textproduct: Chicago

KEY MESSAGES

- Scattered showers through mid-morning near the I-80 and I-88/290 corridors. Showers today into early this evening south of I-80, especially well south of I-80.

- Much colder, though only slightly below average temps today and Wednesday, followed by a moderating trend closer to average toward and over the weekend.

DISCUSSION

Issued at 244 AM CDT Tue May 5 2026

Cold front has cleared most of our CWA early this morning, stretching from NW IN southwest to near STL. Pre-frontal band of showers and thunderstorms has now pushed well south of the area with skies clearing out over most of the area. This clearing will be temporary as an area of over IA likely spreads eastward into northern IL early this morning. These showers are forming in a region strengthening 800-600mb frontogenesis within the entrance region of a 120kt upper level jet streak. These features are progged to quickly translate east this morning with a band of showers likely affecting an east-west corridor roughly near I-80 and I-88/290. These showers will likely quickly shift east of our CWA by late morning with generally dry conditions expected north of I-80.

Later this morning as that first northern stream jet streak zips off to the east, the another jet streak is progged to develop over the Cornbelt as northern stream trough begins to phase with the sub-tropical jet. The result will be a blossoming area of showers and thunderstorms from central Missouri east into downstate IL. The northern flanks of this precipitation does look like it will brush by our southern CWA, but quite a large spread in guidance in how far north this shower activity gets today. Continued chance pops up to about I-80, but the better rain chances look to stay closer to the Hwy 24 corridor and points south. This rain should end tonight with dry conditions expected until later this week.

Cloud cover should be pretty extensive today into tonight, especially southern CWA, which should keep temps in the 50s southern CWA today to perhaps getting close to 60 north. Overall, it looks like we'll keep a good deal of cloudiness around tonight into Wednesday, though there will likely be breaks in the overcast and some mixed sunshine Wednesday. Despite a bit more sun on Wednesday, highs should still hold in the 50s as the heart of this colder air mass settles into the area.

Opted to hold off on introducing frost into the grids Wednesday night. GFS and ECMWF soundings both have fairly saturated conditions between 500-300 mb, suggesting that high cloudiness could be extensive enough to hinder strong radiational cooling and frost development. Also, ECMWF and NAM both have the surface high centered farther south, resulting some light southerly wind. That slight gradient would likely not be much of a factor if skies end up clearer, but given the cloud cover and potential for 5kt or so of wind, the threat of frost over interior northern IL doesn't look particularly high. If guidance trends clearer, then later forecast updates will need to consider frost Wed night for areas well removed from the urban heat island(s).

A trough is progged to move across the region Friday, but with the Gulf shut down, this system won't have much moisture and subsequent instability to work with. Nevertheless, medium range guidance does suggest forcing with this wave will be sufficient for showers Friday. If current model trends continue, then pops will likely need to be boosted Friday into Friday night.

The next shortwave trough looks to be fast on the heels of Friday's with warm air advection in advance of the next wave looking to provide milder temperatures Saturday. That will be followed by another chance of showers Sat night with a frontal passage, followed by a return to cooler temps Sunday into Monday.

- Izzi

AVIATION /06Z TAFS THROUGH 12Z WEDNESDAY/

Issued at 104 AM CDT Tue May 5 2026

Key Messages:

- A period of rain showers is expected this morning.

- A lake breeze will likely cause a northeasterly wind shift at GYY this afternoon and may cause an easterly wind shift at ORD and MDW late this afternoon/early this evening.

A band of post-frontal rain showers is expected to develop/move into the area late overnight and linger for a bit past sunrise. VFR conditions are generally favored to prevail during this rain, but there is about a 30-40% chance that MVFR visibilities are observed for a brief period of time while the rain band passes over the Chicago metro terminals. Couldn't rule out an additional period of rain later today at GYY, but it still appears that the broader swath of rain in the region this afternoon will remain south of the terminals.

A lake breeze is also expected to develop this afternoon and will likely cause a northeasterly wind shift at GYY at some point. Uncertainty remains regarding when/whether it will reach ORD and MDW before washing out this evening as west-northwesterly winds around 10 kts are expected west of the lake breeze through much of this afternoon, but if it does end up reaching ORD and MDW, then an easterly wind shift will occur before light winds settle into a northerly/north-northwesterly direction a few hours later.

Ogorek

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.