Definition
A METAR remark indicating that lightning is occurring occasionally, with discharges observed both within clouds (in-cloud) and between clouds and the ground (cloud-to-ground). 'Occasional' in this context means lightning is observed at a rate of less than one flash per minute.
Plain English
The weather report is saying that less than once a minute, lightning is flashing — some of it inside the clouds, and some of it striking the ground.
Context Anchor
Seen in the remarks section of a METAR when lightning is observed near the reporting station.
Derivation
A compressed code built from four standard METAR shorthand pieces: OCNL (occasional), LTG (lightning), IC (in-cloud), and CG (cloud-to-ground). Multiple lightning-type codes are run together without spaces, so LTGICCG means lightning of both IC and CG types.
Why Pilots Care
Signals active convective weather that can produce turbulence, icing, and electrical hazards near the airport or flight path.
Grounding Statement
Lightning activity of this type means thunderstorms are present and the area should be avoided.
Intuition Check
Do not read OCNL as a casual guess meaning “some lightning.” In METAR use, OCNL has a set meaning: less than one flash per minute.
Example Sentence 1
The METAR remarks included OCNL LTGICCG, so the crew delayed pushback until the cell moved clear of the field.
Example Sentence 2
With OCNL LTGICCG reported at the destination, the crew diverted to an alternate airport.