Learn Spanish (Latin America): La tormenta será violentísima en esa atmósfera densa. - The storm will be extremely violent in that dense atmosphere.

You take the base adjective, violento, drop the final vowel, and add the suffix. Because it modifies the feminine singular noun tormenta, you use the feminine singular ending -ísima. This suffix elevates the meaning from 'violent' to 'extremely violent' or 'highly violent.'

Why do we use será (from ser) instead of estará (from estar) to describe a storm? Aren't storms temporary?

While a storm is a temporary event, we are defining its inherent characteristics here. When you describe the core nature of an event or how it is expected to unfold, you use ser. Using estar describes a temporary state or a change in state, but 'extremely violent' defines exactly what kind of storm this is.

Since atmósfera starts with an 'a', shouldn't it use a masculine determiner like ese to avoid the double 'a' sound with esa?

No, the rule for switching to masculine determiners only applies to feminine nouns that begin with a stressed 'a' or 'ha' sound, like el agua or un águila. In atmósfera, the stress falls on the o (as shown by the accent mark), so it takes standard feminine words like esa and densa.

Is there a difference in meaning if I say densa atmósfera instead of ?