Flash Flood Warning 49, NWS Chicago, IL

920 
WGUS73 KLOT 111835
FFSLOT

Flash Flood Statement
National Weather Service Chicago IL
135 PM CDT Tue Aug 11 2026

INC089-127-111845-
/O.CAN.KLOT.FF.W.0049.000000T0000Z-260811T1930Z/
/00000.0.ER.000000T0000Z.000000T0000Z.000000T0000Z.OO/
Lake IN-Porter IN-
135 PM CDT Tue Aug 11 2026

...FLASH FLOOD WARNING FOR EAST CENTRAL LAKE IN AND CENTRAL PORTER
COUNTIES HAS BEEN REPLACED WITH A FLOOD WARNING...

The heavy rain has ended. Flash flooding is no longer expected to
pose an imminent threat. Please continue to heed remaining road
closures.

&&

LAT...LON 4136 8722 4136 8725 4140 8725 4143 8728
      4149 8728 4151 8726 4151 8693 4135 8693
      4135 8700 4133 8704 4133 8715 4135 8718

$$

Donofrio

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
819 AM CDT Tue Aug 11 2026

.KEY MESSAGES...

- Scattered thunderstorms will affect far northern IL/northwest
  IN early this morning, with isolated instances of large hail
  and damaging wind as well as localized flooding are possible.

- Possible line/complex of severe thunderstorms to affect the
  area by mid/late morning into early this afternoon. Severe
  wind gusts to 80 mph possible, along with heavy rain and
  localized flooding.

- Pattern remains active with additional thunderstorm chances
  through the end of the week/weekend.

&&

.MESOSCALE...
Issued at 819 AM CDT Tue Aug 11 2026

While it took quite some time overnight, the convection over IA
has evolved into a more well organized MCS since 11z. DVN WSR-88D
imagery shows an impressive rear inflow jet has developed with
strengthening cold already resulting in an acceleration of in
forward speed, now moving at around 55 kt.

Morning soundings from ILX and DVN depict a very unstable air
mass in advance of this MCS with steep lapse rates and very
moist boundary layer. While there is still CINH with surface
based parcels, some weakening of this CINH will take place this
morning as sfc warms and dewpoints creep up this morning. Even
with some sfc based CINH, strongly forced ascent on leading edge
of this MCS should have no problem forcing parcels up to the
LFC.

KDMX 88D VWP shows a nice 50kt mid level speed max feeding into
the backside of this MCS, providing ample deep layer shear for
continued mesoscale organization. SPC mesoanalysis depicts an
axis of >3000 J/kg of MLCAPE from northwest IL east-southeast to
west central Indiana. This is in close proximity to a diffuse
surface boundary that remains draped across a very similar area.

Expectation this morning is MCS should continue maintain its
intensity and probably begin to intensify by mid-late morning as
it encounters weaker CINH and diurnally destabilizing boundary
layer. Environment appears primed for a higher-end bow echo and
possible derecho to evolve from this MCS as it rips across our
CWA this morning. Given the orientation of the diffuse sfc
boundary and instability axis, some southward propagation to
this line is likely.

Primary hazard will obviously be damaging winds, potentially
swaths in excess of 80 mph as this complex races across our CWA.
Presence of the weak surface boundary running perpendicular
this QLCS could provide a source of low level vorticity for
mesovortices and potential QLCS tornadoes as well. This threat
is likely to increase as the activity gets into our eastern CWA
as boundary layer destabilizes further.

- Izzi

&&

.DISCUSSION...
Issued at 253 AM CDT Tue Aug 11 2026

Forecast remains complex today, with ongoing storms early this
morning and the potential evolution of a more significant
widespread severe weather/damaging wind threat later this
morning and early afternoon.

Early morning surface analysis depicts a rather diffuse warm
frontal zone stretching from north-central IA east-southeast
across northern IL and northwest IN, roughly from north of the
Quad Cities to LOT and VPZ. KLOT VAD wind profile and SPC/RAP
mesoanalysis indicates a ~25 kt west-southwesterly low level jet
impinging on the frontal zone, with associated isentropic
ascent atop the shallow frontal slope resulting in isolated
thunderstorms from near ARX over to the Lake Michigan shore near
Michigan City IN at this hour. Low-level jet forcing may
increase a bit during the predawn hours as a subtle mid-level
short wave tracks across the area. There has been a signal in
CAM guidance for potential additional development along the
front prior to sunrise, and we continue to monitor for that.
MUCAPE near 3000 J/kg and ~40 kts of effective shear per SPC
mesoanalysis would support storm organization and an associated
severe (primarily wind/hail) threat as evidenced by the
scattered supercell storms currently over LM/IWX's cwa.

