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

The most common answer is thundershower (13 letters).

Crossword clue: a shower 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).

thundershower 13 letters web-def

a short rainstorm accompanied by thunder and lightning

a storm accompanied with lightning and thunder. 60% similar → thunderstorm