Learn Spanish (Spain): La tormenta ha vuelto hoy.

Vuelto is the irregular past participle of the verb volver (to return). While many regular verbs simply add "-ido" to form this part of the verb, volver belongs to a special group of verbs whose past participles are irregular and end in "-to".

Since la tormenta is feminine, shouldn't the past participle be vuelta?

No, because when a past participle is used with the helping verb haber (like in ha vuelto), it always ends in "-o". It never changes to match the gender or number of the subject. It would only change to match gender and number if it were being used as an adjective instead of a verb.

Why do we use ha for "has" here instead of a form of the verb tener?

In Spanish, there are two verbs for "to have". You use tener for possession (like having a dog). But when "have" or "has" is used as a helping verb to build a past tense (as in "has returned"), you must always use forms of the verb haber. Here, ha is the "it" form of haber matching la tormenta.

Why use ha vuelto (has returned) instead of just the simple past tense volvió (returned)?

In Spanish from Spain, this tense (ha vuelto) is the standard choice when talking about an event that happened in an unfinished time period. Because the sentence uses hoy (today), and today isn't over yet, you use the present perfect tense instead of the simple past.