Why does the TAF provide forecasts at irregular intervals?

A TAF is published 4 times a day at 6 hour intervals. However if you look at the forecasts within a TAF one might be at a 4 hour interval and the next at an 8. Just look at this example:

TAF KPDK 071124Z 0712/0812 32009KT P6SM SKC
FM071600 33011G18KT P6SM FEW045
FM080000 32008KT P6SM SKC


Why not publish at consistent intervals?

I assume you're referring to the FM (FROM) groups. The TAF itself is published at regular intervals but sometimes the conditions are expected to change significantly in between those times; obviously, the weather doesn't operate on a neat schedule. So the TAF will show the expected time of those changes.

FROM Group: ie. (FM1600)

The FM group is used when a rapid change, usually occuring in less than one hour, in prevailing conditions is expected. Typically, a rapid change of prevailing conditions to more or less a completely new set of prevailing conditions is associated with a synoptic feature passing through the terminal area (cold or warm frontal passage). Appended to the FM indicator is the four-digit hour and minute the change is expected to begin and continues until the next change group or until the end of the current forecast.

A FM group will mark the beginning of a new line in a TAF report. Each FM group contains all the required elements -- wind, visibility, weather, and sky condition. Weather will be omitted in FM groups when it is not significant to aviation. FM groups will not include the contraction NSW.

• What does the contraction "NSW" mean and why wouldn't it be included? I've watched enough Australian V8 Supercars racing that the only thing coming to mind is "New South Wales", but I'm pretty sure that ain't it. – FreeMan Jun 7 '16 at 20:33
• @FreeMan No Significant Weather. It makes no sense in a FROM group because if there was no significant weather, there wouldn't need to be a FROM group in the first place. – Pondlife Jun 7 '16 at 20:39

This is actually very simple.

The reason is because the conditions shown on a given line are implied to be current until the next line provides a change. If the conditions forecast in a given line are expected for 3 hours, then the next line will be three hours later. If it is 5 hours, then the next line will be five hours later.

When you see TEMPO, then those conditions replace the previous FM line, but only for the block of time in the TEMPO, and then it reverts back to the previous FM line's information until a new FM line replaces it for good.