Greetings all and apologies for the long winded ramble in advance….
In the context of a VFR flight, I’d be interested to get other people’s views on how you approach TAFs with PROB40 TEMPO of non-VFR conditions?
Today I was due to fly to Ostend for a day trip but based on the very early morning weather inspection it was clear it was going to be a no-go for a VFR flight so I decided to cancel the trip.
At this point, I would ordinarily start looking at where else I could go for a day trip however my closest TAF(EGSS) was reporting PROB40 TEMPO 2812/2819 4000 +SHRA TSGS as were other TAFs in the vicinity. On this basis, and the fact I needed to be home this evening without fail, I decided to cancel flying today altogether and go back to bed!
As it turns out, in subsequent TAFs the PROB40 became PROB30 but more importantly the heavy showers and thunderstorms did not materialise at all. Aside from some lingering murk and low bases, by midday it was perfectly decent flying conditions for a local jaunt.
I know this is a personal thing, but in hindsight I think I may have been over cautious and paid too much respect to the initial PROB40 TEMPO and missed out on some flying today. Of course the opposing view will be that we are playing with probabilities and there is a chance it could have turned bad. Anecdotally however I see this in my local TAFs quite frequently and the condition rarely prevails. I am fully aware of what each TAF component means individually but I have a strong suspicion that actually this is a case of the two combined being greater (or less) than the “sum of the parts”
To this end, I’d really like to get people’s views and experiences of this topic
Thanks
Aviator Delta
I used to be of the opinion that “PROB30 CB’s” etc should be interpreted as 70% chance of no CB,s, until the day I found out different. I now look at “PROB30” as a warning to stay in VMC below any cloud bases. Have a look at my video link. The lightning flashes were all around although you can’t see them on the video.
I personally assume PROB40 as “will happen” and PROB30 as “most probably will happen” when the trend is negative and both as “might happen” if the trend is positive. The result is that I sometimes reject a possible flight (as happened yesterday for me as well) but this is better than going and suddenly being caught in a situation worse than you are prepared for.
One of my flying instructors once told me, almost in jest: Prob 30s don’t happen. Prob 40s always happen. It’s surprising how close that appears to be to the truth and it makes a mockery of the difference in probability supposedly being only 10%. (That can’t be right btw : how can the forecasters get to such a small difference in probability with their forecasting when they never use any other percentages? (We never see a Prob20 or a Prob60. Why is that?)
Secondly, as I’m sure you learned too during your studies, there was always a one-word lesson for thunderstorms: Avoid. They are uniquely dangerous things and I have in the past been flying whilst they have been around and they are scary from the air even from afar. If I see TS in the TAFs enroute or at the destination, I don’t fly. The actual geographic incidence of thunderstorms is apparently very difficult to forecast, and seeing them in the TAFs just means that the atmosphere is charged up and thunderstorms will occur in rather unpredictable places at rather unpredictable times (within a few hours). Personally, I fly mostly for fun, and flying in such weather is not fun.
Howard
AviatorDelta wrote:
I am fully aware of what each TAF component means individually but I have a strong suspicion that actually this is a case of the two combined being greater (or less) than the “sum of the parts”
OTOH, knowing how convective weather works, with a TAF like PROB40 TEMPO I would expect that the enroute conditions would be more like PROB40 with no TEMPO.
Howard wrote:
(We never see a Prob20 or a Prob60. Why is that?)
You don’t get PROB60 because the WX with the largest probability should be the main weather – you would get a PROB40 instead.
There used to be both PROB10 and PROB20, but there was a change of rules for forecasting so that PROB10/20 are not used. I guess that the general uncertainty of a TAF is great enough that PROB10/20 doesn’t make sense. I know that at least Swedish airlines disregarded a PROB10 completely.
Thanks for your responses guys and I am completely in agreement regarding paying the required respect to TS.
Like you Howard, I only fly for fun and will always cancel a flight if the fun bit is in jeopardy. Certainly a PROB40 (and even a PROB30) on its own of TS will always lead me to cancel a flight.
I guess for me the bigger question is when the PROB is combined with the TEMPO. I have also heard the jestful saying about Prob30’s don’t happen and Prob40’s always happen. When you throw the TEMPO in, in real terms does it make the Prob condition much less likely?
For this reason, I’d really like to get inside the head of the forecaster to know what the “real story” is when making a PROB TEMPO forecast.
I’m not sure if it would change my approach but at least some of the mystery would be removed 
AviatorDelta wrote:
I have also heard the jestful saying about Prob30’s don’t happen and Prob40’s always happen.
If I remember correctly in (some) commercial operations PROB40 should be considered as happening and PROB30 can be taken as information only. I think the saying you’ve heard comes from these (official) rules of some operators and probably has little to do with statistics.
As AA says, a Prob60 means that that’s what the forecasters believe will happen, and therefore it is the forecast! If the forecasters think there is a 60% chance of conditions A and a 40% chance of conditions B, then you’d never forecast conditions B with a 60% chance of conditions A!
Once the conditions go over 50% prob, they become the forecast conditions.
Prob 20 is probably too unlikely to be useful.
Using the above, you can realise why there is such a big difference between Prob 30 & Prob 40.
Prob 40, means that the forecasters where looking at conditions that were very close to 50/50. (Given as 60/40)
Prob 30, means that there’s a 70/30 % chance. That’s more than 2:1, which makes it unlikely.
But all the focus here is on the Prob in the original post. What strikes me instantly is the TEMPO. As yourself if the Prob wasn’t there, would you fly.
AviatorDelta wrote:
TEMPO 2812/2819 4000 +SHRA TSGS
This means that for less than half the period (12-19), and lasting no longer than 30 minutes each, there will be (ignoring the Prob) thunderstorms. But basically they are moving through. If the prevailing conditions are good (10Km, good cloud base) then you can expect them to be isolated. So the questions are:
1. Can you see them (ie flying outside cloud in good visibility) &
2. Can you navigate around them, or is your route narrowed by terrain and airspace, &
3. Can you hold for 30 mins (if one is located over your destination when you arrive), to wait for it to clear off &
4. If the TEMPO part it wrong and they become more widespread, do you have a diversion place to go.
I see these quite regularly here. They usually (at least here) mean isolated CB’s and heavy showers, but they are easy to go around. I imagine in other areas they might means heavy afternoon TS.
Dublinpilot nailed it.
I would also react differently to that forecast in the UK in May to Italy in August.
