Get your TAFs right
 

G'day chaps,

I see a lot of people (including myself) looking at 'PROB30s and PROB40s' on their TAFs and remarking, "30% probability of this, 40% probability of that..." etc etc.

However I was recently enlightened/corrected to believe that this is in fact nonsense and that a '30' can be thought of as 'low probability' and a '40' as 'high probability'.

This does seem more logical for two reasons I can think of, 1) Why is it ONLY ever '30' or '40', and 2) A forecast cannot generally be absolutely certain of the future weather anyway, let alone label it with a quantitative value in the form of a percentage.

Any ideas?

Cheers, Jack.

Roughly right mate. PROB40 is indeed a 40% probability. However, up to now only PROB30 and PROB40 are allowed to be quoted by international agreement. PROB40 can be thought of as more likely than a PROB30, but only a 'moderate' chance all the same. Anything more likely than that will probably(!) be forecast as a TEMPO - i.e. lasting for periods < 1 hour.

I would interpret PROB30 as relatively unlikely, but possible. It would probably not stop me going somewhere, depending on what the rest of the forecast was (you should look at the whole picture an build the weather 'story', rather than studying individual terms in the forecast IMO). PROB40 is the same, but I would pay a bit more attention, particularly if it was TS forecast.

In looking at these things in general, always remember to look at when the forecast was published - it may be several hours before you get it and looking at other sources (TV, internet, out the window etc) can and should be used to pin down forecast accuracy and likely developments.

Good question though.

Always fly with the assumption that the very worst conditions forecast in the TAF (regardless of probability) are going to occur. That way you have the best chance to avoid getting yourself into major difficulties en-route. Local weather conditions have a nasty habit of doing things that just don't appear in the general area forecasts or in the airfield TAFs or METARs.

....unless you're answering an ATPL meteorology question!!!!

Ha! How true. But then we know how little resemblence to reality there is in some of those questions...

I always try to be optimistic, and when I see Prob 30 or 40 I view it that 70% or 60% chance it aint gonna happen. But I am not completey stoopid as the mere mention of TS or CB will ensure my full attention

If they posted PROB50 then they would basically be saying they don't know; and where does that get anyone?
Anything higher than 50% is just an inverse of anything lower.
Clear as Mud, I thought so?

H.

Here's my take on it:

prob 30 - it might happen but we think probably not

prob 40 - it probably will happen but might not

This seems to work really well with the Lyneham, Brize and Bristol TAFS which I rely on daily!

Cheers for now.