Hey @thmarx,
On the engine level, Flowable does support conditional event. However, not as a start event of a regular process. It is supported for an Event Sub Process Start Event, Intermediate Catch Event and Boundary Event.
Are you sure that other engines support it on the top process level, and not an Event Sub Process?
In any case, Flowable has CMMN, which together with BPMN might be a better fit for what you need.
You can read Start Conditional Event not able to trigger - #2 by filiphr, which is related to what you are asking for.
With CMMN you can solve this by using a Process Task that has a Sentry with the expression ${temperature > 0}
Cheers,
Filip