WXSIM Forecast for: O'Fallon
Issued by: OFallonweather.com
Updated: Monday, 17-Aug-2026 04:00 pm
 
Tonight

Chance rain
Chance rain
Tuesday

Chance rain
Chance rain
Tuesday
night
Chance rain
Chance rain
Wednesday

Thunder storm
Thunder storm
Wednesday
night
Thunder storm
Thunder storm
Thursday

Fog a.m.
Fog a.m.
Thursday
night
Fog late
Fog late
Friday

Fog a.m.
Fog a.m.
<.1 in. <.1 in. .1 in. <.25 in. <.1 in. <.1 in.
Low: 72° High: 89° Low: 73° High: 86° Low: 70° High: 84° Low: 65° High: 86°
  Heat: 95°   Heat: 92°        
N→WNW
1-22
mph
SW
3-23
mph
ESE→S
4-16
mph
NE
5-22
mph
ENE
4-18
mph
ENE
5-22
mph
ENE
3-18
mph
ESE→ENE
2-22
mph
UV: 7
 High 
UV: 6
 High 
UV: 7
 High 
UV: 6
 High 

 

Tonight
 
Partly cloudy in the evening, becoming fair to partly cloudy after midnight. Patchy light fog after midnight. A chance of rain. Low 72°. Wind north around 1 mph, gusting to 22 mph, in the evening, becoming west-northwest after midnight. Chance of precipitation 40 percent. Precipitation mostly less than a tenth of an inch.
Tuesday
 
Partly cloudy. Patchy light fog in the morning. A chance of rain. Hot. High 89°. Heat index up to 95°. UV index up to 7. Wind southwest around 3 mph, gusting to 23 mph. Chance of precipitation 40 percent. Precipitation mostly less than a tenth of an inch.
Tuesday night
 
Fair to partly cloudy in the evening, becoming mostly cloudy after midnight. Patchy light fog after midnight. A slight chance of rain in the evening, then a chance of rain after midnight. Precipitation showery or intermittent. Low 73°. Wind east-southeast around 4 mph, gusting to 16 mph, in the evening, becoming south after midnight. Chance of precipitation 40 percent. Precipitation mostly around a tenth of an inch.
Wednesday
 
Partly cloudy in the morning, becoming mostly cloudy in the afternoon. Moderate fog in the morning. A chance of rain. Scattered thundershowers possible. High 86°. Heat index up to 92°. UV index up to 6. Wind northeast around 5 mph, gusting to 22 mph. Chance of precipitation 50 percent. Precipitation mostly less than a quarter of an inch.
Wednesday night
 
Partly cloudy. Patchy light fog in the evening, then light to moderate fog after midnight. A chance of rain. Scattered thundershowers possible. Low 70°. Wind east-northeast around 4 mph, gusting to 18 mph. Chance of precipitation 30 percent. Precipitation mostly less than a tenth of an inch.
Thursday
 
Mostly cloudy in the morning, becoming partly cloudy in the afternoon. Light to 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 84°. UV index up to 7. Wind east-northeast around 5 mph, gusting to 22 mph. Chance of precipitation 30 percent. Precipitation mostly less than a tenth of an inch.
Thursday night
 
Clear. Patchy light fog after midnight. Low 65°. Wind east-northeast around 3 mph, gusting to 18 mph.
Friday
 
Sunny in the morning, becoming partly to mostly sunny in the afternoon. Patchy light fog in the morning. High 86°. UV index up to 6. Wind east-southeast around 2 mph, gusting to 22 mph, in the morning, becoming east-northeast in the afternoon.
Friday night
 
Clear. Patchy light fog after midnight. Low 66°. Wind northeast around 2 mph, gusting to 16 mph, in the evening, becoming south-southeast after midnight.
Saturday
 
Mostly sunny in the morning, becoming partly cloudy in the afternoon. Patchy light fog in the morning. A slight chance of rain. High 86°. UV index up to 6. Wind west-southwest around 3 mph, gusting to 19 mph. Chance of precipitation 20 percent. Precipitation mostly less than a tenth of an inch.
Saturday night
 
Fair in the evening, becoming mostly clear after midnight. Patchy light fog after midnight. Low 67°. Wind northeast around 3 mph.
Sunday
 
Mostly sunny in the morning, becoming partly to mostly cloudy in the afternoon. Patchy light fog in the morning. High 86°. UV index up to 6. Wind southeast around 3 mph, gusting to 15 mph, in the morning, becoming 6 mph, gusting to 19 mph, in the afternoon.
Sunday night
 
Mostly cloudy in the evening, becoming mostly cloudy to cloudy after midnight. Low 70°. Wind southeast around 7 mph, gusting to 17 mph.

WXSIM forecast formatting script by Saratoga-Weather.org.

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