@ Linton Chilcott's question:
I browsed several documents such as the ICAO Annex 3 and the NWS regulations for TAF production and couldn't find a specified definition on how prob30 is defined.
Annex 3 - 6.2.10 reads:
Recommendation: The probability of occurence of an alternative value of a forecast element or elements should be indicated, as necessary, by use of the abbreviation "PROB" followed by the probability in tens of per cent and the time period during which the alternative value(s) is (are) expected to apply. The probability information should be placed after the element or elements and be followed by the alternative value of the element or elements. The probability of a forecast of temporary fluctuations in meteorological conditions should be indicated, as necessary, by use of the abbreviation "PROB" followed by the probability in tens of per cent, placed before the change indicator "TEMPO" and the associated time group.
A probability of an alternative value or change of less than 30 per cent should not be consiedered to be sufficiently significant to be indicated. A probability of an alternative value or change of 50 per cent or more, for aviation purposes, should not be considered a probability but insetad should be indicated, as necessary, by use of the change indicators "BECMG" or "TEMPO" or by subdivision of the validity period using the abbreviation "FM". The probability should group should not be used to qualify the change indicator "BECMG" nor the time indicator "FM".
PROB30 means the probability at a given time. Else the probability for something to happen in a PROB30 group for say a 2 hour period would be higher in comparison to that in a PROB30 group for a 4 hour period. If probability for certain weather conditions becomes higher than 50 %, these conditions are considered to be prevailing weather, temporary or persistent respectively ...