The syntax of PROBs is defined in the WMO Manual of Codes in FM51 (TAF) 51.9. Unfortunately it leaves the application somewhat open to interpretation. Here's mine:
PROB30 TEMPO <change group>
means that there is a 30% chance of the weather described in the change group occuring at some time within the period in circumstances that would fit TEMPO (i.e. less than an hour in each instance and no more than 1/2 the period)
PROB30 <change group>
means that there is a 30% chance of the weather described in the change group occuring at some time within the period in circumstances that would not fit TEMPO. That means that either the weather described in the change group, if it occurs, is expected to persist for more than 1/2 the period, or that it will last for more than an hour at a time.
Thus even in the second case, the chance of it occuring at any given moment may be much less than 30%.
(IANAMG)