textproduct: Chicago

KEY MESSAGES

- Scattered showers are expected today. A few isolated lightning strikes could also be observed well south of I-80.

- A prolonged period of lake effect rain showers (mixed with and possibly changing over to snow at times) is expected Thursday through early Saturday, primarily in northwest Indiana.

- Cooler temperatures are expected late this week into this weekend before another warmup occurs next week.

DISCUSSION

Issued at 333 AM CST Wed Nov 11 2026

A pronounced shortwave trough will dive southeastward into Illinois and Indiana today, bringing along a deepening surface low that will track across central portions of both states. A wedge of dry low-level air will keep the initial radar echoes that encroach upon our forecast area as virga or sprinkles, but within a few hours, sufficient top-down saturation will occur for a period of wetting rain showers to cross our CWA from west to east between the late morning and early evening. A pocket of slightly steepened mid-level lapse rates tied to the incoming vort max could result in a few lightning strikes occurring in our southernmost counties this afternoon near the surface low. If the surface low manages to take a track that is a little farther to the north than the consensus of the latest hi-res model guidance and the associated warm front creeps into our far southern CWA, then an isolated strong to severe storm could not be ruled out near and south of the US-24 corridor this afternoon. However, the much more likely outcome at this time is that the modest warm sector surface-based instability and associated marginal severe thunderstorm threat will remain to the south of our area, near and south of the I-74 corridor.

Later today, a lobe of a sprawling upper-level trough over eastern Canada will veer southward into the Upper Midwest and Great Lakes, bringing a shot of colder air into the region. While the magnitude of the colder temperatures associated with this inbound air mass will not be of the same caliber as the air mass that resulted in a notable lake effect snow event around this time last year, it will still nevertheless be cold enough to get the lake effect machine going and cause some areas to see their first snowflakes of this cool season. Before the main lake effect band becomes established though, a wave or two of lake-enhanced showers (possibly coupled with non-lake-influenced convective showers) will likely affect areas well inland from Lake Michigan on Thursday as an inverted trough axis comes crashing into the area. Rain should be the predominant precipitation type observed with this, but depending on how quickly this initial activity arrives, could not rule out a few snowflakes being observed in some locations Thursday morning. Blustery northerly winds are also expected throughout the day on Thursday and will prevent temperatures from climbing much (if at all) during the daytime.

In the meantime, the mean longwave eastern Canadian trough will be in the process of sagging southward and re-centering itself over the eastern Great Lakes. As this occurs, a secondary embedded shortwave will plunge southward from western Ontario, sending a reinforcing shot of colder air into the western Great Lakes that will only enhance the already favorable lake effect thermodynamic parameter space over Lake Michigan. The prolonged northerly fetch over the lake will favor a robust, dominant single lake effect band that should largely focus into northwest Indiana from Thursday evening all the way through Friday night/early Saturday, though occasional westward wobbles in the primarily lake convergence axis could still steer the associated precipitation towards and potentially across the Illinois- Indiana state line at times. It should also be noted that the deep lake-induced convective cloud depths and water surface-850 mb temperature differentials pushing 20C could also support a potential for waterspouts over Lake Michigan during this lake effect precipitation event.

Air temperatures will cool into the 20s and 30s Thursday night as cold air advection continues off of north-northwesterly winds. This will trend the lake effect band towards a heterogeneous composition of precipitation types, with some locations (especially those farther away from the lakeshore) possibly seeing a changeover to outright snow at some point Thursday night. However, with lake temperatures in the low-mid 50s in the lead up to this event, thermal profiles downwind of the lake look like they will remain marginal at best for wet snow accumulations to occur throughout this lake effect event. Antecedent ground temperatures will be fairly mild as well, which will make it all the more difficult for slushy accumulations to be realized on paved surfaces, including roadways. That said, given the fairly robust lake effect thermodynamics at play, hefty precipitation rates may be observed at times, and that could prove to be just enough for the less-than-favorable air and ground temperatures to be overcome in some locations in the event that temperatures Thursday night (and again Friday night) end up verifying towards the lower end of the spectrum of the latest forecast guidance. Ultimately, while an impactful wet lake effect snow does not appear to be the most likely outcome in our area at this time with this event, forecast trends will need to be monitored closely.

Cool temperatures with daytime highs largely in the 40s will persist through the weekend as the region remains entrenched in the cool air mass associated with the aforementioned mean longwave trough. However, ensemble guidance continues to favor this troughing getting replaced by high-amplitude ridging by early-mid next week. This will result in another stretch of precipitation-free conditions and a warming trend that should send temperatures back up to near to above normal readings for mid-November.

Ogorek

AVIATION /12Z TAFS THROUGH 18Z THURSDAY/

Issued at 610 AM CST Wed Nov 11 2026

Key aviation forecast messages:

- Showers expected through this afternoon with a period of MVFR VSBYs/CIGs possible around midday.

- Gusty N to NNW winds develop early Thursday AM along with the potential for lake effect showers.

Showers associated with a compact system moving across eastern Iowa are expected to continue east southeast across the area today. Have moved up the timing of the TEMPO for heavier showers with associated MVFR VSBYs and CIGs by an hour based on radar and model trends (17-21Z at ORD/MDW/DPA). The lightning threat continues to favor areas south of I-80 and the terminals but will have to monitor radar trends closely in the event that a short-fused VCTS mention becomes warranted at GYY or MDW.

Light and variable winds early this morning will gradually prevail northeast and become breezy into this afternoon as the surface low moves south and east of the terminals. Winds then turn northerly and then north northwesterly this evening, easing slightly before increasing again late tonight, with gusts in excess of 25 kt then expected during the day on Thursday.

Lake effect showers likely develop into northwest Indiana overnight and may end up near GYY at times. The lake effect band then drifts west into northeast Illinois on Thursday. Have introduced PROB30 groups for ORD/MDW Thursday morning for now due to lower confidence on the onset time, however, TEMPOs or a prevailing SHRA mention will eventually be needed with later updates once confidence in the timing increases.

Petr

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from noon today to 4 AM CST Saturday for the IL and IN nearshore waters.