544
FXUS63 KLOT 250009
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
709 PM CDT Tue Jun 24 2025

.KEY MESSAGES...

- Scattered showers and thunderstorms are favored south of I-55
  through this evening. A low threat for localized damaging wind
  gusts exists, with a localized flooding threat persisting
  overnight into Wednesday.

- Peak afternoon heat indices will still be in the upper 90s to
  near 100 through much of the week.

- Periodic shower and storm chances exist through early next
  week with many dry hours in between rounds. The strongest
  storms may be capable of localized damaging winds and flash
  flooding.

&&

.DISCUSSION...
Issued at 255 PM CDT Tue Jun 24 2025

Through Wednesday:

It is another hot and humid day inland, meanwhile an outflow
boundary combined with the lake breeze has helped to bring
relief to Lake and Cook County. Behind these boundaries the
atmosphere is much more stable. We are in between shortwaves,
and thus the main forcing to have a bit more robust cumulus and
thunderstorm development have been along and ahead of these
boundaries in an environment that has over 2000 J/Kg of CAPE.

Expect continued shower development along with isolated to
widely scattered thunderstorms, with these being more likely
confined south of I-55, including southern Cook County and then
into NW Indiana.

Additional shortwave activity will spread from Iowa into
northern Illinois this evening. This will likely bring some
uptick in shower activity. We do anticipate that this wave may
trigger some additional thunderstorm activity this evening,
favored west and then possibly along and south of I-80 ahead of
any lingering boundaries from this afternoon.

The flow will turn more westerly and parallel to the lingering
boundaries, and thus we will want to watch for any localized
areas of heavy rainfall, later tonight into early Wednesday.

Confidence on Wednesday afternoon activity is low, as it is
conceivable that the main forcing shifts NE of the area, and
there will be onshore flow that would confine any scattered
development to inland areas. It will be a significantly more
comfortable day for areas near the lake, though it will still be
warm and humid inland.

KMD


Wednesday Night through Tuesday:

The upper level ridge is expected to continue to flatten
through mid- week, and break down as it shifts eastward into
Friday. While this occurs, a series of mainly low-amplitude mid-
level short waves are expected to track across the upper
Midwest and Great Lakes region, with a quasi-stationary, west-
east oriented frontal boundary across the area. This will set
the stage for periodic shower and thunderstorms to affect the
forecast area through the week, with greatest chances focused
during peak diurnal heating/instability of the afternoon and
evenings. There remains lower confidence on specific timing of
these shorter amplitude waves (or MCVs generated by upstream
convection) as models often struggle with them.

The air mass along and south of the stationary frontal boundary
will remain warm, moist and conditionally unstable through the
period. The column remains relatively warm aloft, maintaining
somewhat weak mid-level lapse rates, while the core of the mid-
upper level jet stream stays largely north of the forecast area.
The modest mid- level flow and lapse rates, coupled with PWATs
over 1.5 inches would support diurnally strong thunderstorms
capable of producing wet downbursts, in addition to the threat
of localized flooding.

While not as hot and oppressive as the past few days have been,
it will remain seasonably warm and humid through the week, with
daytime highs in the mid-80s to lower 90s, with peak heat
indices around 100 south of the front. There will likely be some
days where onshore winds off of Lake Michigan provide some
cooling near the shore, though that will likely depend on the
day to day position of the frontal zone and outflow boundaries.

After the ridge breaks down, zonal flow is expected to develop
aloft over the weekend. Models are starting to come into better
agreement with a long wave trough developing over Ontario as a
ridge goes over the desert southwest. This will allow for
northwest flow to develop over the Plains and bring a renewed
chance for showers and thunderstorms into early next week.

DK

&&

.AVIATION /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 709 PM CDT Tue Jun 24 2025

Forecast concerns for the terminals...

- Periods of scattered showers and thunderstorms through TAF
  period. Highest coverage at the terminals this evening and
  Wednesday morning.

- Potential (20-30% chance) for MVFR ceilings late tonight
  through early Wednesday morning.

A weak upper disturbance continues to drift into northern IL
this evening which has continued to develop widely scattered
showers and a few thunderstorms across the region. At the same
time, a lake breeze continues to advance westward across the
Chicago metro while an outflow boundary moves southward across
north-central IL. Behind these two boundaries, the environment
has started to stabilize making it less favorable for
thunderstorms. Given that all of the TAF sites are behind these
boundaries, the thinking is that the ongoing showers/storms will
weaken to just showers prior to reaching the terminals and thus
have maintained SHRA mention. That said, the shower coverage is
expected to gradually diminish through 06z this evening as the
aforementioned disturbance pivots through resulting in a period
of dry weather overnight.

However, the ongoing showers and thunderstorms in northwest MO
are forecast to move into northern IL and the terminals between
09z and 10z Wednesday morning. While the thunder threat is
expected to diminish prior to the precipitation arriving, there
is still a non-zero (10-15%) chance for an isolated rumble of
thunder with this activity. Given some uncertainty in exact
shower coverage at the terminals Wednesday morning, have decided
to maintain the PROB30s at the Chicago area terminals and VCSH
at RFD. Finally, additional scattered showers and thunderstorms
are forecast to redevelop Wednesday afternoon along another lake
breeze and the outflow boundary from today`s storms that should
be draped across north-central IL. Since it appears that any
showers/storms Wednesday afternoon should develop south and west
of the TAF sites, have maintained a dry forecast for now but
will keep a close eye on trends and adjust as needed.

Outside of the showers/storms, winds will generally remain light
(speeds in the 5-8 kt range) through the period with directions
remaining out of the east-southeast. Though, wind directions at
RFD will remain northwesterly until the lake breeze arrives
around 05z. VFR conditions are also expected for most of the
forecast period, but there is a potential for MVFR ceilings to
develop late tonight into early Wednesday morning. Given the
probability of MVFR ceilings tonight is only around 20-30% have
opted to cover with a SCT020 mention for now.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Heat Advisory until 7 PM CDT this evening for ILZ023-ILZ032-
     ILZ033-ILZ039.

IN...Heat Advisory until 7 PM CDT /8 PM EDT/ this evening for
     INZ010-INZ011-INZ019.

LM...None.

&&

$$

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