I'm on Vancouver Island in BC, and it's cloudy ALOT here, your plants will have minimal stretching at worst. The main problem is they just don't grow much on cloudy days so be very careful not to over water / over fertilize them when it's cloudy because it's EASY to do especially if there are a few in a row. Hope this helps and don't worry over the course of a long summer a few cloudy days won't matter much. Good Luck
2012 Outdoor: Jack Herer & Kalashnikova both from GH Seeds.