WXSIM Forecast for: O'Fallon
Issued by: OFallonweather.com
Updated: Wednesday, 02-Sep-2026 05:00 am
 
Today

Mostly Sunny
Hot!
Mostly Sunny
Hot!
Tonight

Clear
Clear
Thursday

Sunny
Hot!
Sunny
Hot!
Thursday
night
Mostly Clear
Mostly Clear
Friday

Mostly Sunny
Hot!
Mostly Sunny
Hot!
Friday
night
Partly Cloudy
Partly Cloudy
Saturday

Mostly Sunny
Hot!
Mostly Sunny
Hot!
Saturday
night
Fog late
Fog late
<.1 in.
High: 102° Low: 75° High: 100° Low: 74° High: 99° Low: 73° High: 96° Low: 72°
Heat: 110°   Heat: 108°   Heat: 107°   Heat: 105°  
SW
5-24
mph
S
4-19
mph
SW
5-24
mph
S
5-18
mph
WSW
4-21
mph
S
2
mph
SW→N
3-20
mph
E
3
mph
UV: 8
 Very High 
UV: 8
 Very High 
UV: 6
 High 
UV: 8
 Very High 

 

Today
 
Sunny in the morning, becoming mostly sunny in the afternoon. Near record heat. High 102°. Heat index up to 110°. UV index up to 8. Wind southwest around 5 mph, gusting to 24 mph.
Tonight
 
Clear. Warm. Low 75°. Wind south around 4 mph, gusting to 19 mph.
Thursday
 
Sunny. Extremely hot. High 100°. Heat index up to 108°. UV index up to 8. Wind southwest around 5 mph, gusting to 24 mph.
Thursday night
 
Clear in the evening, becoming mostly clear after midnight. Warm. Low 74°. Wind south around 5 mph, gusting to 18 mph.
Friday
 
Sunny in the morning, becoming partly to mostly sunny in the afternoon. Extremely hot. High 99°. Heat index up to 107°. UV index up to 6. Wind west-southwest around 4 mph, gusting to 21 mph.
Friday night
 
Partly cloudy in the evening, becoming fair to partly cloudy after midnight. Warm. Low 73°. Wind south around 2 mph.
Saturday
 
Partly to mostly sunny in the morning, becoming mostly sunny in the afternoon. A chance of rain. Precipitation showery or intermittent. Very hot. High 96°. Heat index up to 105°. UV index up to 8. Wind southwest around 3 mph, gusting to 20 mph, in the morning, becoming north in the afternoon. Chance of precipitation 30 percent. Precipitation mostly less than a tenth of an inch.
Saturday night
 
Partly cloudy in the evening, becoming fair after midnight. Patchy light fog after midnight. Low 72°. Wind east around 3 mph.
Sunday
 
Partly to mostly sunny in the morning, becoming partly cloudy in the afternoon. Patchy light fog in the morning. Very hot. High 93°. Heat index up to 100°. UV index up to 7. Wind east around 5 mph, gusting to 22 mph.
Sunday night
 
Fair to partly cloudy in the evening, becoming mostly clear after midnight. Patchy light fog in the evening, then light to moderate fog after midnight. Low 68°. Wind east-northeast around 5 mph, gusting to 17 mph.
Monday
 
Partly to mostly sunny in the morning, becoming sunny in the afternoon. Patchy light fog in the morning. Very hot. High 91°. Heat index up to 97°. UV index up to 7. Wind east around 5 mph, gusting to 23 mph.
Monday night
 
Partly cloudy in the evening, becoming fair after midnight. Low 68°. Wind southeast around 6 mph, gusting to 17 mph.
Tuesday
 
Partly to mostly sunny in the morning, becoming sunny in the afternoon. Near record heat. High 101°. Heat index up to 108°. UV index up to 7. Wind south-southwest around 9 mph, gusting to 24 mph.
Tuesday night
 
Clear. Low 72°. Wind south-southwest around 8 mph, gusting to 20 mph.

WXSIM forecast formatting script by Saratoga-Weather.org.

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