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I've also had a DE tell me that the Prob30 meant that 30% of the forecast area would see these conditions
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And what's the forecast area for a TAF?
[/ QUOTE ]sounds like one of those "correct, but who cares?" situations.
From a meteorological standpoint, precipitation probability is a number that is the product of two others:
probability that precipitation will occur in the area
X
the amount of the forecast area that is expected to be covered
So, 30% probability for an area can mean a 30% chance that the forecast area will be 100% covered by precipitation or a 50% chance that 60% of the forecast area will be covered by precipitation or a 100% chance that 30% of the forecast area will be covered by precipitation, or anything in between.
=Maybe= some sense in the general Area Forecast since a general knowledge that it includes elements of both rain and area probability can assist our planning. 80% may mean that there's a good chance that we will get wet, but also that we will have some viable alternates enroute, although even then, being a calculation, neither we nor our briefers know which of the infinite possibilities it is. But it's definitely not as meaningful when we're talking about a 5 NM radius TAF (although I have seen a small storm pouring rain down 1 mile east of a runway with not a cloud in sight anywhere else)