Serra Ascendant doesn't gain flying as a state-based effect. It gains flying through a static ability that continuously checks the game state. It will have flying as it enters the battlefield because you will have 30 or more life at that time. Therefore, Dragon Tempest's ability's trigger condition is met.
Additionally, state-based effects check long before the ability resolves; they check right after Serra Ascendant resolves.
May 26, 2015 3:18 p.m. Edited.
The second line on Serra Ascendant isn't a triggered ability, it's a replacement effect that modifies both the power/toughness and grants an ability--flying. It's doesn't ETB then trigger and check for 30 or more life-points and use the stack the way you are thinking. The card enters the battlefield as a 6/6 flyer if the condition is true (you have 30 or more life). Therefore, it would trigger Dragon Tempest and gain haste.
May 26, 2015 3:22 p.m.
@Azaldon: The ability is just a static ability; it isn't a replacement effect. Replacement effects apply to an event and modify it. This ability just grants additional abilities to something based on the game state.
May 26, 2015 3:24 p.m.
@ Epochalyptik: Yep you're right :) I was answering it about the same time as you and when I posted I didn't know how to edit it. I looked at your response and was like, "Yep, that is static isn't it..."