Learn Finnish: Tuuli saattaa kääntyä yllättäen. - The wind might turn unexpectedly.

Here, saattaa acts as an epistemic modal verb, meaning it expresses a possibility or likelihood ("might" or "may") rather than an ability or permission. Like most modal verbs, it requires the main verb to be in the basic dictionary form (1st infinitive), which is why we see kääntyä immediately after it.

Is saattaa an impersonal verb like täytyy, and how does that affect the subject Tuuli?

No, saattaa is a regular personal verb. That is why the subject Tuuli is in the basic nominative case. If you were using an impersonal necessity verb like täytyy (must), the subject would have to be in the genitive case (tuulen).

Why do we use the verb kääntyä here instead of kääntää?

Kääntyä is an intransitive verb, meaning the action happens on its own without a direct object (the wind turns itself). If you used the transitive verb kääntää, it would mean the wind is actively turning something else, like a weather vane or a page.

What kind of word is yllättäen?

It is an adverb meaning "unexpectedly" or "surprisingly." It is derived from the verb yllättää (to surprise). While it originates from a specific grammatical structure (the second infinitive instructive form), in modern everyday Finnish it is simply used as a fixed adverb.