Webhook (or event?) should fire as the last step in issue creation/updation

I have a webhook that triggers a branch-creation script in Bamboo. I want it to fire when an issue with priority = Blocker is created, and/or when an issue is updated to the Blocker priority.

Trouble is, the JQL (priority = Blocker) is assessed before the priority on my issue is set. Thus, the webhook doesn't fire when I want it to. In the update case, it fires only on issues whose priority was already Blocker before the update was made. In the create case, it doesn't fire at all.