Flood and Damaging Winds in Illinois, April 27, 2026

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1236 PM CDT Mon Apr 27 2026

.KEY MESSAGES...

- Strong to locally damaging non-thunderstorm wind gusts near
  and north of the Illinois and Kankakee Rivers through mid
  afternoon.

- There is a threat for severe weather later today, primarily
  between 4 and 11 PM this evening. The main threats are
  damaging winds and damaging hail.

- A transition toward a cooler and drier pattern will commence
  by midweek.

&&

.UPDATE...
Issued at 1215 PM CDT Mon Apr 27 2026

A Wind Advisory is in effect for locations near and north of the
Illinois and Kankakee Rivers until 4 PM this afternoon due to
strong wake low winds. Observed wind gusts west of the
stratiform rain with embedded thunderstorms quickly advancing
east have been in the 40-50+ mph range from the southeast, most
recently 45 kt at DBQ and FEP. Forecast soundings and high
resolution model output suggest brief localized gusts up to 60
mph are possible, which would enhance wind damage and power
outage potential if/where this occurs.

The thunderstorm driven severe wind threat with the ongoing MCS
will likely continue to focus just south of our far southern
counties over the next 1-2 hours.

It's highly uncertain to what extent and how far north air mass
recovery will occur later this afternoon into this evening
regarding any renewed severe threat later on. This will be
detailed with the full afternoon forecast issuance and
discussion once we have a better handle on trends.

Castro

&&

.DISCUSSION...
Issued at 247 AM CDT Mon Apr 27 2026

Early morning radar, satellite, and ASOS/AWOS observations
depict a developing surface low in far southwestern Kansas with
an associated broad warm frontal zone extending eastward through
central Missouri and into far western Tennessee. Broad
southwesterly flow continues to advect an EML plume across the
central Plains, which with a developing low-level jet (some
50-60kt per regional VWPs) and the approach an unbalanced
upper-level short-wavelengh trough propagating into the
Southern Plains, is supporting an expansive region of elevated
gravity wave associated convection (GWAC) across central Kansas
at press time.

Owing to continued synoptic-scale ascent facilitated by the
developing surface low and upper-level trough as well as a steady
northeasterly feed of instability through the night, the expectation
is for the ongoing elevated convection across central Kansas to
coalesce into a bore-driven mesoscale convective system over the
coming hours. The orientation of the low-level jet and instability
axis would then guide the convective system through Kansas City,
Missouri by daybreak and into central Illinois (along/south of I-80)
by mid to late morning. With instability decreasing with eastward
extent, the convective system should begin to weaken as it crosses
the I-39 corridor in central Illinois. With that said, northeasterly-
oriented deep-layer shear vectors will promote a broad stratiform
precipitation region (perhaps with embedded thunder) on the northern
side of the zone of active convection across northern
Illinois/southern Wisconsin. The severe weather threat with the
first batch of activity is currently thought to be low (call it a 5
to 15% chance at any given point) owing to meager instability. With
that said, locally damaging winds cannot be ruled out particularly
south of I-80 with any decaying convective element.

Outside the convection this morning, do have growing concern
for a period of non-thunderstorm damaging winds mainly
along/north of I-80 between noon and 3 PM along the backside of
the decaying stratiform region this afternoon. CAM guidance has
remained remarkably consistent in depicting rapid
drying/subsidence along the backside of the stratiform region,
which would support the development of a very tight pressure
gradient behind the leading meso-high (a wake-low situation). The
HRRR has long had the signal for nearly 24 hours now, including
explicit wind gust output up to 75 mph in the most recent iteration.
It's always tough to go all-in on these sorts of features given
inherent dependencies on how convection evolves and a history of
false alarms of such by CAM guidance. With that said, will give a
very hearty warning to the incoming day shift to keep an eye out for
even the slightest signs of wake low development to get lead time on
what would probably be a short- fused High Wind Warning.

Attention then turns to the threat for severe weather this
afternoon and evening. There is excellent agreement among
deterministic and ensemble guidance that the upper-level
shortwave currently propagating into the Central Plains will
lift into the Great Lakes region this afternoon, allowing the
aforementioned surface low to track from roughly Kansas City, MO
this morning to near La Crosse, Wisconsin this evening. As the
upper-level wave passes overhead, an associated 75kt 500mb speed
max will arc directly across the Mississippi River as pressure
falls in the central Great Lakes induce the rapid
intensification of the 925- to 850mb wind field. The kinematic
environment will thus become very supportive of organized
convection by this evening.

Meanwhile, increasing isentropic ascent atop the (what will be
outflow-reinforced) warm front will set the stage for festering
convection from eastern Missouri into central Illinois during
the afternoon hours. Even with increasing synoptic southwesterly
flow, a steady feed of cool outflow from the festering
convection will likely impede the northeastward progression of
the warm front, and hence, instability, into area area by early
evening. Heavy cloud cover throughout the day will also hamper
the ability for low-level lapse rates to steepen and surface
temperatures to warm, with highs only poised to make it into the
upper 60s to perhaps locally 70. Nevertheless, cooling mid-
level temperatures (by virtue of synoptic-scale ascent and the
continued advection of an EML plume via southwesterly mid-level
flow) will nevertheless support the development of elevated
instability throughout the afternoon.

