Come to think of it, I like the US system better. I mean PROB30 would imply a TEMPO condition, wouldn't it?
No. There's a big difference between "3000 BKN006 PROB30 0710 0200 FG OVC000" and "3000 BKN006 PROB30 TEMPO 0710 0200 FG OVC000".
In the first case, the airport might well be fogged out for the entire period. In the second case, there should be an opportunity to make an approach at some stage. Don't bet your life on it though.