Today | Weather | Wednesday, August 12, 2026
Active Weather Warnings

Weather in O'Fallon, Missouri

06:48 AM
Now
Cloudy Weather
75° F
Feels Like 79°
Precipitation 4 %
Wind 7 mph
UV Index (0 - 11+) 1
Humidity 86 %
Today
Sunny with a Chance of Thunderstorms
Day 93°
Night 73°
Precipitation 57 %
Wind 9 mph
UV Index (0 - 11+) 9

Warning: High UV Index Today: 9

High risk of harm from unprotected sun exposure. Take precautions: wear a hat, sunglasses, and apply sunscreen regularly if you’ll be outdoors for extended periods.

O'Fallon Weather Forecast Updated

34 minutes ago
Last updated on

Air Quality in O'Fallon

NOW
22 AQI
Good air quality Dominant pollutant: O3

Pollutant Concentrations:

PM2.5 3.19 µg/m³
PM10 3.71 µg/m³
O3 0.024 ppm
NO2 0.5 ppb
SO2 0.1 ppb
CO 0.08 ppm
NH3 1.89 µg/m³
NO 0 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)
PM2.53.19 µg/m³
PM103.71 µg/m³
O347.64 µg/m³
NO20.93 µg/m³
SO20.3 µg/m³
CO96.95 µg/m³
NH31.89 µg/m³
NO0 µg/m³

Weather Alerts and Warnings for O'Fallon

Severe Future Likely

Extreme Heat Watch

Extreme Heat Watch issued August 8 at 2:33PM CDT until August 12 at 8:00PM CDT by NWS St Louis MO
Aug 9, 12:00 PM Aug 12, 8:00 PM
WHAT: Dangerously hot conditions with heat index values up to 109
possible.

WHERE: Portions of south central and southwest Illinois and
central and east central Missouri.

WHEN: From Sunday afternoon through Wednesday evening.

IMPACTS: Heat related illnesses increase significantly during
extreme heat and high humidity events.
+ Read More
Drink plenty of fluids, stay in an air-conditioned room, stay out of the sun, and check up on relatives and neighbors.
Updated: Aug 8, 11:42 PM Source: NWS Alert
Severe Expected Likely

Extreme Heat Warning

Extreme Heat Warning issued August 12 at 1:45AM CDT until August 13 at 8:00PM CDT by NWS St Louis MO
Aug 12, 1:45 AM Aug 13, 8:00 PM
WHAT: Dangerously hot conditions with heat index values up to 111.

WHERE: Portions of south central and southwest Illinois and
central and east central Missouri.

WHEN: Until 8 PM CDT Thursday.

IMPACTS: Heat related illnesses increase significantly during
extreme heat and high humidity events.
+ 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: Aug 12, 6:13 AM Source: NWS Alert