TEMPO in a METAR

[Dan Winterland]

Trends in a METAR may only be included if it is written by a forecaster and not an observer. There may be a trend on the way, but it may not be reported as there may just be an observer on duty. When I was taught to read them many years ago, we were told that if there is no trend statement, then the METAR should be considered valid for one hour only.

[MD83FO]

= trend.
two hours or NOSIG

[Checkboard]

The part I dont understand is if a METAR which is a report issued every 30 or 60 mins, is issued with a TTF, which in AUS is valid for 3 hours, what happens to the validity of that same TTF when the metar is updated in 30 mins time?

As any forcast - if it is superceded by a newer forecast, the newer forcast takes precedence. The reason for the overlap is to allow an aircraft to get the forecast before take-off, and for it to last long enough to be useful for the time of landing.

Trends in a METAR may only be included... that if there is no trend statement, then the METAR should be considered valid for one hour only.

NO - a METAR without a trend is simply on observation of conditions at a particular time, and has NO validity period.