Damaging Winds in Illinois, May 4, 2026

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
550 PM CDT Mon May 4 2026

.KEY MESSAGES...

- Gusty showers and storms will move across the area this
  afternoon and evening, favoring areas north of Interstate 80.

- Temperatures will trend cooler for the rest of the week, with
  a threat for patchy frost Wednesday night.

&&

.DISCUSSION...
Issued at 217 PM CDT Mon May 4 2026

The center of a 982 mb surface low pressure system was located
over central Ontario at press time, with a cold front extending
southwestward through central Wisconsin and into central Iowa.
Ahead of the cold front, strong southwest winds (locally gusting
in excess of 40 mph south of Interstate 80), nearly full
sunshine, and a relatively dry low-level moisture column
(surface dew points in the upper 30s to mid 40s) have led to
temperatures rocketing into the upper 70s to lower 80s.

While low-level southwesterly flow should allow for at least
modest moisture recovery through the afternoon (dew points
returning to the lower 50s from south to north), the expectation
is for the boundary layer to remain effectively deep and well-
mixed through this evening. With that said, uncapped thermal profiles
by late afternoon should result in further deepening of the
developing high-based cumulus field with eventual development
into scattered showers and thunderstorms. Coverage may initially
be highest across northwestern Illinois between 4 and 6 PM
before spreading eastward through the remainder of the area this
evening (ending by midnight or 1 AM). With the deep and well-
mixed boundary layer, instability values will not be off the
charts today (MLCAPE of 500 to perhaps locally 1000 J/kg,
largely driven by cooling mid-level temperatures). With that
said, we continue to suspect that that the tallest convection
will be able to produce localized damaging winds via efficient
evaporation in to the dry sub-cloud layer especially if any
conglomeration of individual cells can occur to support a
composite cold pool. Ironically, such a threat may end up where
moisture quality is poorest today and largely along and north
of Interstate 80 where dew points may remain below 50 degrees.
Cannot sleep on the threat for severe hail today as well given
the cool mid-level temperatures and marginal shear profile. With
that said, the threat for tornadoes will be curtailed by large
surface temperatures/dew point spreads and high cloud bases.

Toward and especially after midnight, any remaining
thunderstorms should shift southeast of the area. Increasing
upper-level jet forcing acting upon the broad baroclinic zone
draped across the Lower Great Lakes will support occasional
flares of frontogenesis, altogether leading to episodic showers
after midnight and through the morning hours. Additional waves
of showers and perhaps a storm or two will continue through the
afternoon hours, though with highest coverage shifting southward
through time (if not entirely out of our area by mid-afternoon)
as the low-level baroclinic zone similarly shifts southward.
Tomorrow otherwise looks much cooler than today with mostly
(largely upper-level) cloudy skies and prevail cold air
advection leading to highs in the upper 50s to around 60.
Onshore flow near Lake Michigan will hold shoreline locations to
the upper 40s. Continued mostly cloudy skies tomorrow night
will hold overnight lows in the upper 30s (north) to lower 40s.

The greatest push of cool air will arrive on Wednesday as 850mb
temperatures fall toward -2C. When paired with mostly cloudy
skies, highs will struggle to get out of the 50s. There may be
some attempt for upper-level clouds to scatter Wednesday night,
setting the stage for overnight lows in the mid to upper 30s.
Should clouds end up clearing completely in parts of the area,
lows could make a run for the lower 30s with an associated
threat for frost (especially across interior northern Illinois).

Looking toward the end of the week and beyond, ensemble
guidance favors a gradual warming trend as upper-level cyclonic
flow shifts eastward. With that said, will note that
interquartile ranges of 850mb temperatures get quite large (over
10C spread) by early next week, suggesting uncertainty in the
placement of the baroclinic zone and with overarching
implications on temperatures. Regardless, there is a notable
trend upward in temperatures by the middle to end of the month
advertised by long-range ensemble guidance, in line with
climatology.

Borchardt

&&

.AVIATION /00Z TAFS THROUGH 06Z WEDNESDAY/...
Issued at 550 PM CDT Mon May 4 2026

.Concerns...

-Thunderstorm coverage through mid evening
-Additional showers overnight into Tuesday
-Lake breeze timing on Tuesday afternoon

Expect scattered showers and storms across the Chicago terminals
through about 2z, longest in the vicinity of MDW and GYY where
slightly better instability is in place. Expect ORD will report
some TS, though the coverage should be lower than toward the
southern Tracon. Localized wind gusts to 35 kt or slightly
higher remain possible in any of these cells, along with some
small hail. Cloud bases will remain VFR, though some occasional
reduced visibility to MVFR/IFR may occur.

A cold front will shift winds to northwesterly across the
airspace this evening. This front will also shift the
thunderstorm axis during 2z-5z to the south and east of the
Chicago area. There will still be some impacts on the southeast
arrival corridor.

Another wave will pass through the area along the lingering
frontal boundary. This will drive some lighter rain showers.
VFR conditions still look to prevail. A lake breeze will develop
on Tuesday afternoon. Latest guidance has come in a bit slower
with its arrival, though we did not feel confident to jump on
this yet. Therefore we left timing unchanged, though this is
only of low-medium confidence.

KMD

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Blowing Dust Advisory until 6 PM CDT this evening for ILZ019-
     ILZ020-ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ107-ILZ108.

IN...Blowing Dust Advisory until 6 PM CDT /7 PM EDT/ this evening
     for INZ001-INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 7 PM CDT this evening for the IL
     and IN nearshore waters.

&&

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
235 PM CDT Mon May 4 2026

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-051945-
Winnebago-Boone-McHenry-Lake IL-Ogle-Lee-De Kalb-Kane-DuPage-
La Salle-Kendall-Grundy-Kankakee-Livingston-Iroquois-Ford-
Northern Cook-Central Cook-Southern Cook-Northern Will-
Southern Will-Eastern Will-Lake IN-Porter-Newton-Jasper-Benton-
235 PM CDT Mon May 4 2026 /335 PM EDT Mon May 4 2026/

This Hazardous Weather Outlook is for portions of north central
Illinois...northeast Illinois and northwest Indiana.

.DAY ONE...Tonight.

Weather hazards expected...

  Limited Thunderstorm Risk...with an associated:
     Level 2 of 5 Damaging Wind Risk...up to 65 mph.
     Level 2 of 5 Large Hail Risk...up to 1 inch in diameter.

DISCUSSION...

This afternoon, strong southwest winds and dry conditions may
lead to areas of blowing dust near agricultural areas south of
Interstate 80.

Scattered thunderstorms may produce locally damaging winds and
hail this evening, especially near and north of Interstate 80.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

  No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

  Any reports provided by spotters would be appreciated today.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east at 40 mph.

LMZ740>745-051945-
Winthrop Harbor to Wilmette Harbor IL-
Wilmette Harbor to Northerly Island IL-
Northerly Island to Calumet Harbor IL-
Calumet Harbor IL to Gary IN-Gary to Burns Harbor IN-
Burns Harbor to Michigan City IN-
235 PM CDT Mon May 4 2026

This Hazardous Weather Outlook is for portions of the Illinois
nearshore waters of Lake Michigan and the Indiana nearshore
waters of Lake Michigan.

.DAY ONE...Tonight.

Weather hazards expected...

  Hazardous conditions for small craft.
  Limited Thunderstorm Risk...with an associated:
     Level 1 of 5 Thunderstorm Wind Gust Risk...up to 50 knots.
     Level 2 of 5 Hail Risk...up to 1 inch in diameter.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

  No hazardous weather is expected at this time.

Borchardt

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