High Surf in Illinois, April 16, 2026

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
655 AM CDT Thu Apr 16 2026

.KEY MESSAGES...

- We'll be monitoring for fog development this evening into
  early Friday, with the highest chance for dense fog near the
  lake.

- Following an unseasonably warm and breezy Friday afternoon,
  yet another severe weather threat is anticipated late day
  Friday into Friday evening, especially near and west of I-39.
  There will also be an areawide threat for flash flooding.

- Cooler air arrives into early next week and may bring
  frost/freeze concerns early Sunday and Monday mornings.

&&

.DISCUSSION...
Issued at 340 AM CDT Thu Apr 16 2026

Through Friday Night:

The pattern regionally will be much quieter today through Friday
morning, giving a brief break to dry out ahead of the final
storm system in the very active/stormy pattern over the past
week. As the main mid-upper trough axis over the mid and upper
MS Valley early this morning shifts over the central Great Lakes
by early to mid afternoon, expect widely scattered showers and
perhaps a few thunderstorms to focus over the northeast 1/3 to
1/2 or so of the CWA. A lake breeze slowly pushing inland may
provide additional focus to modestly enhance coverage near the
lake. Precipitable waters merely a bit above average for mid
April (vs. prior summer-like values this week) will still be
sufficient for localized downpours with any deeper cores, but
limited coverage will keep flooding potential below 5%. Highs
will range from the upper 60s to mid 70s (warmest south of I-80)
for most today, though note that temps will fall into the 50s
near the lake this afternoon.

Attention for late today through tonight will turn to pre-
existing fog and very low stratus over southern Lake Michigan.
Onshore winds (and cooling temps/decreasing T/Td spreads) in
the wake of the lake breeze passage this afternoon could enable
patchy fog to bleed inland near the shoreline prior to sunset
this evening. Weak surface high pressure building over the lake
will be conducive for a period of good radiational cooling and
associated fog development and/or lakeshore fog slowly oozing
inland. While confidence isn't all that high in the fog
potential with inland extent, there's been enough of a signal
for patchy fog mention over the northeast half of the CWA. Where
confidence is higher, near the lake in northeast Illinois and
far northwest Indiana, indicated areas of fog. Strengthening
southerly flow aloft in the pre-dawn hours of Friday may help
erode some of the fog, with the possible exception of near the
Illinois shore.

Friday will be the last summer-like warm and breezy day of this
stretch, out ahead of an elongated surface low lifting towards
western Lake Superior. Expect highs in the low to locally mid
80s away from the far northeast IL shore (which will reach the
70s towards evening), and southerly winds gusting up to 30-35
mph. Lingering lower dew points from underneath the departing
surface high, south-southeast winds to offset surface moisture
return with eastward extent, and parched mid-levels will yield
a fairly sharp instability gradient from the MS River to the
longitude of Lake Michigan by mid Friday afternoon.

The big question marks for the severe setup are with respect to
timing of convective initiation near the MS River and storm mode
upon approach into northwest and north central Illinois late
Friday afternoon or early Friday evening. Deep layer shear will
once again be supportive of severe weather. If CI occurs
earlier (by ~3-5pm) and sufficiently close to our far western
CWA counties, there may be a play for initial discrete/semi-
discrete supercells with an attendant damaging hail threat. Even
in northwest IL, fairly large T/Td spreads may serve to limit
an appreciable supercellular tornado threat.

After possible initial discrete mode, there should be a quick
transition to linear/QLCS as cold frontal forcing takes hold.
The CI timing question is thus also important in terms of far
east a damaging wind (and embedded QLCS tornado threat) will
reach Friday evening until the threat wanes overnight Friday
night. The drier profiles into the Chicago metro and points
south will be deeply mixed with inverted V characteristics at
the low levels. So even if storms get going early and transition
to a weakening squall line outracing the instability gradient,
an earlier arrival would still entail a severe wind threat with
outflow dominant segments. All in all, SPC's initial day 2
outlook appears reasonable for the setup, with a 3/5 threat
level (enhanced risk) near/west of I-39 tapering to a 1/5 threat
level (marginal risk) near/east of I-57.

A very progressive MCS/QLCS Friday night would limit the
residence time of torrential rainfall and the associated
flooding threat. However, with the very rainy stretch of late
for much of the area and already elevated river levels, it won't
take much for renewed rises and localized flash flooding.
Backbuilding of convection behind initial outflow boundary, on
the other hand, would correspondingly increase the flood threat,
especially for hardest hit areas this week and/or urban areas.

