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A continuous mesoscale lightning flash with a horizontal path length of approximately 100 km or greater. The tremendous distances covered by megaflashes necessitate long flash durations as well, typically 5 s or greater.

Megaflashes typically occur in the stratiform regions of mesoscale convective systems (MCSs). These expansive flashes can produce one or more cloud-to-ground (CG) strikes along their paths. Some CGs associated with megaflashes have large charge moment changes and/or peak currents resulting in lightning superbolts, sprites, and/or lightning-triggered upward lightning discharges (LTULs) from tall structures.

In June 2020 the World Meteorological Organization announced the certification of new global lightning extreme records for megaflashes after the identification of megaflashes in excess of 700 km in length and 16 s in duration over Argentina and southern Brazil.

Lyons, W. A., and Coauthors, 2020: Megaflashes: Just how long can a lightning discharge get? Bull. Amer. Meteor. Soc., 101, E73–E86, doi:10.1175/BAMS-D-19-0033.1.

Peterson, M. J., and Coauthors, 2020: New World Meteorological Organization certified megaflash lightning extremes for flash distance (709 km) and duration (16.73 s) recorded from space. Geophys. Res. Lett., 47, e2020GL088888, doi:10.1029/2020GL088888.

Term edited 20 January 2021.