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" Use of probability groups

Recommendation

.— The probability of occurrence 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 forecast 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
associated time group. A probability of an alternative value or
change of less than 30 per cent should not be considered
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
instead 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
group should not be used to qualify the change indicator
“BECMG” nor the time indicator “FM”. "

Ref: Annex III - Met Service