Thanks again for your views. The preceding posts from dublinpilot and Josh are getting to the crux of the matter. It’s knowing what this actually means in the real world which should ultimately drive the reaction.
To dublinpilot’s point number 2, flying in the southeast UK mostly between the various Class D’s, my ability to deviate off course in these conditions is not as flexible as I would like it to be. As such, this factor goes into my decision making which means I am more than likely to cancel my flight. On the other hand, this also means I don’t get to experience for myself what actually prevails and therefore I miss out on building up that field knowledge and will always err on the side of caution… It’s a bit of a chicken and egg situation.
Ultimately I am sure this comes down to experience. I am a mere ~200hr PPL so I am hoping this field knowledge will come with time.
Thanks again
Aviator Delta
Aviator. If it makes you feel any better, I was attempting, yesterday, Londonderry, to Exeter. I cancelled, due to exactly your predicament. The South West of England was a number of PROB 40, with TSRA. I also take into consideration the distance I am travelling, combined with my projected arrival time at destination. Nothing worse than taking off, aborting, added to the fact that you can not get back to your home field. As happened yesterday, I had a window to go, then Londonderry closed up with TSRA. So, as with all flying, sound prep, sound planning, and then, sound decisions. Yesterday was a pretty strange day weather wise, all over the UK.
I think there is a big misunderstanding about the nature of a TAF and an area forecast.
PROB30 does not mean there is 30% probability of the phenomenon, it means there is a 30% probability of the phenomenon occurring within 5 miles around the airport.
Let’s take a scattered front with almost certain CBs and thunderstorms, but the thunderstorms are widely spaced.
100% probability of encountering thunderstorms en-route.
30% probability of one of those actually moving through the 5-mile zone around the airport covered by the TAF.
Thanks BeechBaby. It doesn’t make me feel better that you had to cancel your flight but it certainly does make me feel better that others are contemplating the same predicaments. Those are exactly the considerations that I weigh up therefore it’s comforting to hear likeminded views.
I agree, it was a strange weather day yesterday. In fact, it’s felt like a strange weather year so far in 2016!
@Cobalt. That’s exactly the principle I am working on. I assume that it is almost certain that I will encounter a TS en-route and hence my basis for binning the flight. It’s the validity of my own “cautiousness” that I am questioning really.
Yesterday we crossed France from LFRQ to EDSB. Direct routing is about 490 NM. Regarding the TAFs, the SigWX Charts, Weather Radar and Satellite Pictures, we departed early, have chosen the northern route around Paris and were even routed a little more to the north than planned. But we had a 3:35 hours smooth flight, without turbulences and the buildups always in save distance.
What is the point. A negative TAF, even a series of negative TAFs means not necessarily not to go. It is always the whole picture, we should focus on. In flight: go or no go depends also on the equipment of the aircraft. Equipped with stormscope, weather radar and/or the ADL120 and with the knowledge to interpret these tools you might get a nice, smooth flight although the weather is convective arround you.
BTW: Sometimes I did land, waited just half an hour and continued with my flight immediately after the TS has passed passed by with blue sky.
AviatorDelta wrote:
I guess for me the bigger question is when the PROB is combined with the TEMPO.
I interpret the PROB40 TEMPO RASH TS as saying that there is 40% probability that the convective activity will be great enough to create CBs with TS. If it is then by the nature of convective weather CBs are going to form and dissolve — and most likely move — so that showers and TS will come and go. Thus TEMPO.
If I remember correctly in (some) commercial operations PROB40 should be considered as happening and PROB30 can be taken as information only
I don’t know the full details (I know some people on here do so maybe someone will post them) but AOC ops can disregard a PROB30 unless it is a PROB30 of a persistent condition such as fog.
Accordingly, the tail is wagging the donkey here and the PROB30/PROB40 convention is used because it corresponds with AOC ops rules. It isn’t done for GA. Nobody does anything for GA, especially not wx forecasting 
GA services are provided
If one was looking for a simple rule then, yes, a PROB40 is fairly likely to happen, whereas a PROB30 is chucked in as a CYA (cover your ar*se) measure. The combination of these enables the Met Office to claim some very high wx forecasting accuracy, even if many/most of the forecasts don’t actually materialise 
That’s an interesting perspective Peter and certainty food for thought.
On a separate topic, Alderney looks lovely.
FWIW, IMHO, limiting the reports to PROB30 or PROB40 is self evidently a load of rubbish.
There is no way that any computer wx model (which is how all wx forecasting is done today) is going to chuck out only those two probabilities.
So there has to be some totally artificial reason for doing it that way.
And as I said above IMHO it is driven by the required interpretation of specific conditions according to AOC ops manuals.
Many years ago, when my sons were little, and before them, I used to find myself in the USA for weeks at a time, and I recall the wx forecasts on TV there. They would give a 55% probability of rain on a given day, 75% on another day, etc. That made sense.
PROB30 & PROB40 is nonsense in terms of illustrating an actual % probability.
And even assuming 30% means 0 to 30% and 40% means 40% or more, still means nothing, because 0% is almost impossible and 100% is equally impossible.