Attention then turns to a developing area of thunderstorms over
far southeast South Dakota, which may evolve into a more
widespread and potentially significant severe weather threat
across our forecast area later this morning and early afternoon.
CAM guidance has been consistent in this cluster of storms
eventually growing upscale into a forward-propagating MCS across
northwest IA/southwest MN over the next few hours and
eventually tracking east-southeast along the corridor strong
instability along the aforementioned frontal zone into northern
IL toward mid-late morning. The combination of unseasonably
strong westerly mid-level flow and resulting deep shear(50+ kts
at 700 mb), steep mid-level lapse rates (near 8 C/km) MUCAPE
values in excess of 3500 J/kg, the potential for significant
severe weather appears rather high. Several CAMs are depicting
pockets surface wind gusts of 80+ mph with this MCS across
portions of the forecast area. A tornado threat would also
likely exist given sufficient low-level shear and the proximity
of the previously mentioned surface frontal boundary. While the
exact evolution of this MCS remains uncertain, SPC's Day 1
enhanced (level 3 of 5) for out forecast area appears warranted.
Currently timing of this threat looks to be from perhaps 9 AM
in our northwest cwa through around 3 PM in the far east. There
may also be some convective development along the front ahead of
the MCS, if a strong MCV develops prior to reaching the cwa
increases warm-advection ascent ahead of the main cluster as
indicated by some of the CAMs.

As if the significant severe/widespread damaging wind threat
wasn't enough, we'll also need to monitor the location of heavy
rainfall and flooding potential - particularly for the urbanized
Chicago metro area as well as locations which saw 3-5"+ of
rainfall Sunday and evening. Had contemplated issuing a flood
watch, though with current predawn convection slow to fill-in
(so far) and some uncertainty remaining with the evolution of
the MCS later this morning, elected to hold off at this time
pending some further clarity on location of higher rainfall
impacts.

In the wake of today's expected MCS, a lull in convective
activity is expected from later this afternoon into tonight,
though this will likely depend on the size and magnitude of the
remnant cold pool and how far south the effective frontal zone
ends up along it's southern periphery. The overall pattern some
uncertainty is how far south an area of surface high pressure
builds across the western Lakes in the Friday/Saturday period.
This will have significant impacts on thunderstorm trends/timing
for the local area.

Ratzer

&&

.AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/...
Issued at 706 AM CDT Tue Aug 11 2026

Key Messages:

- An organized complex of thunderstorms is likely to impact the
  terminals between mid-morning and mid-afternoon today. Severe
  wind gusts in excess of 50 kts may occur with these storms.

- Sub-VFR ceilings and visibilities are likely to be observed
  again late tonight into Wednesday morning.

A thunderstorm complex in eastern Iowa at press time is
attempting to get organized into a solid line or bow. These
storms will impact the terminals between mid-morning and mid-
afternoon, and if it manages to become well-organized by the
time it reaches the terminals (observational trends seem to
support that occurring), then severe wind gusts in excess (and
potentially well in excess) of 50 kts may be observed as these
storms roll through. Also could see a few storms develop out
ahead of this main thunderstorm line/bow, so have PROB30 groups
for TSRA preceding the main TEMPO TSRA groups in the Chicago
metro TAFs to account for this possibility.

The persisting abundance of low-level moisture allowed for MVFR
stratus and fog/mist to develop once again last tonight, and
sub-VFR conditions are still being observed at some TAF sites as
of 1200Z. These conditions should improve by mid-morning, but
the persisting low-level moisture and light winds tonight will
likely allow for low stratus and fog to develop once again.

Lastly, there is another chance for thunderstorms to affect the
terminals tomorrow. Current thinking is that the main surface
boundary that storms will focus along tomorrow may remain south
of the terminals, but if this boundary ends up a little farther
north than presently expected, then there will likely be a need
for some sort of TSRA mention to be introduced into the TAFs.

