How many letters does the top answer for “a storm accompanied with lightning and thunder.” have?

The most common answer is thunderstorm (12 letters).

Crossword clue: a storm accompanied with lightning and thunder.

1 answer found in Letterora's clue database (Wikidata facts, Open Trivia DB, WordNet and Webster definitions — all licensed sources).

thunderstorm 12 letters web-def

a storm resulting from strong rising air currents; heavy rain or hail along with thunder and lightning

a shower accompanied with lightning and thunder. 60% similar → thundershower