METARs and Lightning

Source: www.pilotworkshop.com
Featuring Scott Dennstaedt

Subscriber question:

"When a METAR reports LGT DSNT N, what does this imply?" - Kenny P.

Scott:

"The simple answer is that this phrase will appear in the remarks section of the METAR and is literally translated lightning distant north. But to understand what this means to the pilot requires an understanding of how lightning detection is processed on an automated observing system such as an ASOS.

Here's how it works...

An ASOS will format a METAR or special observation (SPECI) for lightning in one of three ways: TS, VCTS (for thunderstorms in the vicinity) or lightning in the distance.

1. If at least one 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 present weather field. TS will continue to be carried in subsequent METAR observations until no cloud-to-ground lightning strikes are observed for a 15-minute period within that five mile radius. At that time, the ASOS will make another special observation and end the thunderstorm which removes the TS from the report.
2. If the cloud-to-ground lightning strike is detected 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 present weather field. VCTS will continue to be carried in subsequent METAR observations until no cloud-to-ground lightning strikes are observed in the airport's vicinity for a 15-minute period. At that time, the ASOS will make a special observation to end thunderstorms in the vicinity which removes VCTS from the report.
3. Separate from this, or independently, if the cloud-to-ground lightning strike is between 10 miles and 30 miles of the ASOS, the ASOS will carry a distant lightning remark. For example, if distant lightning is detected to the north and west, the report will carry a remark LTG DSNT N-W. This will be appended as appropriate on all special and/or METAR observations."