Probability Forecast:
The probability forecast, denoted by the contraction PROB in the TAF, is equivalent to "CHC" in the domestic FT. The PROB statement is immediately followed by a two-digit percent (always 40 in NWS-prepared TAFs), a space, and the beginning and ending time (UTC) in which the probability of occurrence is valid. Thus, PROB40 1820 would indicate a chance probability of occurrence (30 to 49 percent) of a forecast condition between the hours of 1800 UTC and 2000 UTC.
It is important to note that the PROB group is to be used by NWS offices only to forecast the probability of occurrence of a thunderstorm and associated precipitation, or a precipitation event with associated changes in wind, visibility and sky condition. As with CHC in the FT, the PROB group is NOT allowed in the first six (6) hours of the TAF. Further, the group is NOT allowed as a direct modifier of another change group in NWS-prepared TAFs. For example, PROB40 TEMPO 1517 is not allowed in our TAFs.
