Tornado and Damaging Winds in Illinois, May 18, 2026

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
633 AM CDT Mon May 18 2026

.KEY MESSAGES...

- A few additional rounds of showers and thunderstorms today and
  Tuesday, some of which could be strong to severe mainly during
  the afternoon hours.

- Summer-like warmth and breezy winds at times will prevail
  through Tuesday, then cooler temperatures return for the
  middle of the week.

- Milder weather returns heading into the weekend, though with
  at least intermittent shower/thunderstorm potential.

&&

.DISCUSSION...
Issued at 347 AM CDT Mon May 18 2026

Regional radar mosaic shows a linear MCS moving across the
Mississippi River into western WI/IL early this morning. There
has been a notable weakening trend to the convection over the
past few hours as the complex propagates into a region of
decreasing deep- layer shear and low-level instability, though
storms should continue to spread eastward this morning along a
composite outflow boundary at the leading edge of the
convection. In fact, recent radar trends indicate a few new
cells developing ahead of the line near the Quad Cities, perhaps
aided by enhanced ascent associated with one or more an
apparent MCVs near the IA/IL/MO border. Gusty winds of 30-40 mph
with the outflow and some small hail are possible with these
storms early this morning, though the overall threat of
organized severe weather appears low for the next several hours.
Later this morning however, diurnal warming and associated
destabilization along the east/southeast periphery of the
complex remnants and outflow boundary may support renewed
development/strengthening of convection with more of a severe
threat into this afternoon. Though depending on how far the
outflow boundary makes it this morning, this would likely be
across the south/far east portions of the forecast area. Farther
north/northwest, lingering stratiform rain and cloud cover this
morning looks to limit destabilization and recent CAM trends
have been consistently dry there during the day/evening.

Farther to the west, early morning GOES vapor imagery indicates
another mid-level short wave rounding the base of a western
CONUS long-wave trough. Widespread strong convection is progged
to develop late this afternoon and evening across KS/NE/IA once
again in response, eventually growing upscale into another
eastward- propagating linear MCS. Similarly to this morning, CAM
runs continue to depict a similar weakening of this system as
it reaches the Mississippi River and spreads into WI/IL early
Tuesday morning with associated relatively low severe potential.
The lingering MCS cold pool footprint and residual cloudiness
will likely delay destabilization into midday Tuesday, though
additional storm development appears possible across the
southeast half of the forecast area Tuesday afternoon/evening as
the cold front finally pushes through the forecast area.
Stronger mid-level flow on the southeastern periphery of the
aforementioned short wave will likely support enough storm
organization for a hail/wind threat during this time.

Following the cold front, surface high pressure is progged to
build east into the western Great Lakes region Wednesday and
Thursday. Dry, but breezy and cooler weather is expected both
days, with temps mainly in the 60s, but only in the 50s near
Lake Michigan with northeast-east flow off the lake. Nighttime
lows in the 40s are expected. The high then drifts off to the
east Thursday night, with the upper level pattern favoring a
series of mid-level disturbances tracking through the area
Friday into the weekend. This will support milder temperatures,
but also some occasional shower/thunderstorm chances into the
holiday weekend.

Ratzer

&&

.AVIATION /12Z TAFS THROUGH 18Z TUESDAY/...
Issued at 632 AM CDT Mon May 18 2026

Forecast concerns for the terminals...

- Line of showers and storms will move through the terminals
  this morning. Strongest storms may produce locally gusty winds
  around 30 kts.

- Period of gusty southeast winds behind the rain this morning
  before winds turn back southwest with gusts around 25 kts.

- Another period of showers and possible thunderstorms late
  tonight into Tuesday morning.

A line of showers and embedded thunderstorms continues to
advance eastward across northwest and western IL this morning.
While the line has been gradually weakening, newer
showers/storms have been developing ahead of the outflow from
the main line and that trend is expected to continue. Therefore,
expect a 2-3 hour period of thunder at the terminals this
morning which will result in some MVFR ceilings and
visibilities. Additionally, the stronger storms will be capable
of producing locally gusty winds upwards of 30 kts and maybe
some small hail.

Behind the line are a couple of small scale areas of low
pressure. The low over northeast MO is expected to lift
northeast into north-central IL and northwest IN later this
morning which should help in either redeveloping or maintaining
any ongoing showers and thunderstorms. Given the low's
trajectory it appears the greater potential for any
thunderstorms with the low will be south of I-80 but cannot
fully rule out an isolated strike or two as far north as ORD and
DPA. Therefore, have opted to introduce a VCTS for this
potential at the Chicago terminals. Once this area of
showers/storms begins to exit there is also a wake low that has
developed in IA. While this low shouldn't result in more rain,
it does look to bring a period of gusty southeast winds around
25-30 kts but higher gusts are certainly possible.

After the wake low moves through around midday/early afternoon,
winds will return from the southwest with persistent gusts
around 25 kts through the rest of the afternoon. Overall dry
conditions should prevail at the terminals through this
afternoon, but there remains a 15-20% chance for some isolated
showers or storms to redevelop. Since the confidence in any
afternoon activity remains low have opted to forego any formal
TAF mentions for now.

Heading into tonight, gusts will diminish after sunset but will
still remain breezy with upper teen to lower 20 kt gusts. At the
same time more showers and thunderstorms are forecast to develop
across IA along a cold front and eventually will move into
northern IL and northwest IN towards daybreak. Given that these
showers/storms should be arriving in a decaying state
confidence on thunder reaching the Chicago terminals is low so
for now have just maintained PROB30s for showers here. That
said, did opt to add a PROB30 for thunder at RFD late tonight
into Tuesday morning where confidence is slightly higher.

Otherwise, expect mostly VFR conditions to prevail through the
TAF period.

Yack

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

Visit us at weather.gov/chicago

Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
425 AM CDT Mon May 18 2026

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-190930-
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-
425 AM CDT Mon May 18 2026 /525 AM EDT Mon May 18 2026/

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

.DAY ONE...Today and Tonight.

Weather hazards expected...

  Limited Thunderstorm Risk...with an associated:
     Level 2 of 5 Damaging Wind Risk...up to 60 mph.
     Level 2 of 5 Large Hail Risk...up to quarter size.
  Limited Flooding Risk.

DISCUSSION...

A weakening line of showers and thunderstorms will move east
across the area this morning. While the threat of severe weather
is fairly low this morning, daytime heating will likely allow for
a few strong to severe storms to develop during the midday and
afternoon hours. At this time, the threat of severe weather would
appear to be greatest along and south of the I-80 corridor.
Damaging winds and large hail would be the main severe weather
threats.

Another line of thunderstorms is expected to approach the area
from Iowa again toward daybreak Tuesday.

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

Tuesday...
  Limited Thunderstorm Risk...with an associated:
     Level 2 of 5 Severe Thunderstorm Risk.

.SPOTTER INFORMATION STATEMENT...

  Spotters may be needed this afternoon.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the northeast at 30 to 35 mph.

LMZ740>745-190930-
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-
425 AM CDT Mon May 18 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...Today and Tonight.

Weather hazards expected...

  Limited Thunderstorm Risk...with an associated:
     Level 2 of 5 Thunderstorm Wind Gust Risk...up to 50 knots.
     Level 1 of 5 Hail Risk...up to quarter size.

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

Tuesday...
  Hazardous conditions for small craft are likely.
  Limited Thunderstorm Risk...with an associated:
     Level 2 of 5 Severe Thunderstorm Risk.

Ratzer

Visit us at www.weather.gov/chicago