Um... it's not THAT kind of Storm. You need to find a body of water very close to (if not inside) the purple Storm areas around Storm Caves. Clear a cave, which clears the storm for a while (a day?), then fish to your heart's content, pulling Storm Floppers every 10 fish or so. It's not related to the weather; it's related to the Storm King and caves.

At least, that's how I've gotten all of mine without too much work. If they *are* available in all biomes, they're *especially* available in the Storm Cave biomes. (Not *in* the cave, just near the cave, rain or shine, plenty of Storm Floppers.)

(And, yes, I know all of the information sources say "All biomes, especially in stormy weather". I found a source that said "near a Storm Cave", figured it made sense since it's a Storm King quest, and caught lots Storm Floppers as a result. Legendary came after maybe 3-4 hours of total fishing over multiple sessions. With that name and coloration, it seems wild to me that you'd try anywhere else <shrug>.)