-level
moisture has advected northward across areas east of I-55. A
washed out mid-level front is also providing subtle support from
around Kankakee and east. Farther west, little to no forcing
has resulted in very sparse and brief shower activity, with the
mid-level front supporting a slight increase in cumulus coverage
around the Illinois River.

The ongoing trends so far this afternoon should persist into
early evening, with isolated convection exhibiting a short life
cycle while cascading downstream to generate additional brief
single or multicell. Decent mid-level dry air and negligible
deep-layer shear has kept the overall coverage in check.

A pair of mid-level waves currently crossing southeast
Minnesota and central Wisconsin have generated robust convection
from Green Bay to northeast Iowa. appreciable forcing will
remain north of the area with an associated rapid decrease in
deep-layer shear toward the IL/WI line this evening. Have
maintained chance PoPs north of I-88 mid-evening through the
early overnight hours as a few decaying multicell clusters could
extend this far southwest. Cannot rule some stronger wind gusts
toward the Rockford metro mid-evening before low-level
stability increases with time through the night. Elsewhere south
of the I-88 corridor, any coverage of precip will likely be
quite sparse through the night, with much of the area remaining
dry and seasonably mild.

Kluber


Monday through Monday night:

The muggy air mass stationed over the region today will still
be in place at the beginning of the day tomorrow, which will
result in tomorrow being another hot and humid day across
northern Illinois and northwest Indiana. While persistently
ample low-level moisture and sufficiently steep mid-level lapse
rates could allow for some convective remnants and associated
cloud cover from overnight to linger past sunrise, the relative
dearth of synoptic-scale forcing early in the day (possibly
coupled with modest subsidence on the backside of a departing
weak MCV) suggests that most of our forecast area should remain
dry through at least mid-morning. If the spatial footprint of
convection and the associated cloud cover during the morning
indeed ends up being as muted as the latest CAM guidance
indicates, then ample diurnal heating should occur through the
morning, setting the stage for a likely greater coverage of
showers and thunderstorms in our forecast area later on in the
day.

Much like today, air mass convection could get going as early
as the late morning/early afternoon as convective temperatures
in the mid-upper 80s are breached. A sharpening lake breeze
and/or any remnant outflow boundaries from the overnight
convection in the region could serve as potential foci for
convective development. With the stronger tropospheric flow not
expected to arrive until later on in the afternoon, any earlier
convection that manages to get going would have a similar pulse-
like character to what has been exhibited by the convection
that has occurred in our area in recent days with attendant
threats for strong to locally severe downburst winds, torrential
rainfall, and lightning.

Showers and thunderstorms should then become increasingly
numerous across our forecast area as the afternoon goes on as an
incoming mid/upper-level trough swings into the western Great
Lakes and an associated cold front approaches from the
northwest. It also appears increasingly likely that an MCV
originating from widespread convection across the central Plains
tonight will arrive in the region during the afternoon as well.
The exact track that this MCV takes and the extent to which it
can maintain its integrity as it tracks farther to the east will
likely play a key role in dictating whether convection tomorrow
will end up being relatively scattered in coverage or whether a
more widespread coverage will end up being realized with most
of our forecast area getting in on the showers and thunderstorms
at some point during the afternoon or evening.

With deep-layer shear increasing closer to 30 kts as stronger
wind fields on the southern periphery of the aforementioned
trough spread over the area, loosely-organized multicell
clusters should tend to become the more predominant storm mode
with time. Climatologically high precipitable water values
peaking in the 1.5-2" range would support a continued threat for
water-loaded downbursts with the most robust convection. MLCAPE
values as high as 2000-2500 J/kg and 0-3 km theta-e deficits in
the 20-30C range suggest that some of these downbursts could be
strong enough to produce wind damage, though subpar mid-level
lapse rates may play a role in keeping this damaging downburst
threat more isolated than widespread. The Marginal (level 1/5)
Risk for our forecast area in the latest Day 2 Convective Outlook
from the Storm Prediction Center continues to look appropriate
based on our latest forecast thoughts and expectations.

Ogorek


Tuesday through Sunday:

After Monday, a three-day break in the humidity looks to follow
with drier northwest flow, so highs in the mid-upper 80s away
from any lake cooling will be more tolerable. The nighttime
periods during the middle of the week should be pleasant as
well. Turning ahead to the 4th of July holiday weekend, medium
range guidance remains in good agreement in the heat and
humidity returning (highs near to a bit above 90F with dew
points increasing back into the 70s). Convective chances may
end up somewhat minimal (better focus north) on the day and
evening of the 4th, with signs then pointing toward a
potentially more stormy Saturday-Sunday period (PoPs still in
the chance range for now).

Castro/Ogorek

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 649 PM CDT Sun Jun 29 2025

Forecast concerns include...

Chance for thunderstorms early this evening.
Chance for thunderstorms late this evening/early overnight.
Chance for thunderstorms Monday afternoon.
Wind directions early this evening.

Outflows along with an outflow enhanced lake breeze still
allowing for isolated thunderstorm development early this
evening for the Chicago terminals. As the primary outflow lifts
north over the next hour along with the slowly loss of peak
heating, expect thunder potential to end in the next 1-2 hours.

Thunderstorms over west central WI will push an outflow boundary
into northern IL later this evening, that will likely lead to at
least isolated thunderstorm development. Confidence is rather
low for coverage with this activity. Its possible the highest
coverage remains north and northwest of the Chicago terminals
and that it weakens into the early overnight hours. Current prob
mention has the potential time period handled well and opted to
maintain prob mention for now but changes/tweaks are likely
with later updates. Once this activity ends or moves east of the
terminals, the rest of the overnight hours through mid/late
morning Monday are expected to be dry.

Convective trends for Monday remain uncertain with isolated,
possibly scattered thunderstorms possible by early afternoon,
similar to today, which may form outflow boundaries that allow
additional storms to develop. A cold front will be moving across
the area in the late afternoon and early evening, which may
allow for a more organized line of stronger storms to move
across the entire area. Confidence for this is also low, but
prob mention seems reasonable given the potential for a more
impactful line of storms in the mid/late afternoon.

South/southeast winds may gust into the 25kt range in the next
hour or so as the outflow lifts north. Winds will settle on a
light southerly direction with sunset and then turn southwest
later this evening. Wind directions on Monday will likely favor
southwest or west/southwest with a shift to the northwest in the
late afternoon/early evening behind the cold front.

There is some potential for mvfr cigs late Monday afternoon
into Monday evening behind the cold front, but confidence for
how prevailing these may become is low. cms

&&

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

&&

$$

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