WXSIM Forecast for: O'Fallon
Issued by: OFallonweather.com
Updated: Wednesday, 15-Jul-2026 04:00 pm
 
Tonight

Chance rain
Chance rain
Thursday

Chance rain
Chance rain
Thursday
night
Chance rain
Chance rain
Friday

Chance rain
Chance rain
Friday
night
Fog late
Fog late
Saturday

Fog a.m.
Hot!
Fog a.m.
Hot!
Saturday
night
Fog late
Fog late
Sunday

Chance rain
Chance rain
.1 in. <.25 in. .1 in. <.1 in. <.1 in. <.1 in.
Low: 75° High: 90° Low: 75° High: 89° Low: 74° High: 95° Low: 75° High: 89°
  Heat: 99°   Heat: 99°   Heat: 104°   Heat: 98°
E
2-20
mph
NE→SE
2-19
mph
SW
3-15
mph
SW
5-23
mph
SW
5-19
mph
WSW
8-26
mph
WSW
5-21
mph
NNE
3-20
mph
UV: 8
 Very High 
UV: 9
 Very High 
UV: 9
 Very High 
UV: 7
 High 

 

Tonight
 
Fair to partly cloudy. Patchy light fog in the evening, then moderate fog after midnight. A chance of rain. Low 75°. Wind east around 2 mph, gusting to 20 mph. Chance of precipitation 40 percent. Precipitation mostly around a tenth of an inch.
Thursday
 
Partly to mostly sunny in the morning, becoming partly cloudy in the afternoon. Light to moderate fog in the morning. A chance of rain. Precipitation showery or intermittent. High 90°. Heat index up to 99°. UV index up to 8. Wind northeast around 2 mph, gusting to 19 mph, in the morning, becoming southeast in the afternoon. Chance of precipitation 50 percent. Precipitation mostly less than a quarter of an inch.
Thursday night
 
Partly cloudy. Light to moderate fog in the evening, then moderate fog after midnight. A chance of rain. Precipitation showery or intermittent. Low 75°. Wind southwest around 3 mph, gusting to 15 mph. Chance of precipitation 50 percent. Precipitation mostly around a tenth of an inch.
Friday
 
Partly to mostly sunny in the morning, becoming partly cloudy in the afternoon. Moderate fog in the morning. A chance of rain in the morning, then a slight chance of rain in the afternoon. Precipitation showery or intermittent. High 89°. Heat index up to 99°. UV index up to 9. Wind southwest around 5 mph, gusting to 23 mph. Chance of precipitation 40 percent. Precipitation mostly less than a tenth of an inch.
Friday night
 
Fair to partly cloudy in the evening, becoming mostly clear after midnight. Light to moderate fog in the evening, then moderate fog after midnight. Low 74°. Wind southwest around 5 mph, gusting to 19 mph.
Saturday
 
Mostly sunny in the morning, becoming partly to mostly sunny in the afternoon. Moderate fog in the morning. Hot. High 95°. Heat index up to 104°. UV index up to 9. Wind west-southwest around 8 mph, gusting to 26 mph.
Saturday night
 
Partly cloudy. Patchy light fog after midnight. A chance of rain. Low 75°. Wind west-southwest around 5 mph, gusting to 21 mph. Chance of precipitation 30 percent. Precipitation mostly less than a tenth of an inch.
Sunday
 
Partly to mostly cloudy. Patchy light fog in the morning. A slight chance of rain in the morning, then a chance of rain in the afternoon. High 89°. Heat index up to 98°. UV index up to 7. Wind north-northeast around 3 mph, gusting to 20 mph. Chance of precipitation 40 percent. Precipitation mostly less than a tenth of an inch.
Sunday night
 
Fair to partly cloudy in the evening, becoming partly cloudy after midnight. Patchy light fog after midnight. Low 72°. Wind northeast around 4 mph, gusting to 19 mph, in the evening, becoming east after midnight.
Monday
 
Partly to mostly cloudy. Patchy light fog in the morning. A slight chance of rain in the morning, then a chance of rain in the afternoon. Scattered thundershowers possible. High 90°. Heat index up to 98°. UV index up to 8. Wind south-southeast around 3 mph, gusting to 18 mph, in the morning, becoming south-southwest in the afternoon. Chance of precipitation 50 percent. Precipitation mostly less than a tenth of an inch.
Monday night
 
Fair in the evening, becoming mostly clear after midnight. Patchy light fog in the evening, then moderate fog after midnight. Low 74°. Wind southwest around 4 mph, gusting to 15 mph.
Tuesday
 
Partly to mostly sunny in the morning, becoming mostly cloudy in the afternoon. Very hot. High 97°. Heat index up to 107°. UV index up to 7. Wind west around 7 mph, gusting to 24 mph.
Tuesday night
 
Partly cloudy. Low 72°. Wind north-northwest around 3 mph, gusting to 18 mph, in the evening, becoming northeast after midnight.

WXSIM forecast formatting script by Saratoga-Weather.org.

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