I was told by a friend who flies for a well known national airline that they assume PROB30 = won't happen, PROB40 = will happen, for flight planning purposes.
PROB30 seems to mean - don't blame us if it happens, i.e. you fly and get ripped apart by a CB.
PROB40 seems to mean - don't blame us if it doesn't, i.e. you cancel the flight because of a PROB40 CB and then sit and look at a blue sky all afternoon.
Tim