Synonyms for Flash flood:

n.

high water, tidal wave, flood, inundation, flooded, deluge, flood plain, tsunami. flash flood (noun)
flashflood.
phenomenon (noun)
flashflood.
sudden rainfall (noun)
deluge.

Other synonyms:

high water, flooded, tsunami, flood plain. tidal wave. deluge. flood. Other relevant words:
deluge.