flash flood vs flashflood

flash flood and flashflood both are nouns.

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

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

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

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