Flash Flood Warning 50, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
719 AM CDT Wed Aug 12 2026

.KEY MESSAGES...

- Isolated strong storms possible early this morning.

- Another fast-moving line of strong/severe thunderstorms is
  expected later this morning with a damaging wind and heavy
  rainfall threat.

- Our active, stormy weather pattern is expected to persist into
  the weekend with periodic chances of severe weather and heavy
  rainfall which could lead to flooding.

- A period of dry weather may finally arrive early next week.

&&

.DISCUSSION...
Issued at 329 AM CDT Wed Aug 12 2026

Forecast remains complex within our persistent pattern of active
weather. Main concern in the near term is the potential for
another fast-moving line of wind-producing storms across at
least a portion of the area later this morning/midday.

A nearly stationary frontal boundary remains in place across
the Midwest early this morning, stretching roughly from near
Omaha to north of Des Moines to Milwaukee. Complicating the
surface pattern however, is the remnant outflow boundary from
Tuesday's derecho, which intersects the front near Waterloo, IA
and extends southeast and bisects IL along a SQI-BMI-MTO-EVV
line. The west/northwest periphery of the cold pool appears to
be fairly shallow however, per 00Z DVN and ILX soundings (as
compared to ILM farther to the east/southeast). Aloft, a subtle
mid-level short wave has been evident in GOES vapor imagery
transiting northern IA and entering northwest IL, inducing a
modest west-southwest 25-30 kt low level jet from northern MO
into northern/central IL/IN. With an axis of 3500-4000 J/kg
MUCAPE along/north of the front/outflow boundary across the
region, there's been concern since last evening for the
potential to develop elevated convection within a northwest-
southeast arc across the area (along/northeast of the remnant
outflow boundary). So far however, only isolated updraft
attempts have been noted and successive CAM runs have backed off
considerably in convective coverage across the forecast area
through daybreak. That said, we'll have to continue to monitor
for any deeper updraft development, as the environment would
support the potential for strong elevated storms with a hail
threat if they develop.

Of greater concern this morning, is an MCS which has been
developing southeast from western South Dakota into northern
Nebraska overnight in association with an attendant mid-level
short wave and MCV. There is notable CAM support indicating this
MCS becoming more organized across eastern NE/western IA toward
daybreak, then propagating quickly eastward along the
aforementioned surface front/outflow boundaries into our IL cwa
after 14/15Z (9-10 AM) this morning. While not quite as robust
as yesterday's derecho, high-res guidance does explicitly
indicate 60-70 mph winds across parts of our forecast area with
this MCS. CAMs indicate an easterly propagation of this system
across the cwa, though with the remnant outflow likely mixing
back north across the southern half of our cwa that may support
more of an east-southeast or southeast track as we saw
yesterday. SPC has pulled their new Day 1 slight (level 2 of 5)
severe risk up to about a Dixon-Renssalear line. It's possible
that could be adjusted further if the evolution of the MCS
supports it later this morning. For now, will be advertising the
severe threat (primarily wind) even a little north of that for
this morning/midday. Thunderstorm threat should then diminish
markedly from the west into this afternoon in the wake of the
MCS and parent short wave.

There may be somewhat of a lull in convection
coverage/potential tonight into early Thursday morning,
depending on the footprint of the anticipated MCS cold pool, and
with some weak mid-level height rises indicated in model
guidance. Have trimmed pops during this period especially up
north along the IL/WI border and Lake Michigan. Another mid-
level wave (currently over NV/UT) is then progged move into the
region on Thursday which look to increase shower and
thunderstorm chances again during the day and with additional
rounds through at least Thursday night. These continued rounds
of storms will maintain at least a marginal severe threat and
a threat of flash flooding if storms train over the same areas.

Unfortunately, medium-range guidance continues to depict our
active pattern remaining in place across the region into the
weekend. There does appear to be decent agreement in the global
models/ensembles that high pressure eventually settles across
the western Lakes region by Monday-Tuesday of next week, pushing
our stalled frontal zone to the south and providing a period of
dry weather at that time.

Ratzer

&&

.AVIATION /12Z TAFS THROUGH 18Z THURSDAY/...
Issued at 719 AM CDT Wed Aug 12 2026

Key Messages:

- A complex of storms is likely to affect the terminals between
  mid-morning and mid-afternoon today. Strong to severe wind
  gusts will be possible with these storms.

- IFR/LIFR ceilings and visibilities possible at RFD this
  morning, while patchy MVFR ceilings remain possible elsewhere.
  Additional low ceilings and reduced visibilities possible
  overnight tonight into Thursday morning.

An organized thunderstorm complex is entering northern Illinois
early this morning and will approach the terminals during the
mid-late morning. Some uncertainty remains with whether these
storm complex will begin to dive southeastward along an
instability gradient or whether it will continue to progress
more towards the east and into the heart of the Chicago metro.
Current thinking is that the strongest winds with this storm
complex (50+ kts) may remain south of the terminals, but the
threat for lightning at the Chicago metro terminals remains high
enough to maintain the going TEMPO groups. Also maintained a
VCTS mention at RFD, but the latest radar trends that the storms
should remain south of RFD. Conditions are likely to dry out by
mid-afternoon.

IFR/LIFR ceilings and some visibility reductions are being
observed at/near RFD this morning, while patchy MVFR ceilings
are being observed elsewhere at press time. These conditions may
stick around for a bit this morning, but a return to prevailing
VFR conditions is favored by this afternoon. Persisting rich
low-level moisture and light winds may allow for another
episode of sub-VFR ceilings and visibilities late tonight into
tomorrow morning. The potential for this appears to be greatest
at RFD, DPA, and GYY, but based on how some forecast guidance
fared poorly with the visibility and low ceiling forecast this
morning, confidence in this occurring is only low-medium.

Ogorek

&&

.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
420 AM CDT Wed Aug 12 2026

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-130930-
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-
420 AM CDT Wed Aug 12 2026 /520 AM EDT Wed Aug 12 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...

  Elevated Thunderstorm Risk...with an associated:
     Level 1 of 5 Tornado Risk.
     Level 2 of 5 Damaging Wind Risk...up to 75 mph.
     Level 1 of 5 Large Hail Risk...up to quarter size.
  Elevated Flooding Risk.

DISCUSSION...

Isolated thunderstorms are possible early this morning, the
strongest of which may be capable of producing brief hail and
localized downpours.

A fast-moving line of strong to severe thunderstorms is then
expected to move east across the area from mid-morning into early
this afternoon. These storms will have the potential to produce
damaging winds and isolated tornadoes, in addition to producing
torrential rains which may lead to localized flooding. At this
time, the greatest risk of severe would appear to be from the I-88
corridor south and eastward.

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

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

DISCUSSION...

An active weather pattern with periodic thunderstorm chances is
expected to persist into the weekend. Some of these storms may be
strong to severe at times, as well as capable of producing heavy
rainfall which could lead to flash flooding.

.SPOTTER INFORMATION STATEMENT...

  Spotters may be needed from mid-morning into early this
  afternoon.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east-southeast at around 35 mph for early morning
  storms. If mid-morning storms form a bowing line, it may move east at
  45 to 55 mph.

LMZ740>745-130930-
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-
420 AM CDT Wed Aug 12 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...

  Elevated Thunderstorm Risk...with an associated:
   Level 1 of 5 Thunderstorm Wind Gust Risk...up to 60 knots.
   Level 1 of 5 Hail Risk...up to quarter size.

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

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

Ratzer

Visit us at www.weather.gov/chicago

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