[daveshorts]

I haven't read all this in enough detail to be sure this hasn't been mentioned but a lot depends on whether the sea is covered in this cloud, as if it is there is going to be very little heat getting to the sea, so there will be very little evaporation so the area above the clouds are going to be immensely dry as there is no moisture rising at all.

 If you leave the sea uncovered then you would be able to get some moisture up to your high altitude area and it would leave the area under the cloud very wet and miserable, as the constant cloud would make it cold so an onshore breeze would meet the cold air under the cloud and probably rain whenever you got an onshore breeze, at the very least you would get a hue amount of dew, fog, drizzle etc. How much would depend on the prevailing wind directions and latitude.

I would have thought that near the equator there would be lots of energy available so you would get thunderstorms, although probably most of them would be off to sea as cold air would fall off the mountains roll under the clouds and then meet the warm air rising out to sea creating huge constant thunder storms off the coast. There would probably be some fairly dry regions at the tropics as on earth, where being under the cloud would probably mean you got a lot of fog and dew making plant life possible. At UK sort of latitudes you could get some very wet areas as strong westerlies pull in moist air from the sea onto the coast so the storms would be on land.

In this general scenario when the cloud lifted you would suddenly heat a huge damp sponge, and if it was summer, you would create some really serious thunderstorms...