020
FXUS63 KLOT 041713
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1213 PM 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 /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 1213 PM CDT Fri Jul 4 2025

The area of remnant showers continues to dissipate across the
area early this afternoon, with only some lingering VFR cloud
cover. This area of cloud cover will scatter through the
afternoon as the ongoing southeasterly winds turn south-
southwesterly. Currently anticipate this south-southwestern
wind shift to occur in the 19 to 20Z timeframe at the main
terminals.

The weather tonight is expected to be quiet, though smoke from
area fireworks emissions may result in a few hour period of
MVFR VSBYs later this evening into the overnight. The previous
forecast had this handled, so I made no changes to this for the
18Z TAFs. Otherwise, expect southwest winds to increase on
Saturday, with gusts up to 20-25 kt expected Saturday afternoon.

KJB

&&

.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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