Ogorek

&&

.MARINE...
Issued at 253 AM CDT Tue Aug 11 2026

A potential exists for a severe thunderstorm complex to produce
widespread damaging winds (in excess of 65 knots) across far
southern Lake Michigan later this morning/midday. In addition to
the wind threat itself, the line of storms are expected to
propagate in an east-southeast direction, which could
potentially lead to a seiche (water is pushed ahead of the
storms, then sloshes back and forth for several hours after the
storms pass) on the southern portion of the Lake. If this were
to occur, it could result in fluctuating lake levels along the
Illinois and Indiana shores for a time from later morning into
this afternoon.

Ratzer

&&

.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
344 AM CDT Tue Aug 11 2026

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-120845-
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-
344 AM CDT Tue Aug 11 2026 /444 AM EDT Tue Aug 11 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...

  Significant Thunderstorm Risk...with an associated:
     Level 1 of 5 Tornado Risk.
     Level 3 of 5 Damaging Wind Risk...up to 80 mph.
  Elevated Flooding Risk.

DISCUSSION...
Scattered thunderstorms early this morning will present a
localized risk of severe weather, primarily damaging winds. A
complex of thunderstorms may then move across the area from mid-
morning into early this afternoon. If this complex materializes,
it will likely be severe with damaging wind gusts as high as 80
mph. In addition, any of these thunderstorms will be capable of
producing torrential downpours which may lead to localized
flooding.

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

Wednesday and Thursday...
  Elevated Flooding Risk.
  Elevated Thunderstorm Risk...with an associated:
     Level 1 of 5 Severe Thunderstorm Risk.
Friday...
  Elevated Flooding Risk.
  Limited Thunderstorm Risk.
Saturday...
  Limited Flooding Risk.
  Limited Thunderstorm Risk.
Sunday...
  Limited Thunderstorm Risk.

DISCUSSION...

An active and potentially stormy weather pattern will persist
through the end of the week, with additional rounds of
thunderstorms possible and an attendant risk of severe weather
and flash flooding.

.SPOTTER INFORMATION STATEMENT...

  Spotters will likely be needed this morning through mid-
  afternoon.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east-southeast at 30 to 40 mph. If a bowing
  complex develops, it could move east at 50 to 65 mph.

LMZ740>745-120845-
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-
344 AM CDT Tue Aug 11 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...

  Significant Thunderstorm Risk...with an associated:
     Level 3 of 5 Thunderstorm Wind Gust Risk...up to 70 knots.

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

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

Ratzer

Visit us at www.weather.gov/chicago

Preliminary Local Storm Report
National Weather Service Chicago IL
1119 AM CDT Tue Aug 11 2026

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

1021 AM     Tstm Wnd Gst     1 ENE Oswego            41.69N 88.33W
08/11/2026  M65 MPH          Kendall            IL   Public

            Private weather station: 0.9 SSE Oswego.

&&

sb

Preliminary Local Storm Report
National Weather Service Chicago IL
1122 AM CDT Tue Aug 11 2026

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

1018 AM     Tstm Wnd Dmg     1 NNW Oswego            41.69N 88.36W
08/11/2026                   Kendall            IL   Trained Spotter

            tree branches reported down.

&&

sb

Preliminary Local Storm Report
National Weather Service Chicago IL
1124 AM CDT Tue Aug 11 2026

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

1053 AM     Tstm Wnd Dmg     Homewood                41.56N 87.67W
08/11/2026                   Cook               IL   Trained Spotter

            widespread tree damage south side of town.

&&

sb

Preliminary Local Storm Report
National Weather Service Chicago IL
1128 AM CDT Tue Aug 11 2026

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

1044 AM     Tstm Wnd Dmg     1 S Hinsdale            41.79N 87.93W
08/11/2026                   DuPage             IL   Trained Spotter

            numerous branches down with trees blocking
            eastbound 55th street.

&&

sb

Preliminary Local Storm Report
National Weather Service Chicago IL
1131 AM CDT Tue Aug 11 2026

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

1029 AM     Tstm Wnd Dmg     3 SW Warrenville        41.79N 88.21W
08/11/2026                   DuPage             IL   Trained Spotter

            large tree branches down.

&&

sb

Preliminary Local Storm Report
National Weather Service Chicago IL
1133 AM CDT Tue Aug 11 2026

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

1107 AM     Tstm Wnd Dmg     2 NW Lake Station       41.60N 87.27W
08/11/2026                   Lake               IN   Trained Spotter

            several trees uprooted.

&&

sb