Plain English Meaning
An off-schedule weather report from an airport, sent out between the regular hourly reports because something at the field changed enough to matter — like the wind shifting, visibility dropping, or a thunderstorm starting.
Full Definition
An unscheduled aviation weather observation issued between routine hourly METARs when a significant change in conditions occurs at the reporting station — for example, a sudden shift in wind, a drop in visibility or ceiling below specified thresholds, the onset or end of thunderstorms or freezing precipitation, or other operationally significant changes. A uses the same coded format as a ; the only is the report- identifier 'SPECI' in place of 'METAR' at the of the report.