Wrote a long story then deleted it because realized the essence of my question is:
In summer, there is a common pattern of afternoon thunderstorms. Do you try to navigate between them?
i.e. forecasts are showing this for tomorrow. Does it mean it’s a no go window completely for VFR pilot, or do you wait for tomorrow and check the actual radar, and decide if it’s a go / nogo depending on the gaps between the cells ?
@par you may have answered your question: it is quite typical that convective activity builds in the afternoon, if of the popcorn, area form, therefore trying to get the flight done before noon is a good tactic. If there is also orographic lift more so.
My SOP is no embedded CB and giving isolated CB a wide berth. I have a stormscope but use it for strategic avoidance, if painting lightning you want to be 40nm away.
A GA light aircraft will not outrun/outclimb build ups so dodging scattered cells in the afternoon is not recommended, especially low level in poor summer haze visibility.
another quick take: if those TS are part of a frontal system, I’d wait for it to pass. If not part of a system, they can usually be slalomed or zigzagged easily.
Of course your own suggestion are more than valid, push the go/no-go decision as late as possible (the old adage, better on the ground wishing…), and perform the flight early in the day, are good tactical tools.
Talking showers, not sure about your experience, but an easy game leading into IMC is pushing a flight into increasing rain. To maintain VFR, straight forward view is necessary, and having to look thru the side glass is a terrific pointer that the time to turn around has come…
Mostly, this stuff, common over mainland Europe for much of the summer, is not frontal. This is today’s MSLP; there is a bit of a front in this case but mostly it is the troughs and these translate into a PROB30 TEMPO TSRA.
It is OK to do a little local flight, avoiding the stuff visually, but on anything longer you could find yourself cornered. There was a famous case of a twin turboprop commercial flight which this happened to; after several go-arounds at various airports it landed eventually on a disused mil runway which had a load of earth dumped in the middle to make sure anyone landing broke their aircraft, and it broke off its landing gear. I avoid this wx because flying below cloud is deeply dodgy, and one cannot outclimb it; the tops are likely to be FL300.
Best approach is to fly very early in the morning, but beware: when the clouds start to build up, the tops tend to rise at 50-100fpm and as you approach, it climbs faster than you can reach it 
This tomorrow is not what you seem to be asking about. I mean it’s not summer and these will not be afternoon thunderstorms. Now if your question is about that, then we can talk about that. But if your question is about tomorrow that’s a whole other topic. Tomorrow you have a low pressure over Germany, with associated bad weather, you’ll have poor visibility of sub 10km, you have lots of thunderstorms foretasted and low ceilings with gusts. Tomorrow I wouldn’t fly. Now locally you might be able to fly and do some t&gs at your home base but I wouldn’t. Not in the forecasted weather.
As to dodging isolated CB’s do bear in mind that airspace restrictions could catch you out.
par wrote:
In summer, there is a common pattern of afternoon thunderstorms. Do you try to navigate between them?
In Summer, the usual tactics is to fly in the morning. In those situations where TS/SH are not part of a frontal passage but what is known as “Tagesgang” (daily pattern) where convection is observed in the afternoon/early evening, it makes sense to fly as early as you can in order to avoid them.
If a front is involved, i’d plan to fly either before with a good time period before it is predicted to arrive (to be on ground at least 2 hours prior) or behind it if the direction of flight allows that. Fronts mean embedded CB’s as well as possible Squall Lines, so that is weather where you really want to stay out of.
par wrote:
Does it mean it’s a no go window completely for VFR pilot, or do you wait for tomorrow and check the actual radar, and decide if it’s a go / nogo depending on the gaps between the cells ?
In the situation depicted on this pic I would not fly at all. As this is 1700, it refers to the evening. Looking at the actual situation today, there are already significant lines around on a line between Luxembourg and Frankfurt approximately as well as other shower cells. On the other hand most of Southern Germany is still ok. Local flying or shorter enroute legs outside the areas where convection is detected may be possible but with caution.
While looking at the radar gives you a good situational awareness for the actual time, predicting where this stuff is going is a different matter. If you wish to fly in such situations, again the earlier the better but with caution and giving whatever spots you see on the radar a wide detour. It can also help to talk to the meteorologist on duty for specific routes.
Peter wrote:
There was a famous case of a twin turboprop commercial flight which this happened to; after several go-arounds at various airports it landed eventually on a disused mil runway which had a load of earth dumped in the middle to make sure anyone landing broke their aircraft, and it broke off its landing gear.
The case you refer to was a Saab 2000 which got itself messed up in a cold front. IMHO it was a case where the crew was not fully on top of what they were dealing with. They arrived over their destination right as the front reached it and then tried to outrun the front in easterly direction rather than divert behind it towards the west. Arriving over their diversion airport, they again were faced with the same weather they diverted from in the first place and diverted to a disused military airfield, whose runway had been shortened and the closed parts had obstacles put to avoid illegal car racing. In the light they had, they never saw the obstacles until it was too late, also information given to ATC about those obstacles was not passed on to the crew and the markings were insufficient. But the core reason they got to this point was that their tactics for diversion meant they were not really aware what they were dealing with. Behind the front, there were many diversion airports which were or became open. Even holding over the destination would have worked better, as by the time they landed near Berlin, the front had passed their destination.
IMHO it was a case where the crew was not fully on top of what they were dealing with.
I doubt the average GA pilot would do any better. He doesn’t have CAT3 either.
While looking at the radar gives you a good situational awareness for the actual time, predicting where this stuff is going is a different matter.
Radar images are not a problem. You can get them for free from meteox.com, or during flight with the Golze ADL products. The problem is doing that over any distance, and with the inherent delays in delivery.
I’ve just scrapped a flight to Deauville (1500Z pic)
and this kind of convective wx can be found for month after month over mainland Europe. As posted already, fly in the morning And yes this can be a problem if trying to fly say UK to Croatia when one cannot get airborne until say 9am!
TAF LFRG 190500Z 1906/1915 VRB05KT 2000 BR OVC002
BECMG 1906/1908 9999 NSW BKN008
BECMG 1908/1911 BKN020
PROB40
TEMPO 1912/1915 4000 SHRA BKN020CB
And yes this can be a problem if trying to fly say UK to Croatia when one cannot get airborne until say 9am!
I’d split this into two legs, spending night somewhere nice in France or Italy and continuing on the next morning. However, this doesn’t work if you’re planning only weekend trip and obviously long PPRs in France don’t help.