Saturday through Wednesday:

The most recent model guidance for Saturday has pretty
unanimously sped up the timing of the cold frontal passage to
yield a mostly dry day for much of if not the entire area. Best
chance (30-50%) for any lingering showers past sunrise will be
for areas southeast of I-55. Saturday afternoon will trend to
partly cloudy to mostly sunny, albeit notably cooler (50s) with
blustery west-northwest winds. Cold air advection and dry
advection through Saturday night-early Sunday will set the stage
for lows in the 30s, possibly flirting with freezing across
parts of interior northern IL. Following a cool and breezy
Sunday, ~1030 mb high pressure over the region will likely be
conducive to at least areas of frost as temps dip to the low-mid
30s outside of Chicago, if not colder in spots. Agricultural
interests are advised to monitor forecasts through early next
week.

Cooler conditions into Monday morning will be short-lived
however, with extended guidance favoring a return to high
amplitude ridging across the central CONUS and unseasonably warm
temperatures returning to the region by midweek next week.
Despite the returning warmth next week away from Lake Michigan,
a blocky mid-upper pattern will favor much less stormy
conditions over the local area, giving a longer period to dry
out from this week's rain.

Castro

&&

.AVIATION /12Z TAFS THROUGH 18Z FRIDAY/...
Issued at 655 AM CDT Thu Apr 16 2026

Main Concerns:

- Scattered SHRA through early to mid afternoon, with a few TS
  possible.

- Lake breeze wind shift timing this afternoon.

- Potential for low flight categories near the lake tonight into
  early Friday morning.

Additional scattered SHRA should focus over northeast IL and
northwest Indiana late this morning through early to mid
afternoon. Can't rule out an isolated TS during this time (~20%
chance), which will be assessed for subsequent updates. Current
MVFR CIGs should scatter out sometime this afternoon.

The next item of interest is a lake breeze shifting inland this
afternoon. Timing is uncertain based on the most recent
guidance, and west-northwest winds can be expected until its
passage. Opted to hint at the possibility of a later timing at
ORD and/or the airport being bisected by the boundary with
temporary WNW winds.

Finally, there's a pretty strong signal for fog and very low
stratus to ooze in off the lake this evening. Have the highest
confidence at GYY, but ORD and MDW certainly have the potential
to be impacted as well until strengthening southeast winds
develop Friday morning.

Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Dense Fog Advisory until 9 AM CDT this morning for Winthrop
     Harbor to Wilmette Harbor IL.

&&

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Hazardous Weather Outlook
National Weather Service Chicago/Romeoville IL
1134 AM CDT Thu Apr 16 2026

ILZ003>006-008-010>013-019>021-023-032-033-039-103>108-INZ001-002-
010-011-019-171645-
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-
1134 AM CDT Thu Apr 16 2026 /1234 PM EDT Thu Apr 16 2026/

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

.DAY ONE...This Afternoon and Tonight.

Weather hazards expected...

  Limited Thunderstorm Risk.
  Limited Fog Risk.

DISCUSSION...

An isolated thunderstorm or two is possible through early to
mid afternoon. No severe weather is expected.

Some dense fog is possible tonight, particularly closer to Lake
Michigan.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Friday...
  Elevated Thunderstorm Risk...with an associated:
     Level 3 of 5 Severe Thunderstorm Risk.
  Elevated Flooding Risk.
Late Saturday night into Sunday morning...
  Elevated Frost and Freeze Risk.

DISCUSSION...

A line of thunderstorms is expected to move across the area late
Friday afternoon through Friday night, some of which could be
severe, especially near and west of Interstate-39.

.SPOTTER INFORMATION STATEMENT...

  Spotter activation will not be necessary through tonight.

GENERAL STORM MOTION OF THE DAY:

  Moving toward the east at 25 mph.

LMZ740>745-171645-
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-
1134 AM CDT Thu Apr 16 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...This Afternoon and Tonight.

Weather hazards expected...

  Limited Thunderstorm Risk.
  Elevated Fog Risk.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

Friday...
  Hazardous conditions for small craft are likely.
  Elevated Thunderstorm Risk...with an associated:
     Level 1 of 5 Severe Thunderstorm Risk.
  Limited Fog Risk.
Saturday through Tuesday...
  Hazardous conditions for small craft are likely.

- Izzi

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