Learn Dutch: Zo'n bizarre overstroming is extreem. - Such a bizarre flood is extreme.

Zo'n translates to "such a". It is a common contraction of zo (so/such) and the indefinite article een (a/an). You use zo'n before singular countable nouns, like overstroming (flood).

Since zo'n contains een, why does the adjective bizarre get an -e at the end?

Adjectives after een (or words containing it, like zo'n) get an -e added to them if the noun is a de-word. Nouns ending in -ing, like overstroming, are always de-words. If it were a het-word instead, there would be no extra -e (for example: zo'n bizar probleem).

Why doesn't extreem get an -e ending?

Extreem is used here after the verb is to describe the subject, rather than sitting directly in front of a noun. In Dutch, adjectives in this position never get an -e ending.

If I wanted to talk about plural "floods", would I still use zo'n?