Even with a lack of surface-based instability, we still do
expect the rapid development and expansion of convection across
our area this afternoon as the arrival of the upper-level
shortwave interacts with the elevated instability axis.
Accordingly, we continue to see a threat for severe weather this
afternoon and evening in our local area mainly in the form of
locally damaging winds and hail. The inability for the surface-
based instability axis to make it into our area should relegate
the (what will be a higher-end) tornado threat south of our area
across central and southern Illinois, but tracking the exact
location of the warm front will be important in case it does
sneak into our area. So, today still looks active, just not
necessarily what initially looked like a very concerning
scenario in our local area. In collaboration with SPC, the Level
3/5 threat area has been adjusted southward to be closer to the
expected position of the outflow-reinforce warm front this
afternoon.

In spite of the most likely outcome outlined above, do still
feel the need to discuss the low-probability scenario in which
the surface-based instability does make it into our area.
Should convection across central Nebraska fail to coalesce into
a bore-driven MCS (as it has struggled to do so thus far and
instead remain as GWAC), or the convective footprint into
central Illinois is minimized by early afternoon (so no outflow
to counteract southwesterly flow), the safeguard of festering
convection limiting the northeastward-movement of the warm
front would fail to materialize. In such a scenario, the warm
front would be able to lift into at least northwestern Illinois,
leading to a narrow zone where surface-based instability would
overlap with an intense kinematic environment. Should this
occur, a band of supercells with a threat for all hazards
including tornadoes would materialize near the Mississippi River
with upscale growth into an intense squall line thereafter.
Would have to think that instability would still decrease with
eastward extent toward Lake Michigan... though there would still
a more pronounced threat for an all-hazard severe weather
event. Most guidance does not support this outcome at this point
in time, though it does remain completely realistic provided a
lot of dominoes fall in the wrong direction.

Tuesday onward:

The upper-level pattern will undergo a substantial pattern
shift through the rest of the week with predominant aggregate
troughing along the US/Canadian border. As a result, the
expectation is for cooler and drier conditions to prevail with
highs in the 60s and overnight lows in the upper 30s to lower
40s. We can easily envision a threat for frost by the weekend.
Thereafter, long-range ensembles advertise warming trend during
the first week of May.

Borchardt

&&

.AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/...
Issued at 1236 PM CDT Mon Apr 27 2026

Key Aviation Messages for the current TAF period:

- Strong and gusty winds through most of the TAF period.
  Strongest winds out of the southeast this afternoon in excess
  of 40 knots.

- Another round of showers and storms possible this evening.

A line of showers and storms is currently moving east-northeast
over Illinois. This line is expected to be out over Lake
Michigan after 1830Z, with a period of dry VFR conditions after
the rain ends.

More notably, in the wake of the area of convection, a small
scale low pressure area will increase winds out of the
southeast this afternoon. Strongest wind gusts in the TAF were
listed at 40 kt, but obs to the west have already recorded
around 45 kt.

Winds will turn to the south as the low moves east later
tonight. Models have consistently shown another line of showers
and storms that could develop this evening. With lower
confidence in development behind the low, KRFD was converted
down to a PROB30. However, the TEMPO for the Chicago terminals
were adjusted for timing, with chances for it to linger around
KMDW and KGYY for longer.

As the threat for showers and storms diminish, drier conditions
are expected overnight, but there is a chance for lingering MVFR
cigs. Winds will turn to the southwest and eventually west with
gusts 20 to 25 knots. Breezy conditions are expected to remain
through much of Tuesday, becoming VFR in the afternoon. Models
are hinting at a weak lake breeze development tomorrow
afternoon, but not with enough confidence to introduce it into
the TAF presently.

DK

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Wind Advisory until 4 PM CDT this afternoon for ILZ003-ILZ004-
     ILZ005-ILZ006-ILZ008-ILZ010-ILZ011-ILZ012-ILZ013-ILZ019-
     ILZ020-ILZ021-ILZ023-ILZ103-ILZ104-ILZ105-ILZ106-ILZ107-
     ILZ108.

IN...Wind Advisory until 4 PM CDT this afternoon for INZ001-INZ002.

LM...Gale Warning until 7 PM CDT this evening for the IL and IN
     nearshore waters.

     Small Craft Advisory from 7 PM this evening to 10 AM CDT
     Tuesday for the IL and IN nearshore waters.

&&

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
316 AM CDT Mon Apr 27 2026

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-280830-
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-
316 AM CDT Mon Apr 27 2026 /416 AM EDT Mon Apr 27 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 3 of 5 Damaging Wind Risk...up to 70 mph.
     Level 2 of 5 Tornado Risk mainly south of Interstate 80.
     Level 1 of 5 Large Hail Risk...up to quarter size.

  Elevated Flooding Risk.

DISCUSSION...

Severe thunderstorms are possible this morning through this
evening. The greatest risk for severe thunderstorms is mainly
south of Interstate 80. The highest tornado risk is mainly
south of the Kankakee and Illinois River Valleys.

Heavy rainfall may lead to localized flooding and possible
renewed rises on area rivers.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

Friday Night into Saturday morning...
  Limited Frost Risk.

.SPOTTER INFORMATION STATEMENT...

Spotters will likely be needed today and this evening.

GENERAL STORM MOTION OF THE DAY:

Moving toward the east at 40 to 50 mph.

LMZ740>745-280830-
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-
316 AM CDT Mon Apr 27 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...

  Hazardous conditions for small craft.
  Limited Thunderstorm Risk...with an associated:
     Level 2 of 5 Thunderstorm Wind Gust Risk...up to 50 knots.
     Level 1 of 5 Hail Risk...up to quarter size.

.DAYS TWO THROUGH SEVEN...Tuesday through Sunday.

No hazardous weather is expected at this time.

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