On the back of What happens when a Storm sorcerer uses Tempestuous Magic to fly 10 feet straight up? if a Storm Sorcerer uses Tempestuous Magic to move vertically 10' and then casts Levitate on themselves for a vertical 20' would they then remain 30' off the ground?
Yes, you will levitate
Tempestuous Magic (XGtE, 52) states (my emphasis):
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 or higher.
In the case you describe, you are triggering the whirling gusts immediately before you cast levitate (PHB, 255). This will allow you to rise up 10' and then have the spell increase your height by up to 20'.