121
FXUS63 KLOT 040719
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
219 AM CDT Fri Jul 4 2025

.KEY MESSAGES...

- Chance (30-40%) of thunderstorms this morning, mainly across
  northeast IL, along with an outflow boundary and a northerly
  wind shift.

- 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 219 AM CDT Fri Jul 4 2025

Through Saturday Night:

Primary forecast concern and challenge is ongoing convection
along the IL/WI state line back through western WI and its
evolution/affects over the next 6-9 hours.

Much of the model guidance is handling the convection over west
central WI fairly well and has this moving southeast into far
northeast IL later this morning in a weakening phase. However,
current showers and isolated thunder along the IL/WI state line
are not being handled too well at this time and lowers
confidence to the overall coverage and timing of thunderstorm
chances this morning. Its possible this current activity
continues to fester until the rest of the convection catches up
to it and while the instability gradient significantly drops off
along/south of I-88/I-80, there may end up being a bit more
coverage of storms through the mid morning across northeast IL
and trends will need to be monitored. For now, have maintained
chance pops through late morning and have included slight chance
pops into northwest IN as well.

Much of the high-res guidance shows an outflow boundary from
this convection moving south across southern Lake Michigan
during the mid morning hours and its likely there will be a
northerly wind shift with gusts into the 25-30kt range and
quickly building waves. Have included this in the gridded and
nearshore forecast. While this may be short lived, a short
period of waves 2-4 feet and a moderate swim risk is possible.
Winds would then turn more southeasterly by this afternoon and
diminish along with subsiding waves.

This outflow boundary is expected to keep some onshore/easterly
winds along Lake Michigan or its possible an actual lake breeze
may develop. Either way, this boundary may allow an isolated
thunderstorm to develop this afternoon and maintained slight
chance pops for areas near the lake, with the rest of the area
expected to remain dry.

Quite a bit of cloud cover is expected this morning, especially
across the northern cwa, which will allow for a slowly temp
climb. Depending on how strong the outflow boundary is, that
may also keep temps cooler than previously expected through
midday. However, the expectation is that mostly sunny skies will
return this afternoon and other than areas right along the Lake
Michigan shore, high temps back in the lower 90s look on track
for most areas.

Tonight into Saturday morning is expected to by dry and mild
with lows potentially only in the mid/upper 70s for the Chicago
metro area. Southwest winds steadily increase Saturday morning
with gusts into the 25-30 mph range possible in the afternoon
with high temps back into the lower/mid 90s.

A cold front will being moving across the upper Midwest on
Saturday with thunderstorms expected to develop along this
front, well west and northwest of the local area. Some of these
storms may make into northwest IL late Saturday afternoon into
Saturday evening and low chance pops seem reasonable for the
northwest cwa Saturday afternoon. Confidence remains low for
coverage and whatever activity does make into the local area,
will probably be on a weakening trend Saturday night. However,
with the models now showing the front nearby Saturday night,
some low chance pop mention is probably warranted. cms

Sunday through Thursday:

On Sunday, the frontal boundary and upper-level trough
responsible for showers and storms across northwestern IL
Saturday evening should be somewhere in the central to northern
Illinois/Indiana region. Another round of showers and storms
should commence along the front during the afternoon hours,
wherever that front may be. Forecast kinematic profiles along
the front don`t look particularly impressive, though high PWATs
near 2" weak mid- level lapse rates will be supportive of
localized microbursts and torrential rain rates. Note there is a
scenario where the front/trough are positioned over central IL
or northwestern Indiana, meaning part of our area may once again
miss out on much-needed rainfall. With the expectation for at
least partly cloudy skies but also areas of showers ands storms,
highs should be cooler than Saturday and in the mid to upper
80s. Humidity levels will be on the uncomfortable side with dew
points in the mid 70s.

Confidence is increasing that the front/trough will be well
southeast of our area by Monday, allowing for a surface high
pressure system to build into the Great Lakes. While highs will
be similar to the day before and in the mid to upper 80s,
humidity levels will be noticeably lower. Onshore flow will lead
to a pronounced lake shadow characterized by highs in the upper
70s. In fact, with northerly flow of 10 to 15kt down the spine
of the lake, wouldn`t be surprised if a Beach Hazards Statement
ends up being issued for at least northwestern Indiana beaches
on Monday.

Forecast confidence lowers Tuesday onward as somewhat
discombobulated northwesterly flow becomes established across
the central US. Such a pattern will probably support periods of
showers and storms across the broad/general region, though
whether or not any impact our area will be something to discern
in later forecast packages. At this point, ensemble model
guidance seems to favor near to above average temperatures
continuing through at least the end of the 2nd week of July.

Borchardt

&&

.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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