207
FXUS63 KLOT 040037
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
737 PM CDT Thu Jul 3 2025

.KEY MESSAGES...

- A widely isolated thunderstorm with gusty winds cannot be
  completely ruled out across northeast Illinois and northwest
  Indiana through mid-evening.

- An area of scattered showers may shift from southern Wisconsin
  into northern Illinois Friday morning. Isolated embedded
  thunderstorms are possible with these showers.

- Hot July 4th and Saturday but not as oppressively humid as the
  previous rounds of heat recently (~95-100F heat indices).
  Temperatures on the 4th may be slightly cooler than forecast
  if cloud cover and showers persist well into the morning.

- More appreciable thunderstorm chances return Saturday night
  and particularly Sunday with a slowly approaching cold front.
  There may be a day or two of quieter weather to start next
  work week before thunderstorm chances return midweek.

&&

.DISCUSSION...
Issued at 300 PM CDT Thu Jul 3 2025

Through Friday:

Mostly sunny conditions will continue for much of the area this
afternoon as a surface ridge exits to the east and mid-level ridge
approaches from the west. Dew points have mixed out into the low to
mid 60s, which has limited overall cumulus coverage. Meanwhile, 500-
600 hPa warming and existing mid-level dry air noted in forecast
profiles and AMDAR data has stunted the overall growth of existing
cumulus. The lake breeze has remained the only notable focus for
deeper cumulus growth, but this support has so far been insufficient
to overcome these negative factors for sustained convective growth.
Have maintained a slight chance thunderstorms for much of the area
through mid-evening as there have been a couple attempts at deeper
growth, but latest trends suggest coverage of storms will be quite
sparse or even nil through the evening.

A compact wave over eastern North Dakota this afternoon is expected
to pivot SE around the periphery of the approaching wave through
Friday, ultimately crossing southern Wisconsin early Friday morning
and southern Lake Michigan Friday afternoon. The wave will interact
with a pocket of relatively higher mid-level moisture advecting from
Iowa toward the southern half of Wisconsin through tonight. A
localized area of low to mid-level isentropic ascent may produce
sufficient forcing for scattered elevated convection across southern
Wisconsin late this evening and overnight, even with poor mid-level
lapse rates. Overall expectations are for any upstream convection to
gradually weaken while drifting southeastward toward the IL/WI state
line by sunrise, with scattered mid-level showers and perhaps a few
embedded thunderstorms diminishing in coverage over northern
Illinois through the morning. Similar to the past several days, the
thermodynamic environment will remain more or less uncapped on
Friday. So any outflows or differential heating boundaries
(particularly from cloud cover) Friday morning could provide a
subtle focus for isolated convection to redevelop heading into
Friday afternoon. Diurnal mixing of dew points and persistent mid-
level warming and drying should greatly limit coverage of
thunderstorms, with profiles likely becoming too dry to support deep
convection by early Friday evening.

Friday has the potential to be a hot day across the area if upstream
convection and widespread cloud cover fail to materialize or quickly
dissipate through the morning. Areas with abundant sunshine through
the day will likely see temps in the lower to locally mid 90s, with
heat index values roughly matching air temps due to slightly less
humid conditions.

Kluber


Friday Night through Thursday:

The main weather notes of interest for the Holiday weekend
continue to center around the hot weather (particularly through
Saturday), as well as the threat for scattered showers and
thunderstorms on Sunday.

A 592+ DAM 500 mb ridge axis will foster another hot day on
Saturday as it shifts overhead. Considering capping is likely to
keep the daylight hours dry across a majority of the area (only
exception is a low 20% chance for some late day storms across
far northeastern IL), dry weather is anticipated through most,
if not all of the daylight hours of Saturday. Accordingly, this
will aid in the development of a hot and somewhat breezy
afternoon as the very warm lower level airmass overhead (925 mb
temperatures around +26C) diurnally mixes. Afternoon high
temperatures in this setup should thus have no problem topping
out in the low to mid 90s. We would also not be surprised to see
readings top out into the upper 90s (to near 100), particularly
across the Chicago urban heat island. This potential would be
especially favored if dew points do mix down into the upper 50s
and lower 60s as the ECMWF and GEM suggest. Assuming dewpoints
do mix down into the 60s Saturday afternoon during peak heating,
heat indices would not be much different than the actual
temperature. For this reason, in spite of the hot weather, it
appears unlikely that heat headline will be needed at this time
as heat indices don`t look to climb much above 100 degrees.

