WXSIM Forecast for: O'Fallon
Issued by: OFallonweather.com
Updated: Thursday, 20-Aug-2026 04:00 pm
 
Tonight

Chance rain
Chance rain
Friday

Fog a.m.
Fog a.m.
Friday
night
Fog late
Fog late
Saturday

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

Fog a.m.
Fog a.m.
Sunday
night
Fog late
Fog late
Monday

Chance rain
Chance rain
<.1 in. <.1 in. <.1 in. <.1 in. <.25 in.
Low: 68° High: 87° Low: 68° High: 86° Low: 65° High: 84° Low: 64° High: 83°
  Heat: 91°            
NE
3-19
mph
SSE→WSW
2-21
mph
W
3
mph
NW
5-22
mph
N
4-18
mph
N
3-22
mph
E
2-17
mph
E
5-23
mph
UV: 7
 High 
UV: 7
 High 
UV: 7
 High 
UV: 7
 High 

 

Tonight
 
Fair to partly cloudy in the evening, becoming fair after midnight. Patchy light fog in the evening, then light to moderate fog after midnight. A chance of rain. Low 68°. Wind northeast around 3 mph, gusting to 19 mph. Chance of precipitation 40 percent. Precipitation mostly less than a tenth of an inch.
Friday
 
Mostly sunny. Light to moderate fog in the morning. Hot. High 87°. Heat index up to 91°. UV index up to 7. Wind south-southeast around 2 mph, gusting to 21 mph, in the morning, becoming west-southwest in the afternoon.
Friday night
 
Fair in the evening, becoming fair to partly cloudy after midnight. Patchy light fog after midnight. A slight chance of rain after midnight. Low 68°. Wind west around 3 mph. Chance of precipitation 20 percent. Precipitation mostly less than a tenth of an inch.
Saturday
 
Partly to mostly sunny in the morning, becoming sunny in the afternoon. Light to moderate fog in the morning. A chance of rain. High 86°. UV index up to 7. Wind northwest around 5 mph, gusting to 22 mph. Chance of precipitation 30 percent. Precipitation mostly less than a tenth of an inch.
Saturday night
 
Clear. Patchy light fog after midnight. Low 65°. Wind north around 4 mph, gusting to 18 mph.
Sunday
 
Sunny in the morning, becoming partly cloudy in the afternoon. Patchy light fog in the morning. High 84°. UV index up to 7. Wind north around 3 mph, gusting to 22 mph.
Sunday night
 
Partly cloudy. Patchy light fog after midnight. A slight chance of rain. Low 64°. Wind east around 2 mph, gusting to 17 mph. Chance of precipitation 20 percent. Precipitation mostly less than a tenth of an inch.
Monday
 
Partly cloudy in the morning, becoming partly to mostly cloudy in the afternoon. Patchy light fog in the morning. A slight chance of rain in the morning, then a chance of rain in the afternoon. Precipitation showery or intermittent. High 83°. UV index up to 7. Wind east around 5 mph, gusting to 23 mph. Chance of precipitation 50 percent. Precipitation mostly less than a quarter of an inch.
Monday night
 
Partly cloudy. Moderate fog. A chance of rain in the evening, then rain likely after midnight. Low 67°. Wind east around 6 mph, gusting to 17 mph. Chance of precipitation 70 percent. Precipitation mostly between half an inch and one inch.
Tuesday
 
Mostly cloudy to cloudy. Moderate fog in the morning. A chance of rain. Precipitation showery or intermittent. High 83°. UV index up to 5. Wind east-southeast around 5 mph, gusting to 19 mph. Chance of precipitation 30 percent. Precipitation mostly less than a tenth of an inch.
Tuesday night
 
Partly cloudy in the evening, becoming partly to mostly cloudy after midnight. Low 70°. Wind southeast around 2 mph.
Wednesday
 
Partly to mostly cloudy in the morning, becoming mostly cloudy in the afternoon. A chance of rain. Scattered thunderstorms likely. High 85°. UV index up to 6. Wind west around 3 mph, gusting to 17 mph. Chance of precipitation 30 percent. Precipitation mostly less than a tenth of an inch.
Wednesday night
 
Fair in the evening, becoming clear after midnight. Patchy light fog in the evening, then moderate fog after midnight. Low 67°. Wind west-northwest around 3 mph, gusting to 15 mph, in the evening, becoming north-northwest after midnight.

WXSIM forecast formatting script by Saratoga-Weather.org.

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