Count the mana values of all other spells you've cast this turn before Call Forth the Tempest resolved, including any spells that you've cast due to Call Forth the Tempest's cascade abilities. It doesn't matter whether those spells resolved; you only need to have cast them. Conversely, if you copy a spell without casting it, you won't include that copy's mana value in the amount of damage Call Forth the Tempest deals.

Each instance of cascade triggers and resolves separately. The spell you cast due to the first cascade ability will go on the stack on top of the second cascade ability. That spell will resolve before you exile cards for the second cascade ability.

No matter what spell you cast with the first cascade ability (or with any cascade abilities that result from casting that spell), the second cascade ability will look for a card with mana value less than Call Forth the Tempest's mana value of 8.

A spell's mana value is determined only by its mana cost. Ignore any alternative costs, additional costs, cost increases, or cost reductions.

Cascade triggers when you cast the spell, meaning that it resolves before that spell. If you end up casting the exiled card, it will go on the stack above the spell with cascade.

When the cascade ability resolves, you must exile cards. The only optional part of the ability is whether or not you cast the last card exiled.

If a spell with cascade is countered, the cascade ability will still resolve normally.

You exile the cards face up. All players will be able to see them.

If you cast a card "without paying its mana cost," you can't choose to cast it for any alternative costs. You can, however, pay additional costs. If the card has any mandatory additional costs, you must pay those to cast the card.

If the card has {X} in its mana cost, you must choose 0 as the value of X when casting it without paying its mana cost.

Due to a 2021 rules change to cascade, not only do you stop exiling cards if you exile a nonland card with lesser mana value than the spell with cascade, but the resulting spell you cast must also have lesser mana value. Previously, in cases where a card's mana value differed from the resulting spell, such as with some modal double-faced cards or cards with an Adventure, you could cast a spell with a higher mana value than the exiled card.