What does "PROB40 2102 1/2SM +TSRA" indicate in a TAF?

The indication "PROB40 2102 1/2SM +TSRA" in a Terminal Aerodrome Forecast (TAF) provides specific information about anticipated weather conditions during a particular period. The "PROB40" signifies a 40% probability of the weather event occurring within the specified timeframe.

The following part of the notation, "2102," refers to the forecast period, which typically means from the 21st to the 24th hour of the given TAF. The "1/2SM" indicates visibility, specifically that the visibility could be reduced to half a mile. The "+TSRA" indicates the presence of thunderstorms (TS) and heavy rain (RA), denoted by the plus sign (+) before the "TS."

This combination reveals that there is a significant probability (40%) of experiencing low visibility (1/2 mile) along with the occurrence of thunderstorms and heavy rain, which could have major implications for flight operations. The specificity of the visibility measurement alongside the weather phenomena described supports a comprehensive understanding of potential adverse weather conditions, making it crucial for flight planning and safety assessments.