The
Bulgarian National Institute of Meteorology & Hydrology is probably the most prestigious official site, unfortunately their written forecast is in Bulgarian - they are forecasting rain & thunderstorms in Western & central regions of the country until around midnight tonight, resuming again in those areas tomorrow. Eastern Bulgaria is forecast to be mainly sunny in the morning with maximum temperatures between 27 and 30 degrees Celsius.
The extended local forecast
HERE is forecasting continued thunderstorms in some areas as far ahead as Saturday. Any forecast beyond a few days is, at best, educated guesswork.
You'll find accuweather's 15-day forecast for Sozopol
HERE, if you choose to believe that a forecast that far in advance can have any accuracy.