Where has this thread been hiding for the last few montsh? I'm sure I don't remember it the first time around...
Seems like the general advice is to treat any forecast which contains CBs with lots of caution, which makes sense - but I'd also add that there are so many other factors to take into account it's impossible to give a definitive answer.
First of all, it's much easier to see the weather from the air than the ground. It's always worth calling anyone and everyone en route to see what their weather is, but failing that nothing can beat going up to have a look. Do one circuit, land, then evaluate what you saw, and make a decision. (Easier to make a decision on the ground than in the air, I find, which is why I'm suggesting just one circuit to start with).
Other things to take into account include: Where are you going? I would say that, in general (and I'm sure someone can find an exception), doing circuits with this type of forecast is fine - you can get down before the weather arrives. Flying across a deserted part of the country probably isn't. If you are flying cross country, how many alternates are there? Is there any high ground that may make it difficult to turn back? Controlled or restricted airpsace that makes it difficult to fly around any weather?
As well as flying considerations, there are practical considerations. If it's a hired or shared aircraft, do you have to have it back for someone else? Do you have to be at work the next day? Of course if you do get caught out by the weather, these things musn't influence your decision to stay on the ground and find a hotel.... but that doesn't mean that they can't influence your decision to go or not in the first place.
Someone mentioned the difference between the single-cell CBs we often get in the UK, and huge lines of CBs that occur in other parts of the world. I've not yet encountered CBs outside of the UK, so all my experience and suggestions are entirely based on UK weather - but you should certainly take local weather patterns into account.
Also, are you going VFR or IFR? I would never consider flying in IMC if CBs are forecast unless I had weather radar (and I've never yet been allowed anywhere near the controls of anything that does have weather radar!)
And probably loads more stuff that I can't think of right now!
FFF
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PS -
@ Kolibear