The wettest month in Bulgaria is May so occasionally we get storms across the Balkan peninsular. In the month of May it is not raining every day but when it does the wind picks up and the rain can be very heavy. So, if there is a storm warning then make sure your tents and awnings are well pegged into the ground.
Thankfully the storms are brief only lasting one to two hours and then it is back to clear skies and warm sunny weather. They rarely cause any damage but can be a bit of a surprise for those first time visitors to the Balkan peninsular. By mid June the storms stop and it is hot and dry often until the end of September or early October.