Tornado Warning 16, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
303 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 303 PM CDT Fri Apr 17 2026

Latest radar and satellite trends indicate that storms are
following a mostly discrete supercell structure in the vicinity
of far northwest Illinois. Recent model guidance has also
trended to favor this scenario, suggesting the bulk of
convection through at least 5pm in our area will remain
generally northwest of a Dixon to Boone County line. A less
favorable environment highlighted by diminishing theta-e from
modest diurnal mixing and limited near-term upstream moisture
advection does raise some questions about how far east the more
organized convection will survive over the next couple hours.
However, modestly favorable low-level hodographs amid slightly
less-backed flow do support the potential for supercell modes to
survive with any convection that does clip the far northwest
CWA. Primary threat will be large hail and possibly a tornado.

Focus then turns to an expected expansion of convection along
the cold front (currently from just east of Des Moines to Kansas
City) as it quickly moves eastward into a more favorable
environment. As this convection marches eastward through late
afternoon, increasing coverage of pre-frontal convection
associated with deeper low-level moisture over Missouri will
surge toward west-central Illinois. This should result in a
well-formed QLCS with the potential for damaging winds and
embedded brief tornadoes across a large swath of Illinois this
evening. Will need to closely monitor any bowing segments,
particularly around and after sunset, as low-level flow
increases and lengthens hodographs.

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 /18Z TAFS THROUGH 00Z SUNDAY/...
Issued at 116 PM CDT Fri Apr 17 2026

Key Messages:

- High confidence in a line of thunderstorms impacting the
  terminals late this afternoon and evening. Localized wind
  gusts in excess of 50 kts may occur in some portions of the
  line of storms.

- Southerly winds this afternoon will gust to around 30 kts at
  times before a gusty westerly to northwesterly wind shift
  occurs with both the line of storms and again with a trailing
  cold front later tonight.

- Post-frontal MVFR ceilings are expected tonight.

Confidence remains high in all terminals being affected by
thunderstorms today. Uncertainty remains with the precise timing
of the storms, but this evening still appears to the most likely
time frame for storms at the Chicago metro terminals, while RFD
could see storms as early as the mid-late afternoon. The storms
will likely affect the terminals as a solid line, though some
potential exists for RFD to be affected by more cellular or
supercellular convection before storms grow upscale into a line
with time. A gusty westerly to northwesterly wind shift and
localized wind gusts in excess of 50 kts may occur in some
portions of the thunderstorm line, and some hail and tornado
threat exists too -- primarily at RFD. The storms should last
for about 2-3 hours at any one location (possibly longer at RFD
if multiple rounds of storms materialize there) with rain then
continuing for a bit longer until a cold front pushes across
the area.

Prior to the arrival of the storms, southerly winds will gust
to around 30 kts at times this afternoon. Another gusty
northwesterly wind shift is expected behind the aforementioned
cold front with blustery post-frontal west-northwesterly winds
then continuing through the remainder of the TAF period.

Lastly, MVFR (and potentially IFR) ceilings are likely to occur
during the thunderstorms and again behind the cold frontal
passage. A return to VFR conditions is then likely to occur
sometime tomorrow morning.

Ogorek

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch through late tonight 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 through late tonight 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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