Flash Flood and Flooding in Illinois, April 15, 2026

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1210 AM CDT Wed Apr 15 2026

.KEY MESSAGES...

- Late spring to summer-like temperatures continue through
  Friday with highs in the 70s and 80s and lows in the 60s.

- Threat for additional isolated to scattered severe weather
  overnight and on Wednesday PM, along with a heavy rain and
  associated flash flooding threat from repeated rounds of
  thunderstorms overnight into Wednesday.

- A brief break in the pattern is expected late Thursday into or
  through the day on Friday before another storm system arrives
  in the Friday evening to Saturday timeframe.

- In the wake of the Saturday cold front, a brief, potentially
  sharp, shot of much cooler air could bring frost/freeze
  concerns into next Sunday and Monday.

&&

.DISCUSSION...
Issued at 312 PM CDT Tue Apr 14 2026

Through Wednesday night...

The primary weather focus in the near term continues to revolve
around the continued threat for a significant severe weather event
this evening. This includes the threat of tornadoes, instances of
very large hail (2"+ diameter) and damaging winds in excess of 70
mph. The threat of flash flooding is also of increasing concern
tonight into Wednesday, due to repeated rounds of very heavy rain
producing storms.

A recent surface analysis indicates that the stalled west-to-
east oriented frontal boundary continues to reside north of the
area this afternoon, generally from northeastern IA eastward
across southern WI. Meanwhile, the outflow boundary from this
mornings convection has largely washed out, with a notable airmass
recovery across northeastern IL, noted by temperatures rebounding
into the low 80s amidst surface dewpoints in the mid 60s.
Interestingly, while this afternoon's airmass recovery beneath a
corridor of very steep mid-level lapse rates has resulted in
moderate to strong MLCAPE values in excess of 3,000 j/kg, a notable
capping EML inversion remains in place. This is apparent on the 18Z
DVN RAOB, with a sharp EML inversion noted around 800 mb. While this
cap is currently curtailing convective development, the sampled
thermodynamic and kinematic environment is quite volatile and will
be supportive of significant severe weather as we head through the
evening.

While some isolated to widely scattered supercells capable of
producing large hail will be possible this afternoon as a subtle
impulse continues to slide eastward across central and eastern
IL into northwestern IN, the primary severe thunderstorm threat
continues to be this evening, particularly across northern IL
and southern Wisconsin. Initial severe storm development is
expected near the stalled frontal boundary in northeastern IA
within the next hour or two. In fact, current satellite and
radar imagery indicate several convective attempts already
underway in this area.

As development occurs, storm modes will initially favor
supercells, though a eventual upscale growth is anticipated
through the evening. Very steep mid-level lapse rates and long
straight hodographs will favor instances of very large
destructive hail with these initial supercells. However,
strengthening low- level flow and the associated enlarging low-
level hodographs into early this evening will will support a
notable tornado threat for a few hours this evening (6-9 pm).
This tornado threat is expected to be the highest across far
northern IL into southern WI (generally along and north of
I-88). Upscale growth into a severe wind producing MCS, with
potential embedded QLCS tornadoes is then likely to occur
through the evening as this activity progresses eastward through
southern WI and far northern IL. Accordingly, the main severe
threats look to transition to damaging wind gusts in excess of
70 mph and isolated tornadoes later this evening. Increasingly
confidence has prompted the recent issuance of a tornado watch
for most of northern IL through this evening.

While the severe threat should largely wane by midnight
tonight, the threat for training showers and thunderstorms may
continue across parts of northern IL into the overnight hours. A
veering low-level jet overnight may set up favorable conditions
for training and backbuilding storms as the flow offsets the
easterly movement. This adds concern for flash flooding,
particularly given that this activity could train over the urban
areas of Chicago. At this time we have opted to hold off on a
flash flood watch, though as trends become more clear this
evening one may be needed. We did highlight the threat for
possible flooding concerns in a Hydro outlook (ESF), but again,
messaging may need to be ramped up to a Watch this evening.

Additional waves of showers and thunderstorms are expected on
Wednesday. The amount of cloud cover and periods of rain does
make it unclear as to the extent and coverage of severe storms
due to questions of diurnal destabilization. Nevertheless, a
level 1 to 2 out of 5 severe weather threat continues on
Wednesday afternoon and evening. Regardless of the overall
severe threat, flooding and flash flooding will continue to be
of concern Wednesday into Wednesday night, owing to the repeated
instances of very heavy rainfall. Accordingly, a future flood
watch may be needed for the tonight through Wednesday time
period.

Thursday onward...

A short period of drier weather is expected late Thursday into
Friday morning following the eastward departure of the mid-
level impulse ejecting out of the Desert Southwest. However, yet
another storm system and associated cold front is favored to
shift across the central CONUS Friday night into Saturday
morning. Locally, another threat of strong to severe
thunderstorms will exist in advance of cold front, which should
move across our area sometime Saturday morning.

In the wake of this cold front, a period of much cooler weather
will move in across the area Sunday into Monday. The magnitude
of this colder airmass may result in freezing overnight low
temperatures and afternoon highs only in the low 50s for Sunday.

KJB

&&

.AVIATION /06Z TAFS THROUGH 12Z THURSDAY/...
Issued at 1210 AM CDT Wed Apr 15 2026

Aviation forecast concerns for the 06Z TAFs:

- Periods of TSRA, particularly remainder of overnight, and
  again late Wednesday afternoon/evening. Brief IFR/MVFR
  possible in precipitation. Scattered SHRA possible at other
  times.

- Winds likely convectively disturbed with somewhat variable
  directions for the next several hours, otherwise mainly
  southwest to south winds (breezy at times with gusts 20-25 kts).

