Today | Weather | Tuesday, September 1, 2026
Active Weather Warnings

Weather in O'Fallon, Missouri

10:24 AM
Now
Sunny Weather
86° F
Feels Like 93°
Precipitation 0 %
Wind 7 mph
UV Index (0 - 11+) 4
Humidity 58 %
Today
Hot Day
Day 100°
Night 73°
Precipitation 1 %
Wind 6 mph
UV Index (0 - 11+) 7

O'Fallon Weather Forecast Updated

7 minutes ago
Last updated on

Air Quality in O'Fallon

NOW
29 AQI
Good air quality Dominant pollutant: O3

Pollutant Concentrations:

PM2.5 5.88 µg/m³
PM10 6.32 µg/m³
O3 0.032 ppm
NO2 0.5 ppb
SO2 0.3 ppb
CO 0.10 ppm
NH3 0.78 µg/m³
NO 0.16 µg/m³
PM2.5 – Fine particles <2.5μm
PM10 – Inhalable particles <10μm
O3 – Ozone
NO2 – Nitrogen dioxide
SO2 – Sulfur dioxide
CO – Carbon monoxide
NH3 – Ammonia
NO – Nitric oxide

Health Recommendations:

Air quality is considered satisfactory, and air pollution poses little or no risk.

Show raw pollutant concentrations (µg/m³)
PM2.55.88 µg/m³
PM106.32 µg/m³
O362.39 µg/m³
NO20.87 µg/m³
SO20.74 µg/m³
CO113.95 µg/m³
NH30.78 µg/m³
NO0.16 µg/m³

Weather Alerts and Warnings for O'Fallon

Severe Future Likely

Extreme Heat Watch

Extreme Heat Watch issued August 30 at 12:27PM CDT until September 4 at 8:00PM CDT by NWS St Louis MO
Aug 31, 12:00 PM Sep 4, 8:00 PM
WHAT: Dangerously hot conditions with heat index values of 102 to
108 degrees, and morning lows from 75 to 80 degrees, possible for
several consecutive days.

WHERE: In Illinois, Clinton IL, Washington IL, Madison IL, Monroe
IL, Randolph IL, and Saint Clair IL Counties. In Missouri, Saint
Charles MO, Saint Louis City MO, and Saint Louis MO Counties.

WHEN: From Monday afternoon through Friday evening.

IMPACTS: Heat related illnesses increase significantly during
extreme heat and high humidity events.

ADDITIONAL DETAILS: In addition to near-record afternoon
temperatures, warm morning lows may provide little relief,
especially Wednesday through Friday.
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Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors. Monitor the latest forecasts and warnings for updates.
Updated: Aug 30, 10:20 PM Source: NWS Alert
Moderate Expected Likely

Heat Advisory

Heat Advisory issued September 1 at 3:22AM CDT until September 4 at 8:00PM CDT by NWS St Louis MO
Sep 1, 3:22 AM Sep 4, 8:00 PM
WHAT: Heat index values up to 108.

WHERE: In Illinois, Madison IL, Monroe IL, and Saint Clair IL
Counties. In Missouri, Jefferson MO, Saint Charles MO, Saint Louis
City MO, and Saint Louis MO Counties.

WHEN: Until 8 PM CDT Friday.

IMPACTS: Hot temperatures and high humidity may cause heat
illnesses.
+ Read More
Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors. Young children and pets should never be left unattended in vehicles under any circumstances. Take extra precautions if you work or spend time outside. When possible reschedule strenuous activities to early morning or evening. Know the signs and symptoms of heat exhaustion and heat stroke. Wear lightweight and loose fitting clothing when possible. To reduce risk during outdoor work, the Occupational Safety and Health Administration recommends scheduling frequent rest breaks in shaded or air conditioned environments. Anyone overcome by heat should be moved to a cool and shaded location. Heat stroke is an emergency! Call 9 1 1. People in the Saint Louis Metropolitan Area can get information about cooling centers or energy assistance related to the excessive heat by calling the United Way of Greater Saint Louis at 800-427-4626, or if calling from a land line phone dial 2-1-1. You can also call Cool Down Saint Louis at 314-241-7668.
Updated: Sep 1, 10:16 AM Source: NWS Alert