they can mean the same thing but not necessarily all the time. also, they are not synonyms because flash is a noun and suddenly is an adverb so they belong to different grammatical categories.
brighten, fire, flash, floodlight, highlight, hit with a light, ignite, illume, illumine, irradiate, kindle, light, light up, lighten, limelight, spot, spotlight
They are synonyms.
Synonyms
right away suddenly in a blink at once immediately instantaneously right away in a flash right there and then
synonyms for prominence:importancefamestatureprestige
The Floods that happen suddenly are called Flash Floods.
move abruptly
abruptly, unexpectedly
(immediately) following, then, suddenly, etc.
synonyms of flame : blaze, brightness, conflagration, flare, flash, holocaust, light, rapid oxidation, wildfire
They can break in use.
Here's a few: erupt, flame, flash, irrupt,
burn, glow, flare, be ablaze, shine, flash, radiate
twinkle, flash, sparkle, flare, gutter, glimmer, shimmer
Flash floods are dangerous because they happen suddenly It rains VERY hard, and then it floods up.
Unexpectedly, an adverb, means surprisingly. Some synonyms of the term "unexpectedly" include out of the blue, quickly, startlingly, suddenly, and unusually.
immediately, instantaneously, instantly, straightaway, promptly, abruptly, swiftly; unexpectedly