National Oceanic and Atmospheric Administration National Weather Service United States Department of Commerce

Dangerous Heat and Heavy Rainfall into the Weekend

Hot temperatures and high humidity are forecast to continue across the mid-Mississippi Valley into this weekend. Heat Advisories remain in effect. Heavy to excessive rainfall occurring at times through the weekend could bring areas of flooding to parts of the northern Rockies, upper Midwest, central Great Lakes, and southern New England. Read More >

Hazardous Weather Conditions

Current conditions at

St. Louis Downtown Airport (KCPS)

Lat: 38.56°NLon: 90.15°WElev: 410ft.

Fair

77°F

25°C

Humidity 88%
Wind Speed Calm
Barometer 30.01 in (1015.9 mb)
Dewpoint 73°F (23°C)
Visibility 10.00 mi
Heat Index78°F (26°C)
Last update 18 Sep 10:53 pm CDT
Extended Forecast for

O'Fallon IL

Heat Advisory September 18, 11:42pm until September 19, 08:00pm
Click here for hazard details and duration
Heat Advisory
  •  
    Heat Advisory September 18, 11:42pm until September 19, 08:00pm

    NOW until 8:00pm Sat

    Heat Advisory

  • Tonight

    Low: 73 °F

    Partly Cloudy

  • Saturday

    Saturday: Increasing clouds and hot, with a high near 97. Heat index values as high as 101. Light and variable wind becoming west 5 to 8 mph in the morning.

    High: 97 °F

    Hot

  • Saturday Night

    Saturday Night: Partly cloudy, with a low around 73. South wind 3 to 5 mph.

    Low: 73 °F

    Partly Cloudy

  • Sunday

    Sunday: A slight chance of showers between 1pm and 4pm, then a slight chance of showers and thunderstorms after 4pm. Mostly sunny, with a high near 94. West wind 3 to 8 mph. Chance of precipitation is 20%.

    High: 94 °F

    Mostly Sunny
    then Slight
    Chance
    T-storms

  • Sunday Night

    Sunday Night: A chance of showers and thunderstorms before 1am, then showers likely and possibly a thunderstorm between 1am and 4am, then showers likely after 4am. Mostly cloudy, with a low around 68. North wind 6 to 8 mph. Chance of precipitation is 60%.

    Low: 68 °F

    Chance
    Showers then
    Showers
    Likely

  • Monday

    Monday: A chance of showers, then showers likely and possibly a thunderstorm after 1pm. Mostly cloudy, with a high near 81. Chance of precipitation is 70%.

    High: 81 °F

    Showers
    Likely

  • Monday Night

    Monday Night: A slight chance of showers and thunderstorms, then a chance of showers after 1am. Mostly cloudy, with a low around 61. Chance of precipitation is 30%.

    Low: 61 °F

    Slight Chance
    T-storms then
    Chance
    Showers

  • Tuesday

    Tuesday: Mostly cloudy, with a high near 72.

    High: 72 °F

    Mostly Cloudy

Detailed Forecast

Tonight
Partly cloudy, with a low around 73. Southeast wind around 6 mph.
Saturday
Increasing clouds and hot, with a high near 97. Heat index values as high as 101. Light and variable wind becoming west 5 to 8 mph in the morning.
Saturday Night
Partly cloudy, with a low around 73. South wind 3 to 5 mph.
Sunday
A slight chance of showers between 1pm and 4pm, then a slight chance of showers and thunderstorms after 4pm. Mostly sunny, with a high near 94. West wind 3 to 8 mph. Chance of precipitation is 20%.
Sunday Night
A chance of showers and thunderstorms before 1am, then showers likely and possibly a thunderstorm between 1am and 4am, then showers likely after 4am. Mostly cloudy, with a low around 68. North wind 6 to 8 mph. Chance of precipitation is 60%.
Monday
A chance of showers, then showers likely and possibly a thunderstorm after 1pm. Mostly cloudy, with a high near 81. Chance of precipitation is 70%.
Monday Night
A slight chance of showers and thunderstorms, then a chance of showers after 1am. Mostly cloudy, with a low around 61. Chance of precipitation is 30%.
Tuesday
Mostly cloudy, with a high near 72.
Tuesday Night
Mostly cloudy, with a low around 55.
Wednesday
Mostly sunny, with a high near 74.
Wednesday Night
Partly cloudy, with a low around 56.
Thursday
Mostly sunny, with a high near 76.
Thursday Night
Partly cloudy, with a low around 58.
Friday
Mostly sunny, with a high near 76.
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Forecast Area
Tiles ©ESRI

ABOUT THIS FORECAST

 
Point Forecast:
O'Fallon IL
 38.59°N 89.92°W (Elev. 551 ft)
Last Update:
9:26 pm CDT Sep 18, 2026
Forecast Valid:
11pm CDT Sep 18, 2026-6pm CDT Sep 25, 2026
 
Forecast Discussion
 
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Additional Resources

Radar & Satellite Image

Link to Local Radar Data Link to Satellite Data

Hourly Weather Forecast

National Digital Forecast Database

National Digital Forecast Database Maximum Temperature Forecast

High Temperature

National Digital Forecast Database Weather Element Forecast

Chance of Precipitation