Formally, a number of airline's ops manuals allow crews to disregard PROB30 forecasts for a fuel decision. For example PROB30 TEMPO 1217 +TSRA could theoretically be ignored in terms of taking extra holding fuel.
In practice, any sensible crew would work out that PROB30 really means 1 time in 3 the met man will get it right so it's probably worth sticking a bit extra on.
In Private Flying terms, 1 time in 3 you'll find one of these looming over your turning point or destination so either don't fly or be prepared to vary your route or hold-off. Also remember these are very localised forecasts. Flexibility is the key to air power, someone once told me.....
