Can any of you met guys clarify how the "PROB 30 / 40" info is defined and even derived?
They seem to me to have a rather nebulous definition. I have been unable to find a written resource that explains precisely what they mean. To me, "PROB 30" could mean:
1. AT ANY GIVEN MOMENT, there is a 30% chance of ...
or
2. There is a 30% chance of ... occurring AT SOME STAGE throughout the period.
Two slightly different possibilities. In general, it probably doesn't make a lot of difference, I know - in effect it means "a feeling in our water suggests that ... might happen". However, somewhere in this highly defined business of ours, there must surely be a clear definition of what it means and how it is derived. Or not.
Thanks.
Also, about once a fortnight or so, I find myself comparing a METAR with a TAF covering the period of the METAR, and find the METAR to be already worse than anything in the TAF. (But I guess it's a black art, as much as a science!) Certainly, while JAR permits ignoring certain PROB items in a TAF while selecting the Alternate, these occurences make me think it is a very good idea to temper this with common sense!