Tempestuous Magic reads:
Sword Coast Adventurer's Guide, pg. 137
Starting at 1st level, you can use a bonus action on your turn to cause whirling gusts of elemental air to briefly surround you, immediately before or after you cast a spell of 1st level of higher. Doing so allows you to fly up to 10 feet without provoking opportunity attacks.
I've read the PHB on the subject, and I'm still not clear how much the character can move based on the wording.
PHB, pg. 190
However you're moving, you deduct the distance of each part of your move from your speed until it is used up or until you are done moving.
Using different speeds is also listed, but this talks about the fly spell which specifically grants a fly speed of 60.
PHB, pg. 243
The target gains a flying speed of 60 feet for the duration.
Assuming that the character has a 30 speed (base), does this mean the that:
- The storm sorcerer gets 10 additional feet of moment, and therefor can move his full distance then cast a spell to move further. (40 total feet, 10 of which is flown).
- Is still limited by his speed, but can "fly" for 10 feet. (30 total feet, 10 of which can be flown).
- Something else entirely.
You can move your normal rate during your turn because you always can and this can be broken up before and after the casting of the spell.
This flying movement is additional movement using your bonus action (which implies that you must use your action to cast the spell). It must be taken as a block before or after the spell casting; it can't be split like normal movement.
Essentially, your option 1 with some additional nuances.