032
FXUS63 KLOT 300538
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1238 AM CDT Mon Jun 30 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 /06Z TAFS THROUGH 12Z TUESDAY/...
Issued at 1238 AM CDT Mon Jun 30 2025

Main Concerns:

- Scattered TS with gusty, variable winds early this afternoon
  through the late afternoon, with another round of TS possible
  into the early evening.

- Lake breeze wind shift likely at MDW and GYY, and possible at
  ORD, with TS trends likely to influence wind directions.

A potentially impactful flying day looks to be in store later
today, albeit one fraught with forecast uncertainty. As of this
writing, an outflow boundary is sagging south early this
morning, bringing a brief period of north to northeast winds to
the terminals. The potential for TS appears to have ended for
the overnight period.

Similar to today, a very warm and humid air mass and lake breeze
convergence should quickly foster the development of TCU and
then TS over the Chicago metro. It`s a bit more unclear out by
RFD. Any storms causing direct terminal impacts will bring
strong, VRB downburst winds and brief heavy rainfall. Had enough
confidence to maintain VCTS mention (starting at 17z) with
PROB30s for direct impacts due to coverage uncertainty. The
biggest question mark into this evening is whether a second
round of more organized TS will impact the terminals, or develop
farther south. Added in one last PROB30 for this potential.

Finally, owing to the convective uncertainty plus lake breeze
proximity, forecast confidence in exact wind trends is on the
lower side. It does appear likely that GYY and then MDW will
shift to northeast, but then the boundary may stay just east of
ORD, unless it`s ushered through by TS outflow. Observational
trends will need to be closely monitored. Winds will then shift
back to west-northwest in the late evening and overnight as
conditions quiet down.

Castro

&&

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

&&

$$

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