Flash Flooding and Flood in Illinois, April 14 to 15, 2026

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
636 PM CDT Tue Apr 14 2026

.KEY MESSAGES...

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

- Significant all hazards severe weather threat this evening,
  particularly north of I-80.

- Heavy rain and flash flooding threat from repeated rounds of
  thunderstorms tonight 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.

&&

.MESOSCALE...
Issued at 505 PM CDT Tue Apr 14 2026

Convection has taken off over IN in advance of that subtle
shortwave that brought some scattered showers and storms to our
southern CWA. In the wake of this shortwave, there has been
evidence of subsidence, both in satellite imagery and in the ILX
12z to 18z soundings. The 18z soundings from DVN and ILX both
have MLCAPE over 2000 J/kg, but with a pronounced capping
inversion on both soundings. That strong cap suggests that we it
will likely be another couple-few hours before the threat of
convection makes it into our area.

Convection has been increasing in coverage across Wisconsin
closer to the warm front as well as across central/eastern IA
near the sfc low and south along the dryline. Strong deep layer
shear and strong instability will continue to support supercells
with potential for large to very large hail with this activity
in the near term. Given the intense nature of the convection
already, plus a glancing shot of synoptic ascent associated with
a shortwave pass well to our northwest, this convection should
survive and break through the capping inversion as it progresses
east toward and across the MS River early this evening.

As sunset approaches, developing nocturnal low level jet should
result in a rapid increase in low level shear in advance of this
convection across northern IL and southern WI. This will result
in storms moving into a much more favorable environment for
tornadoes, potentially strong as they get into southwest WI and
northwest IL. By this point, there are uncertainties regarding
storm mode, but the strong cap in advance of these storms could
slow the upscale growth into a QLCS. If any well developed
supercell(s) move into this environment the potential would
exist for strong tornado(s) for at least a couple of hours.

Eventually, there are indications that this convection should
congeal into more of a QLCS with severe threat transitioning
to more of a damaging wind with an attendant QLCS tornado
threat. Given the very strong low level shear, the threat for a
strong tornado or two could even persist even after the
convective mode transitions to QLCS.

It still appears the areas most at risk in our CWA is near/north
of I-88/I-290 corridors. Given its current motion, the storms
over IA may not reach our CWA until after 0100 or 0130z.

- Izzi

&&

.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 /00Z TAFS THROUGH 06Z THURSDAY/...
Issued at 636 PM CDT Tue Apr 14 2026

Aviation forecast concerns for 00Z TAFs:

- Period of TSRA likely later this evening into the overnight
  hours. Some TS may be strong-severe. Brief IFR vis/MVFR cigs
  in TS.

- SHRA/embedded TS may linger for some time overnight, if linear
  orientation becomes more west-east. Low confidence in end
  timing of TS overnight.

- Additional TSRA possible especially late Wednesday into
  Wednesday night, however timing/location will likely be
  affected by lingering cold pool/boundaries from tonight's
  storms.

- Breezy S-SW winds through period, though potentially disturbed
  at times by thunderstorms clusters especially tonight.

Latest surface analysis places low pressure over central IA,
with a nearly stationary frontal boundary extending northeast
and east across southern WI. Clusters of severe thunderstorms
have developed in the vicinity of the low and frontal boundary
from central/eastern IA into southern/central WI. Farther south,
our terminals are in a warm, humid and breezy warm sector,
generally capped by an inversion associated warm EML layer
aloft. While can't completely rule out isolated TS developing
ahead of current activity this evening, current expectation is
that the cluster of TSRA over northeast IA will track east
across far northern IL and move into the RFD area after 02Z and
Chicago area after 03Z with attendant severe weather threat
(wind/hail and isolated tornado) through late evening.
Thereafter, additional TS may develop as the low level jet turns
more west-southwest and increases and potentially leads to some
training of storms across part of the area. Exact location and
footprint of this is of somewhat low confidence at this time,
though have maintained at least a VCTS mention through 09-10Z
for this. At some point TS coverage should wane with a somewhat
stable cold pool likely resulting in a lull in activity
Wednesday morning.

Redevelopment of SHRA/TSRA appears likely later in the day
Wednesday into Wednesday evening, though effects of tonight's
cold pool and residual stabilizing effects (and location of any
aggregate outflow boundaries) will likely have impacts on
destabilization and location of stronger low-level forcing. Have
indicated VCTS/prob30 after 20-21Z Wednesday afternoon and into
the evening with lower confidence in details, though coverage
may ultimately require a tempo/prevailing mention.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory until 9 PM CDT this evening for Winthrop
     Harbor IL to Burns Harbor IN.

&&

Visit us at weather.gov/chicago

Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
404 PM CDT Tue Apr 14 2026

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-152115-
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-
404 PM CDT Tue Apr 14 2026 /504 PM EDT Tue Apr 14 2026/

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

.DAY ONE...Tonight.

Weather hazards expected...

  Elevated Thunderstorm Risk...with an associated:
     Level 3 of 5 Tornado Risk.
     Level 3 of 5 Large Hail Risk...up to Tennis Ball size.
     Level 3 of 5 Damaging Wind Risk...up to 75 mph.
     Level 2 of 4 Flash Flood Risk.

DISCUSSION...

An all hazardous significant severe weather threat exists across
northern IL this evening, particularly along and north of I-88.

Multiple rounds of heavy rain producing storms moving over the
same areas may also result in areas of flash flooding tonight.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday.

Wednesday...
  Elevated Thunderstorm Risk...with an associated:
     Level 2 of 5 Severe Thunderstorm Risk.
     Level 2 of 4 Flash Flood Risk.
Thursday...
  Limited Thunderstorm Risk southeast of Interstate-55.
Friday Night into Saturday morning...
  Limited Thunderstorm Risk...with an associated:
     Level 2 of 5 Severe Thunderstorm Risk.
     Level 1 of 4 Flash Flood Risk.
Sunday Night...
  Significant Frost Risk.

DISCUSSION...

Additional waves of storms are expected at times through the end
of the week, some of which could be severe and produce heavy rain
and localized flash flooding.

.SPOTTER INFORMATION STATEMENT...

  Spotters will likely be needed this evening.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east northeast at 45 mph.

LMZ740>745-152115-
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-
404 PM CDT Tue Apr 14 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...

  Elevated Thunderstorm Risk...with an associated:
     Level 3 of 5 Thunderstorm Wind Gust Risk...up to 55 kt.
     Level 3 of 5 Hail Risk...up to Tennis Ball size.
     Level 3 of 5 Thunderstorm Waterspout Risk.

.DAYS TWO THROUGH SEVEN...Wednesday through Monday.

Wednesday...
  Elevated Thunderstorm Risk...with an associated:
     Level 2 of 5 Severe Thunderstorm Risk.
Friday and Saturday...
  Hazardous conditions for small craft are likely.
  Limited Thunderstorm Risk.
Sunday...
  Hazardous conditions for small craft are likely.

KJB

Visit us at www.weather.gov/chicago