Priority... not really a sequence.

On down, which occurs before up.
If its off of the GCD, and the runes are available...
I will Howling Blast.

So Howling Blast has higher priority than anything else.

If I'm on a GCD, or Howling Blast could not fire off on the down press.
Then we move on to the release.
Which tries to do Howling Blast, Frost Strike, Frost Strike or Necrotic Strike.
There's a higher chance for a Frost Strike than a Necrotic Strike.

So the priority is Howling Blast, Frost Strike, Necrotic Strike.

Priority system might be a better term than Priority Sequence.