textproduct: Chicago

KEY MESSAGES

- Showers likely late Friday into Friday night.

- Return to somewhat above average temperatures expected next week, though cooler temperatures expected near the lake.

DISCUSSION

Issued at 202 PM CDT Wed May 20 2026

High pressure over Lake Superior providing for northerly winds and a steady feed of rather chilly temperatures, especially near and just downwind of Lake Michigan. Thursday looks to be pretty similar to today, though with more mid-high level cloudiness northern CWA and with slightly less wind. While the winds won't be quite as gusty, they'll remain off the lake and keep temps in the 50s again near the lake. Southern CWA, where there should be a bit more sunshine, should see temps several degrees warmer than today, probably making a run at 70 degrees.

Shortwave trough over northern Mexico is progged to lift northeastward into the mid-Mississippi to lower Ohio valley regions Friday into Friday night. Associated sfc low is progged to track from central IL northeast into lower Michigan Friday night with the warm sector expected to remain south of most, if not all, of our CWA. Rain associated with this system is progged to begin spreading into our southern CWA during the afternoon, spreading north to the WI border early Friday evening. As mentioned in the previous AFD, instability is expected to remain well south of our CWA with seemingly very little threat of thunderstorms, so we're looking for primarily just a chilly rain.

Given the currently progged track of the sfc low, it is possible that our extreme southeast CWA could briefly poke into the warm sector Friday evening. Despite this, GFS and ECMWF forecast soundings exhibit little or no CAPE due to the rather cool low levels and warmer than average mid-level temps (-8 to -10C at 500mb). Opted to only retain the slight chance of thunderstorms Friday evening over the far southeast corner of our CWA and even there the chances appear meager.

Rain should end by or shortly after sunrise Saturday, but lingering cloud cover will likely keep temps from getting too warm. Winds off the lake will again keep it chilly near the lake. Fast on the heels of this southern stream shortwave will be a northern stream shortwave wave by Sunday. Warm air advection ahead of this wave should result in warmer temps Sunday, possibly right up to the lakefront. NBM pops were pretty low Sunday and didn't make any changes at this time, given some model variability in handling of this system as well as the progged weaker forcing and questionable moisture. This will be something we'll have to look at in coming days to see how models trend with that wave.

Medium range guidance is in pretty good agreement on a large scale pattern change over North America next week. Both EPS and GEFS develop high amplitude, high latitude upper ridging over central Canada south into the north central U.S. with a developing omega block pattern. Medium range guidance can struggle at times in dealing with blocking patterns, but the signal has been and continues to be pretty strong. This would likely result in warmer than average temperatures and likely drier than normal conditions locally. However, the northern displacement of the upper ridge would probably mean a surface high to our north as well, which would open to the door to winds off Lake Michigan and the typical spring/early summer "cooler near the lake" forecast for much of next week.

- Izzi

AVIATION /06Z TAFS THROUGH 12Z FRIDAY/

Issued at 1254 AM CDT Thu May 21 2026

Forecast concerns for the terminals...

- Breezy east-northeast winds Thursday afternoon with gusts around 25 kts.

- Low chance (10-15%) for a few sprinkles/light showers Friday morning with better rain chances Friday afternoon/evening.

A broad area of high pressure over the Great Lakes will keep tranquil but cooler weather conditions in play through the TAF period. Light northeast winds around 5-7 kts will prevail tonight before speeds increase Thursday morning. A period of east- northeasterly winds gusts around 25 kts is expected Thursday afternoon. Gusts will taper Thursday evening with easterly 7-10 kt winds expected Thursday night into Friday.

As for clouds, FEW to SCT VFR skies are expected through the majority of the TAF period. However, recent satellite trends do show some 2500-3000 ft clouds developing in central IN and southern Lower MI. While confidence on how widespread these MVFR clouds become is still somewhat low, there is chance for a period of MVFR ceilings to materialize at the Chicago area terminals around daybreak and linger through at least the mid- morning hours Thursday. For now have opted to maintain a SCT020 mention in the TAFs for this potential and will tactically update as trends warrant.

Finally, there is a low chance (10-15%) for some sprinkles or light showers to develop Friday morning ahead of an approaching disturbance. While forecast soundings do show some decent moisture to be present, forcing is rather modest so confidence in coverage of any showers to too low for any formal mention yet. That said, better rain chances are expected Friday afternoon and evening as the aforementioned disturbance lifts through the region but arrival time looks to be after 18z Friday at this point.

Yack

LOT WATCHES/WARNINGS/ADVISORIES

IL...Beach Hazards Statement through this afternoon for ILZ006- ILZ103-ILZ104.

IN...Beach Hazards Statement until 10 AM CDT this morning for INZ001.

Beach Hazards Statement until 7 AM CDT this morning for INZ002.

LM...Small Craft Advisory until 1 PM CDT this afternoon for the IL nearshore waters.

Small Craft Advisory until 7 AM CDT this morning for the IN nearshore waters.