WXSIM Forecast for: O'Fallon
Issued by: OFallonweather.com
Updated: Monday, 27-Jul-2026 05:00 am
 
Today

Mostly Sunny
Hot!
Mostly Sunny
Hot!
Tonight

Thunder storm
Thunder storm
Tuesday

Thunder storm
Hot!
Thunder storm
Hot!
Tuesday
night
Fog late
Fog late
Wednesday

Fog a.m.
Fog a.m.
Wednesday
night
Fog late
Fog late
Thursday

Fog a.m.
Fog a.m.
Thursday
night
Fog late
Fog late
.1 in. .1 in. <.1 in. <.1 in.
High: 103° Low: 79° High: 93° Low: 72° High: 90° Low: 68° High: 92° Low: 70°
Heat: 113°   Heat: 103°   Heat: 96°   Heat: 97°  
WSW
6-25
mph
WSW
5-20
mph
NNW
5-23
mph
NE
4-19
mph
E
5-24
mph
E
4-22
mph
SSE
5-24
mph
E→SE
4-21
mph
UV: 10
 Very High 
UV: 7
 High 
UV: 9
 Very High 
UV: 9
 Very High 

 

Today
 
Mostly sunny. Extremely hot. High 103°. Heat index up to 113°. UV index up to 10. Wind west-southwest around 6 mph, gusting to 25 mph.
Tonight
 
Partly to mostly cloudy in the evening, becoming partly cloudy after midnight. A chance of rain in the evening, then a slight chance of rain after midnight. Scattered thunderstorms likely, some possibly severe. Warm. Low 79°. Wind west-southwest around 5 mph, gusting to 20 mph. Chance of precipitation 40 percent. Precipitation mostly around a tenth of an inch.
Tuesday
 
Partly to mostly cloudy in the morning, becoming mostly cloudy in the afternoon. A chance of rain in the morning, then a slight chance of rain in the afternoon. Scattered thundershowers possible. Hot. High 93°. Heat index up to 103°. UV index up to 7. Wind north-northwest around 5 mph, gusting to 23 mph. Chance of precipitation 40 percent. Precipitation mostly around a tenth of an inch.
Tuesday night
 
Fair to partly cloudy in the evening, becoming mostly clear after midnight. Patchy light fog in the evening, then moderate fog after midnight. A slight chance of rain. Low 72°. Wind northeast around 4 mph, gusting to 19 mph. Chance of precipitation 20 percent. Precipitation mostly less than a tenth of an inch.
Wednesday
 
Sunny. Light to moderate fog in the morning. Hot. High 90°. Heat index up to 96°. UV index up to 9. Wind east around 5 mph, gusting to 24 mph.
Wednesday night
 
Clear. Patchy light fog after midnight. Low 68°. Wind east around 4 mph, gusting to 22 mph.
Thursday
 
Mostly sunny. Patchy light fog in the morning. Hot. High 92°. Heat index up to 97°. UV index up to 9. Wind south-southeast around 5 mph, gusting to 24 mph.
Thursday night
 
Partly cloudy. Patchy light fog after midnight. A slight chance of rain after midnight. Low 70°. Wind east around 4 mph, gusting to 21 mph, in the evening, becoming southeast after midnight. Chance of precipitation 20 percent. Precipitation mostly less than a tenth of an inch.
Friday
 
Partly cloudy in the morning, becoming 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. Scattered thundershowers possible. High 88°. Heat index up to 93°. UV index up to 5. Wind south-southeast around 7 mph, gusting to 25 mph. Chance of precipitation 50 percent. Precipitation mostly less than a quarter of an inch.
Friday night
 
Mostly cloudy in the evening, becoming cloudy after midnight. Light to moderate fog. Rain very likely. Thunderstorms very likely, some possibly severe. Low 74°. Wind south-southeast around 7 mph, gusting to 19 mph. Chance of precipitation 80 percent. Precipitation mostly around an inch.
Saturday
 
Cloudy in the morning, becoming mostly cloudy in the afternoon. Light to moderate fog in the morning, then patchy light fog in the afternoon. Rain likely. Thunderstorms very likely, some possibly severe. High 81°. UV index up to 5. Wind south-southeast around 5 mph, gusting to 21 mph, in the morning, becoming north-northeast in the afternoon. Chance of precipitation 70 percent. Precipitation mostly between a quarter and half an inch.
Saturday night
 
Moderate to dense fog in the evening, then moderate fog after midnight. A chance of rain. Scattered thunderstorms likely. Low 71°. Wind northeast around 5 mph, gusting to 18 mph. Chance of precipitation 40 percent. Precipitation mostly around a tenth of an inch.
Sunday
 
Sunny. Moderate fog in the morning. High 85°. UV index up to 9. Wind north around 8 mph, gusting to 26 mph.
Sunday night
 
Clear. Light to moderate fog after midnight. Low 68°. Wind east-northeast around 4 mph, gusting to 22 mph.

WXSIM forecast formatting script by Saratoga-Weather.org.

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