Learn German: Sie sagt, dass es heute wahrscheinlich regnet. - She says that it will probably rain today.

This is the main rule for the word dass (that). It is a subordinating conjunction, meaning it kicks the conjugated verb in its clause all the way to the end. So instead of es regnet heute, it becomes dass es heute wahrscheinlich regnet.

Do I always need a comma before dass?

Yes! In German, you must always put a comma between the main clause (Sie sagt) and a subordinate clause starting with dass.

The translation says "will probably rain", but isn't regnet in the present tense?

Exactly! German very often uses the present tense to talk about things happening in the future, especially when context or a time word like heute (today) makes it clear. You don't need to use a complex future tense here.

Why does the time word heute come before wahrscheinlich?

German sentences generally follow the Time-Manner-Place rule. Since heute tells us when (Time), and wahrscheinlich tells us the likelihood (Manner), heute comes first.