SPECI

A SPECI is a special report. It uses exactly the same format as a METAR and carries exactly the same kinds of information; the only difference is why it exists. A METAR is the scheduled routine observation, issued every hour or half hour. A SPECI is issued between them, because something changed.

The triggers are defined thresholds rather than the observer's judgement: visibility or ceiling crossing specified values in either direction, the onset or cessation of precipitation, freezing precipitation or a thunderstorm, the beginning or end of significant wind shifts, and similar events. Crossings in the improving direction count as much as deterioration, so a SPECI can be good news.

The practical significance of a SPECI is its timing. Its existence tells you that conditions moved through a threshold since the last routine report, which is information the next scheduled METAR would have delivered too late. When both are available, the SPECI is the more current picture and supersedes the METAR before it.