625
FXUS63 KLOT 161109
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
609 AM CDT Thu Jul 16 2026

.KEY MESSAGES...

- Warm and humid conditions will continue into Saturday with
  peak heat indices around 100 degrees at times.

- A period of reduced visibility due to smoke is expected today
  and tonight, generally along and northeast of Interstate 90.

- A few storms are possible this afternoon, with better chances
  Friday and then again late Saturday afternoon into Saturday
  evening.

&&

.DISCUSSION...
Issued at 213 AM CDT Thu Jul 16 2026

The main focus this morning is on the incoming plume of
wildfire smoke, which has crept in along a lake-enhanced
front. This boundary continues to slowly shift southwestward,
but has become increasingly diffuse/washed out in the vicinity
of a Freeport to Joliet to Valparaiso line. In the wake of this
boundary, smoke is gradually working its way westward and
towards the surface. Backward model trajectories continue to
indicate the primary source region for near-surface parcels
later this morning originating primarily from western lower
Michigan where visibilities continue to slowly fall towards 2-3
miles. Recent webcam imagery from Chicago does reveal a slowly-
worsening pall drifting off the lake though, and have been
watching visibilities slowly work their way towards 4-6 SM near
the lake.

Observations from Wednesday afternoon across central Wisconsin
showed widespread BKN-OVC ceilings with cloud layers around 3000
feet. HRRR/RRFS smoke output suggests slightly lower total
smoke concentrations in our area, but given the footprint of the
observed ceilings, pushed sky cover grids towards the partly
sunny range across northeast Illinois and northwest Indiana
through the day. It`s unclear to what extent (if any) this will
impact surface insolation and subsequent high temperatures. With
the notably thicker plume across central lower Michigan on
Wednesday, observed high temperatures were some 4 to 9 degrees
cooler than guidance. At this time, don`t suspect things will be
thick enough to result in that drastic of a temperature
difference, but do think the NBM delivered highs in the mid 90s
were a touch overdone. Additionally, onshore flow will prevail
to start the day, which is a big change from the past few
mornings. This will allow for a notably more expansive lake
cooling footprint today, which will be reinforced by an
additional lake breeze push this afternoon. Lakeside highs are
expected to be in the lower 80s today. Peak heat indices will
still likely flirt with the century mark, but mainly south and
west of a Rockford to Aurora to Kouts, IN line.

Boundary layer moisture will pool along the aforementioned
quasistationary boundary this afternoon. Assuming smoke isn`t so
dense that it stunts insolation, we should see an increase in
cumulus with heating. Forecast soundings do show the presence of
a bit of a "CAPE robber" upstairs today (the previous
subsidence inversion which is gradually being lifted and cooled)
which will reduce overall instability a bit and may tend to
keep some degree of inhibition present. That being the case,
convergence on this axis looks respectable, and modest upper-
level divergence is forecast to develop on the southern flanks
of a jet streak situated to our north. With otherwise negligible
large scale forcing but a moist and generally uncapped
environment, isolated airmass-type convection will be possible
this afternoon in the vicinity of a Rockford to Joliet to
DeMotte, IN line. Skinny CAPE profiles and the previously
mentioned mid-level warming may work a bit against lightning
production, but guidance is nearly unanimous in developing at
least isolated activity during peak heating. Little/no shear
will result in any updrafts quickly raining themselves out,
perhaps with some locally gusty winds. Finally, given the
forecast steep low-level lapse rates, light mid-level flow, and
environmental vorticity along the surface boundary, can`t rule
out the potential for a few funnel clouds today.

Lingering activity should tend to diminish this evening but may
continue to fester in the moist environment. Smoke output
depicts a potential secondary surge across northeast Illinois
and northwest Indiana this evening and overnight, and have
continued the smoke mention in the gridded forecast through
Friday morning. By Friday afternoon, boundary layer flow will
turn southwesterly, and should gradually push lingering near-
surface smoke back towards the lake.

Tropospheric moisture is forecast to increase further on Friday,
with PWATs over 2 inches across a broad swath of the CWA. While
most CAM guidance remains decidedly unimpressive regarding
convective coverage, do continue to wonder if coverage will end
up a bit higher given the eastward advancement of a low-
amplitude shortwave into this richly moist environment. At least
scattered showers and storms are expected to develop midday
into the afternoon. Slow storm motions, deep warm cloud layers,
etc. will lead to a threat for heavy rainfall and localized
flooding.

A cold front will then move through the region sometime Saturday
afternoon and evening. Ahead of the front, afternoon heat
indices may once again push into the 100 to locally 105 degree
range as dewpoints rise into the mid to possibly upper 70s.
While the bulk of the large scale ascent will slide into
Michigan, height falls/DCVA with the frontal passage may be
sufficient to ignite isolated to perhaps widely scattered
convection during the afternoon. Instability is forecast to be
fairly robust, with MLCAPE values around 3,000 J/kg (or more)
along with a hint of additional mid/upper level flow. Could
envision a multicell cluster or two with a potential localized wind
threat as a result, but still quite a ways out with a good deal
of uncertainty, and ultimately expect the greatest coverage to
our east.

Northeasterly winds will increase Saturday evening which will
lead to at least a moderate swim risk. Whether waves build
further remains a bit unclear due to a significant spread in the
low-level wind field.

Sunday looks like a nice day with seasonable temperatures in the
mid 80s (a bit cooler near the lake) and more comfortable
humidity levels. A stronger synoptic-scale trough will then move
across the region Monday afternoon and evening, sending another
cold front through the area. Saturday`s front will only send the
low-level moist axis into Iowa, so this may not have much of a
problem surging back east across the forecast area on Monday.
Dewpoints are progged to rise back into the mid to upper 70s
ahead of the front while a plume of steeper mid-level lapse
rates build in aloft. While capping could pose a challenge to
convective development, significant flow through the column,
coupled with building instability suggests this will be a
period to monitor closely for more of a strong to severe weather
threat in the region. Depending on timing, the threat for storms
could continue into Tuesday.

Carlaw

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 609 AM CDT Thu Jul 16 2026

Key Aviation Messages for the current TAF period:

- Smoke from wildfires is reducing visibilities across the
  region

- Isolated shower/storm chances this afternoon, but chances at
  any specific terminal remains less than 30 percent and thus
  kept out of the TAF

Vis sensors continue to drop this morning with webcams showing
the smoke spreading over northern Illinois. Most obs north of a
KRFD to KGYY line are 3 to 5 SM; however, localized obs at KMKE
and KRAC are down to 1SM. There is a chance that diurnal mixing
helps improve things slightly during the day, but the TAFs were
written more conservative and kept MVFR vis (localized IFR
through at least 15Z) from smoke through the day. Gradual
improvement is expected through the evening, but with winds
becoming light, there is lower confidence in exact improvement
timing. Better chances for a return to P6SM tomorrow by mid
morning as winds turn to the southwest.

Winds will remain light and northeasterly through the current
TAF window, up to 10 knots in the afternoon. Light southwesterly
winds are expected tomorrow morning.

There remains a non-zero chance for daytime showers and storms
this afternoon along a moisture axis to the west. Any storm
that develop would have a very slow storm motion to the east.
Coverage should remain isolated, so the chance for a shower or
storm at any particular terminal is too low for formal TAF
mention. Radar trends in the afternoon will need to be monitored
for quick AMDs if needed.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Air Quality Alert until midnight CDT tonight for ILZ003-ILZ004-
     ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-
     ILZ020-ILZ021-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-ILZ108.

IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002-
     INZ010-INZ011.

LM...None.

&&

$$

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