top of page

ASOS Lightning Reports

Updated: Aug 15, 2023

Does an Automated Surface Observating 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.

Here are the gory details of how this is accomplished. The sensor can detect both cloud-to-ground and intracloud strikes. Even though all strikes are counted, only the cloud-to-ground strikes are used when determining how far the strike was away from the sensor for the classification above (e.g., TS, VCTS or distant lightning). These strikes are grouped into three buckets that includes 0 to 5 miles, 5 to 10 miles, and 10 to 30 miles. A thunderstorm is declared to start when the number of strikes in the 0 to 5 miles and 5 to 10 buckets is greater than or equal to two.


If you've ever seen a cloud-to-ground lightning strike, you know it's made up of one or more individual strokes. The sensor is programmed to group all strokes occurring within one second of each other and designates that as a single flash. The range is determined by using the distance of the closest stroke within a flash. The sensor keeps track and automatically “ages” each lightning strike for 15 minutes. This means that if no strike occurs after a 15 minute period, then the thunderstorm is deemed to have ended.


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™


Dr. Scott Dennstaedt

Weather Systems Engineer

Founder, EZWxBrief

CFI & former NWS meteorologist

401 views0 comments

Recent Posts

See All

Comments


bottom of page