WXSIM Forecast for: O'Fallon
Issued by: OFallonweather.com
Updated: Saturday, 21-Mar-2026 04:00 pm
 
Tonight

Mostly Clear
Mostly Clear
Sunday

Mostly Cloudy
Mostly Cloudy
Sunday
night
Partly Cloudy
Partly Cloudy
Monday

Partly Cloudy
Partly Cloudy
Monday
night
Mostly Cloudy
Mostly Cloudy
Tuesday

Partly Cloudy
Partly Cloudy
Tuesday
night
Partly Cloudy
Partly Cloudy
Wednesday

Partly Cloudy
Partly Cloudy
Low: 56° High: 87° Low: 40° High: 59° Low: 37° High: 66° Low: 40° High: 76°
               
SSE→SSW
6-21
mph
SSW→NNW
8-30
mph
NNW
11-30
mph
NNE
7-27
mph
ENE
5-20
mph
ESE
6-25
mph
SSE
6-20
mph
SSW
7-25
mph
UV: 5
 Medium 
UV: 5
 Medium 
UV: 5
 Medium 
UV: 6
 High 

 

Tonight
 
Clear in the evening, becoming mostly clear after midnight. Relatively mild. Low 56°. Wind south-southeast around 6 mph, gusting to 21 mph, in the evening, becoming south-southwest after midnight.
Sunday
 
Partly to mostly sunny in the morning, becoming partly to mostly cloudy in the afternoon. Exceptionally hot. High 87°. UV index up to 5. Wind south-southwest around 8 mph, gusting to 30 mph, in the morning, becoming north-northwest in the afternoon.
Sunday night
 
Fair to partly cloudy in the evening, becoming partly cloudy after midnight. Low 40°. Wind north-northwest around 11 mph, gusting to 30 mph.
Monday
 
Partly cloudy. High 59°. UV index up to 5. Wind north-northeast around 7 mph, gusting to 27 mph.
Monday night
 
Partly to mostly cloudy. Low 37°. Wind east-northeast around 5 mph, gusting to 20 mph.
Tuesday
 
Sunny in the morning, becoming partly cloudy in the afternoon. Mild. High 66°. UV index up to 5. Wind east-southeast around 6 mph, gusting to 25 mph.
Tuesday night
 
Fair to partly cloudy in the evening, becoming partly cloudy after midnight. Low 40°. Wind south-southeast around 6 mph, gusting to 20 mph.
Wednesday
 
Partly cloudy. Unusually warm. High 76°. UV index up to 6. Wind south-southwest around 7 mph, gusting to 25 mph.
Wednesday night
 
Partly to mostly cloudy in the evening, becoming mostly clear after midnight. Low 47°. Wind south-southwest around 5 mph, gusting to 17 mph.
Thursday
 
Sunny in the morning, becoming partly to mostly sunny in the afternoon. Near record warmth. High 90°. UV index up to 7. Wind south-southwest around 7 mph, gusting to 24 mph, in the morning, becoming 12 mph, gusting to 29 mph, in the afternoon.
Thursday night
 
Partly cloudy in the evening, becoming dense overcast after midnight. Patchy light fog after midnight. A slight chance of rain in the evening, then rain likely after midnight. Scattered thundershowers possible. Low 49°. Wind north around 8 mph, gusting to 18 mph, in the evening, becoming 11 mph, gusting to 22 mph, after midnight. Chance of precipitation 70 percent. Precipitation mostly around half an inch.
Friday
 
Mostly cloudy to cloudy in the morning, becoming partly cloudy in the afternoon. Patchy light fog in the morning. Rain likely in the morning, then a slight chance of rain in the afternoon. High 54°. UV index up to 5. Wind northeast around 10 mph, gusting to 28 mph. Chance of precipitation 70 percent. Precipitation mostly between half an inch and one inch.
Friday night
 
Clear in the evening, becoming mostly clear after midnight. Low 38°. Wind north-northwest around 4 mph, gusting to 24 mph.

WXSIM forecast formatting script by Saratoga-Weather.org.

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