O'Fallon, IL Monthly Outlook

The first 14 days are a live model forecast. The back half of the month shows the typical range for those dates — useful for planning, not a literal prediction.

First 14 days: a real forecast, blended from NBM, ECMWF and GFS runs. Trust it most inside three days; by the second week treat it as a lean, not a promise.

Rest of the month: past day 14 we stop pretending to forecast and switch to the climate corridor — the range those dates have produced over three decades computed directly for O'Fallon, with mild week-to-week wiggle.

Good question for this page: "Is late August usually humid in Chicago?" Bad question: "Do I need an umbrella on the 29th?" — three weeks out no one knows, and that's the honest answer.