While the primary threat of storms remains west-northwest of the
area on Saturday in closer proximity to a surface cold front and
approaching mid-level impulse, this focus shifts into our area
Saturday night and Sunday. We will thus begin to have some low
(20-30%) chances for showers and storms in the forecast Saturday
night, especially across the northwestern half of the area.
However, the best chance (50+%) for scattered showers and
thunderstorms will be on Sunday as the aforementioned mid-level
impulse and surface front shift into the area, and are
accompanied by a plume of rich low-level moisture (low 70s
dewpoints).

While the threat of severe storms with this activity looks low,
owing to rather poor deep layer shear, the threat of locally
strong downbursts could support some instances of locally strong
winds Sunday afternoon. Otherwise, the main concern with these
storms on Sunday may end up being instances of locally heavy
rainfall. This is especially true given the rich moisture
(precipitable water values ~2"), and slow storm motions (15-25
mph) anticipated along the frontal boundary. The area has been dry
of late, with much of northern IL and northwestern IN in D0 to D1
drought. However, if any excess rainfall falls over the more
urban areas in and around Chicago, hydro concerns could increase.
However, at this time it remains a bit unclear as to which areas
could experience the heaviest rainfall. Stay tuned.

It currently appears more likely than not that the front will
clear most of if not the entire area on Monday, which would keep
most locations dry. A slight slowing of the front would
reintroduce a concern for additional scattered convection, hence
the low (20-30%) PoPs near/south of the Kankakee River appear
reasonable. Seasonable temps and less humid conditions should then
prevail into the start of Tuesday. Shower and thunderstorm
chances will increase again late Tuesday into Wednesday with a
larger trough axis crossing the region.

KJB

&&

.AVIATION /00Z TAFS THROUGH 06Z SATURDAY/...
Issued at 737 PM CDT Thu Jul 3 2025

Key Messages:

- Potential for showers and thunderstorms overnight into
  tomorrow morning. Gusty wind shifts also possible from
  associated thunderstorm outflow.


The main forecast concern for the current TAF period revolves
around the potential for showers and thunderstorms, mainly
from late tonight through tomorrow morning. A disturbance
tracking southeastward through Minnesota and eventually into
Wisconsin tonight will likely be the main driver for this
potential, though ongoing convection in eastern Iowa and
central/eastern Wisconsin complicates the picture and lowers
confidence in how things will evolve tonight into tomorrow
morning.

One point of higher forecast confidence is that the
environment across northern Illinois and northwest Indiana will
be much less favorable for the sustenance of thunderstorms
compared to farther north in Wisconsin. Thus, any convection
that does attempt to cross south of the Illinois-Wisconsin state
line should start to weaken -- and possibly quite rapidly.
While this may be the case, there is still a chance for
thunderstorms to make it to at least RFD, ORD, and DPA. Strongly
considered adding a TSRA or VCTS mention of some sort into the
TAFs for those TAF sites, but there was just enough uncertainty
regarding timing (both onset time and duration) and the overall
evolution of convection tonight to hold off on formally
introducing that mention, at least for now with the 00Z TAF
package. Any thunderstorm complex that at least gets close to
our TAF sites would likely spit out outflow that would result in
a gusty wind shift to a northerly, northeasterly, or easterly
direction. These winds would likely persist for a few hours
before an eventual return to a prevailing southerly or
southwesterly wind direction.

Conditions should dry out beyond tomorrow morning, though
couldn`t completely rule out a stray shower or two popping
along the lake breeze (which may get close to ORD and MDW)
during the mid-late afternoon. With tomorrow being the Fourth
of July, wouldn`t be surprised if there were some visibility
reductions at the terminals tomorrow night as a result of
lingering firework smoke. Low-level flow may end up being
strong enough to disperse the smoke enough for it to not pose
much of an issue, but this will be something to keep tabs on
tomorrow evening.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Air Quality Alert until midnight CDT Friday night for ILZ005-
     ILZ006-ILZ012-ILZ013-ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-
     ILZ106-ILZ107-ILZ108.

IN...Air Quality Alert from midnight tonight to midnight EDT Friday
     night for INZ019.

LM...None.

&&

$$

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