911
FXUS63 KLOT 040524
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1224 AM CDT Fri Jul 4 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 /06Z TAFS THROUGH 12Z SATURDAY/...
Issued at 1224 AM CDT Fri Jul 4 2025

Primary aviation concerns include:

- Threat for thunderstorms from daybreak through early afternoon
  at all terminals.

- Period of northeasterly winds due to outflow at some point
  during the morning hours (regardless of thunderstorms).

- Potential for a lake breeze this afternoon/evening, with
  another or reinforcing northeasterly wind shift.

- Expectation for periods of reduced visibility this evening due
  to firework smoke.

Discussion:

A complex TAF period commences with convection ongoing upstream
of the terminals across west-central Wisconsin. The expectation
is for at least some upscale growth of ongoing convection to
take place over the coming hours, or additional development to
take place in southeast Wisconsin associated with a subtle wave
moving along the IL/WI state line. In either scenario,
thunderstorms may move east-southeastward toward the terminals
or at least the southern half of Lake Michigan after daybreak.
So, opted to convert SHRA to TSRA in the inherited PROB30
groups, which themselves were maintained from 12-18Z. Any
thunderstorm that does make it to the terminals should end by
early afternoon as mid-level height rises effectively
eliminates the attainability of any remaining instability. So,
this afternoon and evening are currently expected to be dry.

The wind forecast through the TAF period is an item of
particularly low confidence. Even if thunderstorms remain over
Lake Michigan and do not make it to the terminals, conceptually
would expect outflow to surge into northern and northeasterly
Illinois at some point this morning. The evolution of convection
over the next 4-8 hours will ultimately influence the footprint
and intensity of associated outflow. Regardless, pattern
recognition certainly leverages support to maintain a mention of
a northeasterly wind shift at all terminals after daybreak. For
now, will keep them in the PROB30 groups in favor of monitoring
how convection evolves overnight. The wind forecast this
afternoon is no more straight forward, but would expect winds
to turn southerly and possibly southwesterly by this evening. A
lake breeze may attempt to develop and move toward ORD/MDW this
evening, but there are too many dominoes at play (e.g.,
influence of morning outflow, etc) to introduce a formal mention
in the outgoing TAF package at this point in time.

Finally, have elected to introduce HZ at all terminals after
sunset this evening due to firework smoke. In the past 3 years,
visibility at each terminal fell to 2 to 4 miles particularly
between 0230-0600Z during the evening of the 4th of July (as
confirmed by examining archived 1-min ASOS data prior to the TAF
issuance). Historically, the lowest visibilities are often
recorded at ORD/MDW, so will use FM groups there and TEMPOS
elsewhere.

Borchardt

&&

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

IN...Air Quality Alert until midnight EDT tonight for INZ019.

LM...None.

&&

$$

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