Tornado Warning 22, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
636 PM CDT Fri Apr 17 2026

.KEY MESSAGES...

- Another round of showers and thunderstorms is expected later
  this evening into tonight. Some of the storms could be
  severe. Locally heavy rainfall could result in flooding
  issues.

- Windy and unseasonably warm weather today will be replaced by
  much colder weather over the weekend into early next week.

- Frost/freeze conditions are expected many areas Sunday and
  especially Monday mornings.

&&

.MESOSCALE...
Issued at 615 PM CDT Fri Apr 17 2026

The cluster of cyclical supercells over northwest Illinois that
ultimately produced a tornado in Winnebago County has shifted
northeastward into southern Wisconsin. Frontal convection is not
currently surging eastward toward the Mississippi River, with
radar trends indicating bowing clusters with embedded supercell
structures. While the 21Z KILX RAOB shows a remaining weak cap
and diurnal mixing has decreased dew points by a couple degrees
over much of the area, the congealing nature of the convection
and modest low-level moisture advection from the southwest
supports convection remaining quite organized well into the
forecast area this evening.

North of the I-80 corridor, a localized area of moisture
convection (aligning with a few showers over the west Chicago
metro), observed slight backing of the winds, and observed
gradual elongation of the low-level hodograph (per KLOT VWP
trends) supports a locally higher QLCS tornado threat into at
least the western Chicago metro and possibly to the lakeshore
with any bowing segments. Otherwise, damaging winds with
possibly a corridor or two of 70+ mph gusts are possible through
mid-evening.

South of the I-80 corridor, the amalgamation of a large area of
convection over over-central Illinois has a potential to
organize into a MCV with eastward extent over the next several
hours. If this were to occur, another localized damaging wind
threat with embedded QLCS tornados would be focused around the
IL/Kankakee River valleys.

Kluber

&&

.DISCUSSION...
Issued at 136 PM CDT Fri Apr 17 2026

For near term details on the convective threats through this
evening, see the mesoscale discussion (and/or additional future
discussions) above. Additional concerns for tonight will be
centered on heavy rainfall and localized flash flooding.
Precipitable water values still lie at the upper echelon of
typically observed values for Mid April, and this coupled with
some enhanced forcing should lead to areas of 1-1.5" rainfall
in a relatively short window of time. We cannot rule out any
higher overall totals (pushing 3"), given that we have several
windows of forcing both in the open warm sector and along the
cold front. Much of this rain will go immediately into runoff.
In addition to any urban locations, it is hard to pinpoint the
more favored areas in the watch area, however the most
susceptible areas overlapping with higher confidence rainfall
would be in the Rock River Basin, and then extending eastward
into the Fox and Des Plaines River Basins across northern
Illinois. A secondary heavy rain axis may setup south, but if we
have a forward propagating system, that may limit the duration
of heavy precipitation.

A taste of reality will return to the lower Great Lakes region
behind tonight's strong cold front. High temperatures will
reached at midnight. While some temperature recoveries will
occur as clouds break through the morning, rather stout cold air
advection through the day will prevent temperatures from
plateauing higher than the mid 40s to low 50s area wide. Normal
highs for mid April are right at 60. A stiff northwest breeze
will make it feel even less pleasant, with "wind chill" readings
in the lower 40 in many areas in the afternoon. This will set
the stage for a cold night Saturday night.

Cold advection gets reinforced as the upper trough axis will be
shifting overhead. The center of the surface high will be off to our
southwest Saturday night, thus we will maintain a bit of a wind
component, however things look to decouple enough to suggest some
areas of frost.

Reinforced northwest flow on the backside of the upper trough will
maintain a similar airmass on Sunday and provide a carbon copy day
from Saturday. With high pressure nearby Sunday night, a widespread
frost appears increasingly likely, with some area in outlying areas
heading for a potential hard freeze.

Upper heights recover nicely and warm advection on the back side for
the departing surface high will encourage temperatures to rebound
back above normal through mid week with generally dry
conditions. There will be periods of onshore flow keeping cool
readings near the shore.

KMD

&&

.AVIATION /00Z TAFS THROUGH 06Z SUNDAY/...
Issued at 636 PM CDT Fri Apr 17 2026

Forecast concerns include...

A broken line of thunderstorms this evening.
Strong/gusty southerly winds early this evening.
Strong/gusty northwest winds overnight/Saturday.
Mvfr cigs overnight.

A broken line of thunderstorms will move across the terminals
this evening. The forward speed of this line may begin to
increase in the next few hours and specific arrival times to
ORD/MDW remain challenging. Opted for prevailing thunder at 130Z
and maintained the 02z-04z tempo, but this may need some
adjustment. Once this line of storms moves east, a period of
showers with isolated thunder will remain possible during the
late evening with showers ending during the early overnight
hours. The main severe weather threat is expected to be locally
high westerly winds.

Ahead of these thunderstorms, southerly winds gusting into the
30kt range will continue. Prevailing speeds/gusts will diminish
behind the storms, likely favoring a south/southwest direction
and then shift to the northwest overnight when a cold front
moves across the area. Gusts into the 25-30kt range are expected
for much of Saturday, diminishing Saturday evening.

Mvfr cigs will be possible both with the thunderstorms and then
prevailing mvfr cigs are possible overnight. Only medium
confidence and cigs look to lift into low vfr by mid/late
Saturday morning. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 5 AM CDT Saturday for ILZ003-ILZ004-ILZ005-
     ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-ILZ020-
     ILZ021-ILZ023-ILZ032-ILZ033-ILZ039-ILZ103-ILZ104-ILZ105-
     ILZ106-ILZ107-ILZ108.

IN...Flood Watch until 5 AM CDT /6 AM EDT/ Saturday for INZ001-
     INZ002-INZ010-INZ011-INZ019.

LM...Small Craft Advisory until 10 PM CDT Saturday for the IL and
     IN nearshore waters.

&&

Visit us at weather.gov/chicago

Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
1218 PM CDT Fri Apr 17 2026

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-181730-
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-
1218 PM CDT Fri Apr 17 2026 /118 PM EDT Fri Apr 17 2026/

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

.DAY ONE...This Afternoon and Tonight.

Weather hazards expected...

 Significant Thunderstorm Risk...with an associated:
	 Level 3 of 5 Damaging Wind Risk...up to 70 mph.
     Level 2 of 5 Tornado Risk.
     Level 1 of 5 Large Hail Risk...up to one inch in diameter.
  Elevated Flooding Risk.

DISCUSSION...

A line of thunderstorms is expected to work across northern
Illinois this evening. Thunderstorms may be severe with damaging
wind gusts, hail, and perhaps a few tornadoes. Heavy rain may
also lead to instances of flash flooding.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

Saturday...
  Elevated Frost and  Freeze Risk.
Sunday...
  Significant Frost and Freeze Risk.

.SPOTTER INFORMATION STATEMENT...

  Spotters will likely be needed this evening.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the northeast at 45 mph and then becoming east.

LMZ740>745-181730-
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-
1218 PM CDT Fri Apr 17 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...This Afternoon and Tonight.

Weather hazards expected...

  Hazardous conditions for small craft.
  Elevated Thunderstorm Risk...with an associated:
     Thunderstorm Waterspout Risk
     Level 3 of 5 Thunderstorm Wind Gust Risk...up to 60 knots.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday.

Saturday...
  Hazardous conditions for small craft are likely.
Monday and Tuesday...
  Hazardous conditions for small craft are likely.

KMD

Visit us at www.weather.gov/chicago

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