WXSIM Forecast for: O'Fallon
Issued by: OFallonweather.com
Updated: Monday, 13-Jul-2026 04:00 pm
 
Tonight

Fog late
Fog late
Tuesday

Fog a.m.
Fog a.m.
Tuesday
night
Fog late
Fog late
Wednesday

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

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

Thunder storm
Thunder storm
<.1 in. <.1 in. <.1 in. <.1 in. .25 in.
Low: 70° High: 91° Low: 72° High: 89° Low: 72° High: 91° Low: 75° High: 89°
  Heat: 96°   Heat: 97°   Heat: 100°   Heat: 97°
NE
3-24
mph
NE
5-24
mph
NE
3-19
mph
NNE→E
3-21
mph
SE→SSW
2-16
mph
S
3-21
mph
SW
4-18
mph
WSW
6-24
mph
UV: 9
 Very High 
UV: 9
 Very High 
UV: 8
 Very High 
UV: 9
 Very High 

 

Tonight
 
Fair. Patchy light fog after midnight. Low 70°. Wind northeast around 3 mph, gusting to 24 mph.
Tuesday
 
Mostly sunny in the morning, becoming partly to mostly sunny in the afternoon. Patchy light fog in the morning. Hot. High 91°. Heat index up to 96°. UV index up to 9. Wind northeast around 5 mph, gusting to 24 mph.
Tuesday night
 
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 72°. Wind northeast around 3 mph, gusting to 19 mph. Chance of precipitation 30 percent. Precipitation mostly less than a tenth of an inch.
Wednesday
 
Partly to mostly sunny. Light to moderate fog in the morning. A chance of rain. High 89°. Heat index up to 97°. UV index up to 9. Wind north-northeast around 3 mph, gusting to 21 mph, in the morning, becoming east in the afternoon. Chance of precipitation 30 percent. Precipitation mostly less than a tenth of an inch.
Wednesday night
 
Fair in the evening, becoming mostly clear after midnight. Patchy light fog in the evening, then moderate fog after midnight. Low 72°. Wind southeast around 2 mph, gusting to 16 mph, in the evening, becoming south-southwest after midnight.
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. Scattered thundershowers possible. Hot. High 91°. Heat index up to 100°. UV index up to 8. Wind south around 3 mph, gusting to 21 mph. Chance of precipitation 30 percent. Precipitation mostly less than a tenth of an inch.
Thursday night
 
Partly to mostly cloudy in the evening, becoming mostly cloudy after midnight. Light to moderate fog. A chance of rain. Scattered thundershowers possible. Low 75°. Wind southwest around 4 mph, gusting to 18 mph. Chance of precipitation 40 percent. Precipitation mostly less than a tenth of an inch.
Friday
 
Partly cloudy. Light to moderate fog in the morning. Rain likely. Scattered thundershowers possible. High 89°. Heat index up to 97°. UV index up to 9. Wind west-southwest around 6 mph, gusting to 24 mph. Chance of precipitation 60 percent. Precipitation mostly around a quarter of an inch.
Friday night
 
Mostly cloudy in the evening, becoming fair after midnight. Moderate fog. A chance of rain. Scattered thunderstorms likely. Low 75°. Wind southwest around 5 mph. Chance of precipitation 40 percent. Precipitation mostly less than a tenth of an inch.
Saturday
 
Mostly sunny. Moderate fog in the morning. Hot. High 93°. Heat index up to 103°. UV index up to 9. Wind southwest around 7 mph, gusting to 25 mph.
Saturday night
 
Clear in the evening, becoming mostly clear after midnight. Moderate fog after midnight. Low 73°. Wind southwest around 5 mph, gusting to 21 mph.
Sunday
 
Partly cloudy. 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 101°. UV index up to 9. Wind west around 5 mph, gusting to 22 mph, in the morning, becoming north-northwest in the afternoon. Chance of precipitation 40 percent. Precipitation mostly less than a tenth of an inch.
Sunday night
 
Partly cloudy in the evening, becoming fair to partly cloudy after midnight. Moderate fog in the evening, then moderate to dense fog after midnight. A chance of rain in the evening, then a slight chance of rain after midnight. Scattered thunderstorms likely. Low 73°. Wind northeast around 5 mph, gusting to 17 mph. Chance of precipitation 60 percent. Precipitation mostly around a quarter of an inch.

WXSIM forecast formatting script by Saratoga-Weather.org.

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