Where required, the ASOS uses the Global Atmospherics Inc. (GAI) Model 924 single site lightning sensor as a source for reporting a thunderstorm. The ASOS Lightning Sensor (ALS) is installed at selected Service-Level "D" ASOS sites that do not have the FAA Automated Lightning Detection And Reporting System (ALDARS). The ALDARS is another source of lightning information provided through the National Lightning Detection Network. The ALS sensor is a single-point omnidirectional system that requires two criteria before reporting a thunderstorm: an optical flash and an electrical field change (radio signal), which occur within milliseconds of each other. The requirement for simultaneous optical and radio signals virtually eliminates the possibility of a false alarm from errant light sources.

The sensor can detect cloud-to-ground and cloud-to-cloud strikes. All strikes are counted, but only the cloud-to-ground strikes are used to generate an estimate of the range. Cloud-to-ground strikes are grouped into three range bins: 0 to 5 miles, 5 to 10 miles, and 10 to 30 miles. Because the cloud-to-cloud detection is less efficient than cloud-to-ground detection, the ASOS considers cloud-to-cloud strikes to be within 5 miles.

A cloud-to-ground strike is made up of one or more individual flashes. Within one flash, numerous discharges can occur; these individual discharges are called strokes. The sensor groups all strokes occurring within 1 second of each other into a single flash. The range of a cloud-to-ground strike is determined by the range of the closest stroke within a flash.

The sensor automatically "ages" each lightning strike for 15 minutes. Because a thunderstorm is defined to be in progress for 15 minutes after the last lightning or thunder occurs, the sensor continues to report each strike in the appropriate bin for 15 minutes after it is first detected. A thunderstorm is determined to end when no strikes are detected within the last 15 minutes.