WXSIM Forecast for: O'Fallon
Issued by: OFallonweather.com
Updated: Tuesday, 30-Jun-2026 04:00 pm
 
Tonight

Fog late
Fog late
Wednesday

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

Thunder storm
Hot!
Thunder storm
Hot!
Thursday
night
Thunder storm
Thunder storm
Friday

Fog a.m.
Hot!
Fog a.m.
Hot!
Friday
night
Thunder storm
Thunder storm
Saturday

Thunder storm
Hot!
Thunder storm
Hot!
.35 in. .35 in. <.1 in. <.1 in.
Low: 74° High: 95° Low: 72° High: 96° Low: 78° High: 95° Low: 76° High: 94°
  Heat: 105°   Heat: 108°   Heat: 111°   Heat: 107°
SSW
6-23
mph
SSW
6-24
mph
SSW
5-19
mph
SSW
5-24
mph
SSW
4
mph
SSW
5-23
mph
SSW
4-17
mph
SSW
4-22
mph
UV: 11
 Extreme 
UV: 11
 Extreme 
UV: 9
 Very High 
UV: 9
 Very High 

 

Tonight
 
Clear. Moderate fog after midnight. Low 74°. Wind south-southwest around 6 mph, gusting to 23 mph.
Wednesday
 
Sunny. Moderate fog in the morning. Hot. High 95°. Heat index up to 105°. UV index up to 11. Wind south-southwest around 6 mph, gusting to 24 mph.
Wednesday night
 
Clear. Moderate fog after midnight. Low 72°. Wind south-southwest around 5 mph, gusting to 19 mph.
Thursday
 
Sunny in the morning, becoming partly cloudy in the afternoon. Light to moderate fog. Rain likely in the afternoon. Thunderstorms very likely, some severe. Very hot. High 96°. Heat index up to 108°. UV index up to 11. Wind south-southwest around 5 mph, gusting to 24 mph. Chance of precipitation 70 percent. Precipitation mostly between a quarter and half an inch.
Thursday night
 
Moderate fog. Rain likely in the evening, then a slight chance of rain after midnight. Scattered thunderstorms likely. Low 78°. Wind south-southwest around 4 mph. Chance of precipitation 70 percent. Precipitation mostly between a quarter and half an inch.
Friday
 
Partly to mostly sunny in the morning, becoming partly cloudy in the afternoon. Patchy light fog in the morning. Hot. High 95°. Heat index up to 111°. UV index up to 9. Wind south-southwest around 5 mph, gusting to 23 mph.
Friday night
 
Partly cloudy. Light to moderate fog in the evening, then moderate fog after midnight. A slight chance of rain in the evening, then a chance of rain after midnight. Thunderstorms very likely, some severe. Low 76°. Wind south-southwest around 4 mph, gusting to 17 mph. Chance of precipitation 40 percent. Precipitation mostly less than a tenth of an inch.
Saturday
 
Partly to mostly cloudy in the morning, becoming mostly sunny in the afternoon. Moderate fog in the morning. A chance of rain. Scattered thunderstorms likely. Hot. High 94°. Heat index up to 107°. UV index up to 9. Wind south-southwest around 4 mph, gusting to 22 mph. Chance of precipitation 30 percent. Precipitation mostly less than a tenth of an inch.
Saturday night
 
Fair in the evening, becoming mostly cloudy after midnight. Patchy light fog in the evening, then moderate fog after midnight. A chance of rain after midnight. Thunderstorms very likely, some possibly severe. Low 76°. Wind south around 4 mph, gusting to 18 mph, in the evening, becoming west-southwest after midnight. Chance of precipitation 50 percent. Precipitation mostly around a tenth of an inch.
Sunday
 
Mostly cloudy in the morning, becoming partly cloudy in the afternoon. Light to moderate fog in the morning. A slight chance of rain in the morning, then a chance of rain in the afternoon. Scattered thunderstorms likely. Hot. High 92°. Heat index up to 104°. UV index up to 8. Wind west-southwest around 4 mph, gusting to 21 mph, in the morning, becoming west-northwest in the afternoon. Chance of precipitation 60 percent. Precipitation mostly less than a quarter of an inch.
Sunday night
 
Moderate fog. Rain likely. Scattered thunderstorms likely. Low 76°. Wind north-northwest around 3 mph in the evening, becoming northeast after midnight. Chance of precipitation 70 percent. Precipitation mostly around a quarter of an inch.
Monday
 
Mostly sunny. Light to moderate fog in the morning. Hot. High 91°. Heat index up to 102°. UV index up to 10. Wind southeast around 3 mph, gusting to 21 mph.
Monday night
 
Partly cloudy in the evening, becoming mostly cloudy to cloudy after midnight. Patchy light fog in the evening, then light to moderate fog after midnight. A slight chance of rain in the evening, then a chance of rain after midnight. Thunderstorms very likely, some possibly severe. Low 75°. Wind east around 3 mph, gusting to 18 mph, in the evening, becoming southwest after midnight. Chance of precipitation 50 percent. Precipitation mostly less than a tenth of an inch.

WXSIM forecast formatting script by Saratoga-Weather.org.

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