textproduct: Chicago

KEY MESSAGES

- Dense fog is possible tonight, especially closer to Lake Michigan.

- Another round of showers and thunderstorms is expected later Friday afternoon into Friday night. Some of the storms could be severe. Locally heavy rainfall could result in flooding issues.

- Windy and unseasonably warm weather Friday will be replaced by much colder weather over the weekend into early next week.

- Frost/freeze conditions are expected many areas Sunday and especially Monday mornings.

UPDATE

Issued at 824 PM CDT Thu Apr 16 2026

Evening surface analyses depict a surface pressure col centered over Lake Michigan between a departing surface low pressure system in the northeastern United States and a new low pressure system developing in the central Plains. The resulting nearly calm pressure gradient has allowed for a marine "bubble" high to develop, leading to onshore winds around the perimeter of southern Lake Michigan. The resulting inland bleed of relatively cool air into the warmer and more moist continental airmass is leading to the development of dense fog along the Lake Michigan shoreline, as confirmed by numerous webcams and GOES-19 nighttime microphysics imagery. At this point, fog is densest across portions of Lake (IL), southern Cook, and Lake (IN) counties, where a Dense Fog Advisory is already in effect.

Going forward, the rapidly cooling boundary layer should allow for the marine airmass to accelerate further inland. As a result, the expectation is for dense fog to rapidly expand inland through the overnight hours. As is typical in these sorts of regimes, determining exactly how far inland fog will spread is an item of low confidence. Nevertheless, gridded visibility guidance is ubiquitous in showing dense fog making it as far south as the Kankakee River Valley and west as the Fox River Valley. Will adopt the usual approach of tacking on counties to the Dense Fog Advisory as observations dictate through the overnight hours.

Taking a quick look at the forecast for tomorrow, am not seeing much different than what is outlined in the Short Term Discussion below. Do continue to highlight the heightened threat for flash flooding and a worsening of river flooding north of Interstate 88 and especially near the Wisconsin state line where repeated rounds of thunderstorms have left soils saturated and many rivers are already in Minor to Moderate flood stage (Rock, Fox, and Des Plaines Rivers in particular).

Updated products will be sent shortly.

Borchardt

DISCUSSION

Issued at 251 PM CDT Thu Apr 16 2026

Through Friday night...

Surface ridge will move across the region tonight. Relatively moist low level air mass remains in place, so as skies clear out and winds diminish, areas of dense fog could develop tonight. The threat appears greatest near and downwind from Lake Michigan as marine chilled air mass bleeds inland into higher dewpoint air mass over land. May very well need a dense fog advisory tonight, but will punt that decision to the evening shift who will benefit from observational trends and be able to better define the areas of greatest dense fog threat.

Friday will be the last summer-like warm and windy 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.

Significant uncertainties remain regarding severe weather threat in our CWA late Friday and Friday night. A number of high resolutions models are developing open warm sector convection across western IL by mid afternoon Friday. It isn't clear what is forcing this convection, which given the stout EML and potential for a cap, raises doubts about the veracity of the model solutions depicting this. Are these models too aggressive in eroding convective inhibition in light of the strong EML? It really isn't clear at this point, leading to lower forecast confidence in the afternoon.

Should this convection develop in the open warm sector during the afternoon, then storm mode would likely be supercellular given the long hodographs. Primary threat from these storms (if they materialize) would likely be large to very large hail. Cannot rule out a tornado threat, however low level shear looks fairly marginal. In addition, it isn't clear how much dewpoints will mix out and even modest mixing out of low level moisture would lead to larger T/Td spreads and a more muted tornado risk.

Regardless of whether this open warm sector convection materializes, an extensive frontal squall line is likely to develop west of the Mississippi River along the approaching cold front. Given the parameter space this convection will develop within, severe weather is likely, especially damaging winds. This QLCS should cross the MS River and move into our western CWA early-mid evening Friday. This line will likely still pose a threat of damaging winds and potentially line embedded tornadoes given the strengthening low level shear during the evening.

Lower confidence in how quickly the convection will decrease in intensity as it moves east across our CWA into less unstable and more capped air mass. The latest SPC SWODY2 nicely depicts the likely west to east gradient in severe weather threat.

There is a pretty strong signal in guidance in a southern stream shortwave slowing the southward progression of the extensive QLCS across central IL Friday night. In fact, there could be a reinvigoration of convection capable of producing heavy rainfall in association with this wave. The southern portions of our CWA seem most susceptible to a more prolonged period of heavy rainfall Friday night.

Soils are saturated and streamflow much above average over much of the region, leaving our CWA susceptible to both flash flooding and river flooding from heavy rainfall. Our WI border counties and the immediate Chicago area are most vulnerable to flooding, and should heavy rain fall in these areas, then a more significant flood threat would exist. Have issued an ESF to highlight this threat for now, but once we are better able to define the areas most likely to potentially see heavy rainfall a flood watch will likely be needed.

- Izzi

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 /06Z TAFS THROUGH 12Z SATURDAY/

Issued at 1226 AM CDT Fri Apr 17 2026

The key aviation weather messages are:

- LIFR/VLIFR cigs and vsbys through early morning, mainly at ORD, MDW, GYY, and possibly DPA. Conditions will improve rapidly after 12-13z.

- SE winds turning S to SSW this afternoon, with some occasional gusts greater than 30 knots possible.

- Line of thunderstorms--some severe--expected to impact all terminals this evening.

- Cold front arrives overnight with a breezy northwest wind shift and MVFR/brief IFR cigs.

An area of low stratus and fog continues to ooze westward off the lake. Over the past hour, cigs and vsbys have deteriorated at ORD/MDW, and expect that trend to continue overnight, with 200-300 foot cigs/VVs prevailing. With RVRs dropping, felt comfortable with continued TEMPO groups for 1/4SM FG through the night. Trends at DPA are more uncertain, with moist easterly trajectories evidently being blocked by high-rise structures in Chicago. It's unclear how cig/vsby trends will evolve as a result, but overall expectation is for eventual LIFR/VLIFR cigs and vsbys to develop overnight. Have left RFD VFR at this time given increasing low-level flow overnight and moist trajectories turning more northwesterly with time (into Wisconsin).

Conditions will rapidly improve in the 12-14z timeframe as SE breezes develop. Winds will eventually turn to a 180-190 direction during the afternoon with increasing gusts. There will be a potential for some intermittent 35 kt gusts towards mid afternoon, particularly INVOF RFD. Thereafter, a line of thunderstorms is expected to roll east across the region. Confidence in overall storm timing is moderate-high. Some storms will be severe.

After storms clear, lingering showers and embedded TS will be possible through the late evening. A cold front will eventually shut any precipitation chances off with a gusty NW wind shift and developing MVFR cigs overnight into Saturday morning.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES

IL...Dense Fog Advisory until 10 AM CDT this morning for ILZ005- ILZ006-ILZ103-ILZ104-ILZ105-ILZ106-ILZ108.

IN...Dense Fog Advisory until 10 AM CDT this morning for INZ001- INZ002.

LM...Dense Fog Advisory until 7 AM CDT this morning for the IL and IN nearshore waters.