CHICAGO, Illinois — If you live in northern Illinois, this weekend is not the time to have outdoor plans you cannot cancel. Multiple rounds of torrential rainfall are targeting northern Illinois from Saturday night through Sunday evening, September 20, 2026, with total rainfall amounts that could easily reach 2 to 4 inches across the region and isolated spots picking up 4 to 6 plus inches by Sunday night. Flash flooding is likely, especially in urban areas, and anyone living near a river, stream or low-lying flood-prone area needs to be prepared for rapid water rises beginning as early as 1:00 AM to 4:00 AM Saturday night. The rainfall map valid through Monday, September 21, 2026 shows the most extreme totals of 5 to 6.8 inches concentrated in a bull’s-eye directly over northern Illinois.
The Two Rainfall Rounds: When to Expect the Worst
Understanding the timing of both rounds will help you stay safe and make smart decisions this weekend across northern Illinois:
Round 1 — Saturday Night Starting 1:00 AM to 4:00 AM:
The first round of torrential rainfall begins between 1:00 AM and 4:00 AM Saturday night and continues through lunchtime Saturday. After lunchtime on Saturday, the rainfall eases up with scattered showers and thunderstorms continuing off and on through Saturday night with some dry hours mixed in. If you have morning outdoor activities planned for Saturday, expect heavy downpours to be a factor from the early morning hours onward.
Round 2 — Late Saturday Night 10:00 PM and After:
The second and potentially more damaging round begins late Saturday night at 10:00 PM or later and continues through the overnight hours and throughout Sunday morning. The heaviest rain and thunderstorms from this second round will move east by lunchtime Sunday, but additional showers and storms will continue developing and moving through Sunday afternoon into Sunday evening. Anyone with outdoor plans for Sunday in northern Illinois should be prepared for heavy downpours at any point throughout the entire day.
Rainfall Totals Across Northern Illinois Through Sunday Night
The model rainfall projection valid through Monday, September 21, 2026 shows the following totals across the region:
- The highest totals on the map are concentrated in the deep orange and red bull’s-eye zone over northern Illinois, where values of 5.2 to 6.8 inches are visible — well above the 4 to 6 plus inch threshold identified in the source data for isolated spots by Sunday night.
- Values of 5.7 to 6.0 inches are visible across multiple grid points within the core rainfall zone over northern Illinois through Sunday night.
- Totals of 4.2 to 5.0 inches cover a broad surrounding area of northern Illinois beyond the immediate core through Sunday night, September 20, 2026.
- The broader northern Illinois region shows widespread totals of 2.1 to 3.7 inches across a large footprint through Sunday night, confirming the source data’s forecast of 2 to 4 inches widespread across the region.
- Lighter totals of 1.0 to 1.4 inches are visible on the eastern edges of the rainfall map through Monday, September 21, 2026, with the heaviest rainfall clearly concentrated over the northern Illinois corridor.
Flash Flooding Likely Sunday Especially in Urban Areas
The source data is direct — flash flooding is likely Sunday across northern Illinois, especially in urban areas. Urban flooding happens when rainfall rates exceed what storm drain systems can handle, sending water rushing through streets, flooding underpasses, basements and low-lying areas faster than it can drain away. With 2 to 4 inches of rain falling across northern Illinois over a 24 to 36 hour period on top of any moisture remaining in the ground from earlier rainfall, urban drainage systems will be overwhelmed in many communities by Sunday morning and Sunday afternoon. If you live near the Chicago metro area or any other urbanized portion of northern Illinois, avoid driving through any standing water on roads at any point this weekend.
Critical Warning for River and Stream Residents
The source data specifically warns residents living near rivers, streams or areas prone to rapid water rises to be prepared. When 4 to 6 plus inches of rain falls over a region in a 36 hour window, river levels can rise dramatically and quickly — sometimes rising several feet in just a few hours during the most intense rainfall phases. If your home is in a floodplain or near any waterway in northern Illinois, do not wait for the water to start rising before acting. Move valuables to upper floors now and know your evacuation route before Saturday night’s first round begins between 1:00 AM and 4:00 AM.
