lightning & Thundering :
The occurrence of a natural electrical discharge of very short duration & high voltage between a cloud & the ground or within a cloud, by a accompanied by a bright flash & a typically also thunder.
A flash or a series of flashes of a light in the sky caused the discharge of atmospheric electricity from one cloud to another or between a cloud & the earth.
A loud explosive resounding noise produced by explosive expansion of air heated by a lightning discharge.
Thundering is correct, but slightly ambiguous as to whether you mean one ongoing peal of thunder or repeated ongoing peals.
