flashflood vs flash flood

flashflood and flash flood both are nouns.

Word	Noun	Adjective	Verb	Adverb
flashflood	Yes	No	No	No
flash flood	Yes	No	No	No

As nouns, flashflood and flash flood are synonyms defined as:

flashflood and flash flood: a sudden local flood of great volume and short duration

flashflood (noun)	flash flood (noun)
a sudden local flood of great volume and short duration	a sudden local flood of great volume and short duration