Does an Automated Surface Observation System (ASOS) have a built in lightning detection system? How does it know to add a TS, VCTS or tell you about lightning in the distance?
Some ASOS sites do have a single-site lightning sensor. If there isn't a lightning sensor at the site, it is still possible for the ASOS to report lightning. For FAA-sponsored ASOS sites without a lightning sensor, lightning data is made available to the ASOS through the Automated Lightning Detection and Ranging System (ALDARS) which is a ground-based lightning detection system. ALDARS is not co-resident with the sensor and sends the data to the ASOS. Here's how it works.
An ASOS will format a METAR or SPECI for lightning in one of three ways: TS, VCTS (thunderstorms in the vicinity) or lightning in the distance.
1) If the cloud-to-ground lightning strike is within five miles of the ASOS, the ASOS will make a special (SPECI) observation and carry "TS" (for thunderstorm) in the body of the special observation in the present weather field. "TS" will continue to be carried in the present weather field in subsequent METAR observations until no cloud-to-ground lightning strikes are observed for a 15 minute period. At that time, the ASOS will make a SPECI observation and end the thunder (removes the TS).
2) If the cloud-to-ground lightning strike is between five miles and ten miles of the ASOS, the ASOS will make a SPECI observation, and carry "VCTS" (for thunderstorm in the vicinity) in the body of the observation in the present weather field. "VCTS" will continue to be carried in the present weather field in subsequent METAR observations until no cloud-to-ground lightning strikes are observed for a 15 minute period. At that time, the ASOS will take a SPECI and end the "VCTS."
3) Separate from above, or independently, if the cloud-to-ground lightning strike is between 10 miles and 30 miles of the ASOS, the ASOS will carry a "LTG DSNT xx" remark indicating distant lightning, with "xx" being the direction of the lightning in octants. This will be appended as appropriate on all SPECI and/or METAR observations.
Of course, at locations with a human observer, the ASOS observation can be overridden including adding remarks such as FRQ LTGICCG OHD TS OHD MOV NE.
Most pilots are weatherwise, but some are otherwise™
Weather Systems Engineer
CFI & former NWS meteorologist