E.g. the 0900 TAF:
The forecasted Temp and QNH can happen from as early as 0730 onwards but can also occur as late as 1030. The prediction covers the three hours bridging / saddling the forecasted time.
With the last Temp and QNH: It can happen from 90 min before the 12 h TAF ends, but it can also happen as late as 90 min after the end (13,5 hrs from the start time of the TAF), but that will then be the updated start time of the new 12 h TAF (TAF's are updated every 6 hrs and therefore overlap, but the futher in the future you go, the more inaccurate and vague they become.
That is why you have to fuel and plan for an alternate that has much better wx and less chance of a mishap. TAF's are for planning.
During your flight you compare the predicted with the actuals (METAR) to determine how accurate the Metman was, and if needed replan, refuel, re-route or divert.
If you look at the TAF:
TAF YXXX 110835 110921 ....
TAF was finalised and published on the 11th of the month at 0835 UTC
covering the period 0900 UTC to 2100 UTC
0835 UTC is the exact time it was published by the Metman (hours and min)
The actual TAF period is just in hours because it can happen either slightly before or slightly after, no Metman can predict right down to the minute, not humanly possible. That is why there is that "bridging / saddling" time covering 90 min before and after.
Last edited by Nirak; 17th Mar 2011 at 13:07.
Reason: Adding last paragraph