textproduct: Chicago

KEY MESSAGES

- Unseasonably mild spring-like temperatures continue through Saturday.

- Much cooler weather returns with a cold front on Sunday, and persists into early next week.

DISCUSSION

Issued at 147 PM CDT Fri Mar 20 2026

A cold front continues to work southward across the area early this afternoon. However, with the lack of a real cold push in its wake, conditions remain unseasonably mild (mid 60s to the 70s) area-wide early this afternoon. However, this will change for areas near Lake Michigan later this afternoon as the low- level flow flips onshore with a lake breeze. As this occurs after 2 or 3 this afternoon, temperatures will quickly drop off into the 40s and 50s, coldest along and near the lakeshore.

On Saturday, another cold front and area of low pressure will shift southward into the Upper Midwest following the quick eastward passage of a surface high across the Great Lakes tonight. This front will be moving across our area into early Sunday morning. However, prior to its arrival, a quick return to breezy and unseasonably mild southwesterly winds across our area for Saturday. Accordingly, Saturday is by far looking to be the best day of the weekend as temperatures top out in the 70s under mainly sunny skies. Also, the offshore flow will also curtail lake breeze development, thus resulting in the mild conditions extending all the way to the lakeshore.

Conditions will remain mild Saturday night in advance of the approaching cold front. However, conditions will quickly turn chilly following the cold frontal passage early Sunday morning as breezy north-northeasterly winds develop. This will especially be the case for areas north of the Illinois and Kankakee Rivers, where high temperatures for the day are expected to occur in the morning, with temperatures quickly falling into the lower 40s near the lake, and into the mid 40s and 50s across interior parts of northern IL. These colder temperatures are also likely to be accompanied by gusty winds up to 30 mph, a lower level deck of stratus and possibly a period or two of light rain.

Broader WNW flow across the northern CONUS will take hold across much of next week, with a continued series of waves crossing somewhere across the Great Lakes region. Cooler but seasonable conditions will persist Monday as high pressure passes across the area. However, a compact wave removed from the stronger WNW flow aloft is progged to pass just southwest of the area on Tuesday and may brush the area with some cloud cover and maybe even a few widely scattered showers. Otherwise, dry conditions and warming temperatures are anticipated into the midweek period ahead of the next approaching cold front. This cold front looks to move across our area sometime around Thursday of next week, with a period of cooler weather to follow for the end of the week.

KJB

AVIATION /00Z TAFS THROUGH 06Z SUNDAY/

Issued at 637 PM CDT Fri Mar 20 2026

Key aviation messages for the 00Z TAFs:

- Northeast winds becoming light east/variable overnight and southeast Saturday morning. Winds then become breezy southwest by early Saturday afternoon.

- Low (<25%) probability for some fog/low stratus to form near Lake Michigan toward/around sunrise Saturday.

Lake breeze has pushed inland through all of the Chicago metro terminals at issuance time, with surface winds to maintain an easterly component into the overnight hours. The pressure gradient is expected to weaken with time as surface high pressure passes north of the forecast area, which may support light/variable (but mainly east) winds by the predawn hours. Winds will then turn southeasterly Saturday morning as the surface high moves off to the east and a warm front approaches from the west. The warm front is forecast to pass through the area midday, with winds then turning southwest and becoming breezy in the afternoon. Gust in the low-mid 20 kt range can be expected by mid-afternoon with a few higher gusts possible late. Winds will ease with sunset, though may remain a bit breezy into early evening with warm temperatures helping to maintain a mixed boundary layer. This should mitigate a brief LLWS potential early in the evening, when a low-level jet of 35-40 kts briefly transits the area above 1500-2000 ft AGL.

Other than a relatively thick VFR high cloud (FL200-250) layer tonight, the only thing of note is the potential for some light fog or patchy low stratus toward sunrise as depicted in some of the available model guidance (most notably the RAP, and the often too-moist NAM-based runs). Thick high cloud cover may help prevent this, but given light east surface winds and a slightly more moist marine layer air mass off of the lake, did hint at this potential with a 6SM BR FEW015 for the Chicago terminals. If any BR/stratus were to develop, it would likely mix out by mid-morning.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

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