Late evening surface analysis depicts an area of low pressure
over north-central KS, with a quasi-stationary frontal boundary
extending northeast across IA into southern WI. A thunderstorm
complex which moved across northern IL late Tuesday evening was
moving east across lower MI, though storms continue to
redevelop back across northwest IN/northeast IL along and
northeast of a west-east oriented outflow boundary trailing the
aforementioned complex. Additional thunderstorms have developed
along the stationary front roughly along a a KOMA-KDSM-KCID
line.

Expectation is that scattered TS will continue to redevelop
across the Chicago terminals as the low-level jet ascends atop
the west-east outflow boundary, at least for another 2-3 hours
or so. High-res guidance (RRFS/HREF) indicates IA TSRA will
spread into parts of northern IL after 09-10Z, potentially
lingering through daybreak or slightly later before weakening
and moving east of the area. Terminals will remain within a
region of broad southwest warm/moist advection however, which
could result in isolated/scattered SHRA just about any time
Wednesday. Can't rule out some isolated TS during the day,
though the next best window for greater TSRA coverage appears to
be late Wednesday afternoon into Wednesday evening/night.

Surface winds have become convectively disturbed across northern
IL at this time in the wake of the departing complex. While
winds should eventually settle back to a south-southwest,
current winds do have some variability in direction north of
the aforementioned outflow boundary. Once winds do shift back to
the south-southwest, breezy conditions are likely with gusts
20-25 kts at times.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Flood Watch until 6 AM CDT Wednesday for ILZ003-ILZ004-ILZ005-
     ILZ006-ILZ008-ILZ011-ILZ012-ILZ013-ILZ020-ILZ103-ILZ104-
     ILZ105-ILZ106-ILZ107-ILZ108.

IN...Flood Watch until 6 AM CDT Wednesday for INZ001.

LM...None.

&&

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
147 AM CDT Wed Apr 15 2026

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-160700-
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-
147 AM CDT Wed Apr 15 2026 /247 AM EDT Wed Apr 15 2026/

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

.DAY ONE...Early this Morning through Tonight.

Weather hazards expected...

  Elevated Thunderstorm Risk...with an associated:
     Level 2 of 5 Damaging Wind Risk...up to 70 mph.
     Level 2 of 5 Large Hail Risk...up to Half Dollar size.
     Level 1 of 5 Tornado Risk.
  Elevated Flooding Risk.

DISCUSSION...

Additional waves of thunderstorms are expected at times early
this morning through tonight, some of which could be strong to
severe, especially in the afternoon and evening.

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Thursday...
  Limited Thunderstorm Risk.
Friday into early Saturday morning...
  Limited Thunderstorm Risk...with an associated:
     Level 2 of 5 Severe Thunderstorm Risk.
Sunday Night...
  Elevated Frost Risk.

DISCUSSION...

Additional waves of storms are expected at times through the end
of the week.

.SPOTTER INFORMATION STATEMENT...

  Spotters may be needed at times through this evening.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east at 40 mph.

LMZ740>745-160700-
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-
147 AM CDT Wed Apr 15 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...Early this Morning through Tonight.

Weather hazards expected...

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

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Thursday...
  Limited Thunderstorm Risk.
Friday...
  Hazardous conditions for small craft are likely.
  Limited Thunderstorm Risk.
Saturday and Sunday...
  Hazardous conditions for small craft are likely.

Petr

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Preliminary Local Storm Report
National Weather Service Chicago IL
1140 PM CDT Tue Apr 14 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1135 PM     Flash Flood      1 NNE Elmwood Park      41.93N 87.81W
04/14/2026                   Cook               IL   911 Call Center

            2600 block of 74th Avenue closed due to
            flooding.

&&

WSL

Preliminary Local Storm Report
National Weather Service Chicago IL
1145 PM CDT Tue Apr 14 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1130 PM     Flash Flood      1 W Lincolnwood         42.00N 87.75W
04/14/2026                   Cook               IL   Public

            I-94 Edens Expressway flooded at Pratt
            Avenue underpass. Relayed via waze.

&&

WSL

Preliminary Local Storm Report
National Weather Service Chicago IL
1151 PM CDT Tue Apr 14 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1125 PM     Flood            2 S Mount Prospect      42.03N 87.94W
04/14/2026                   Cook               IL   Public

            Multiple public reports of flooding near
            Algonquin Road and Elmhurst Road. Relayed
            via waze.

&&

WSL

Preliminary Local Storm Report
National Weather Service Chicago IL
1152 PM CDT Tue Apr 14 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1110 PM     Marine Tstm Wind 3 NE Beverly Shores     41.72N 86.94W
04/14/2026  M43 MPH          LMZ745             IN   C-Man Station

            Measured at the Michigan City GLERL site.

&&

Preliminary Local Storm Report
National Weather Service Chicago IL
112 AM CDT Wed Apr 15 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

0100 AM     Rain             Ohare Airport           41.98N 87.90W
04/15/2026  M2.21 Inch       Cook               IL   ASOS

            O'Hare Airport recorded 2.21 inches of rain
            between 7PM and 1AM, with the heaviest rain
            having occurred between 10PM and 12AM during
            which 1.72 inches fell.

&&

Preliminary Local Storm Report
National Weather Service Chicago IL
156 AM CDT Wed Apr 15 2026

..TIME...   ...EVENT...      ...CITY LOCATION...     ...LAT.LON...
..DATE...   ....MAG....      ..COUNTY LOCATION..ST.. ...SOURCE....
            ..REMARKS..

1100 PM     Rain             1 S Park Ridge          41.99N 87.84W
04/14/2026  M2.50 Inch       Cook               IL   Public

            Multiple personal weather stations in the
            vicinity indicate between 2 and 2.5 inches
            of rain fell in about one hour between 10
            and 11 pm.

&&