Map from Google - Sunny_Beach

The climate of Sunny Beach (Slanchev Bryag), located on the Bulgarian coast of the Black Sea, is slightly continental, with cold winters and hot summers. The average temperatures are similar to those of Northern Adriatic Italian cities like Venice.
Here are the average temperatures.
Average temperatures - Sunny Beach
Sunny BeachJanFebMarAprMayJunJulAugSepOctNovDec
Min (°C)-103712161817151262
Max (°C)681116212528272419138

Precipitation is not abundant, given that it's slightly above 500 millimetres per year; the driest months are July and August, when the rare rains occur in the form of thunderstorm.
Here is the average precipitation.
Average precipitation - Sunny Beach
Sunny BeachJanFebMarAprMayJunJulAugSepOctNovDecYear
Prec. (mm)404540554050353035455550520

Winter, from December to February, is cold: the average temperatures are a few degrees above freezing, but at times a cold wind like the Bora can blow. Snowfall is quite frequent, though usually not abundant. At the worst, the temperature can drop to about -15 °C. There are also relatively mild and stable periods, sometimes foggy.

Spring, from March to May, is an unstable season: there are the first mild days but also sudden returns of cold weather, accompanied by gusts of wind. Typically, the temperatures become permanently pleasant in the second half of May.

Summer, from June to August, is hot and sunny, with a few downpours or thunderstorms, usually of short duration. At night it's quite cool, especially in June, while in July and August there may be hot and muggy periods, but in which sea breezes relieve the heat. Daytime temperatures are generally pleasant, although there can be heatwaves, usually short, with peaks of 40 °C.

Autumn, from September to November, is initially mild and often sunny, but then gradually becomes colder; sunny periods alternate with short periods of bad weather, however, the rains are generally not abundant.

The amount of sunshine in Sunny Beach is good in summer, when good weather prevails, but also in May and September, while in winter it's low, when the sky is often gray or cloudy.

Sunny Beach

The Black Sea, which in winter is very cold, takes a while to heat up, so much so that in June it's still cool, around 20 °C. The sea temperature becomes acceptable in July, when it reaches 23 °C, in August, when it's about 24 °C, and all in all still in September, when it's 22 °C.
Here are the average sea temperatures.
Sea temperature - Sunny Beach
Sunny BeachJanFebMarAprMayJunJulAugSepOctNovDec
Sea (°C)878101520232422181410

When to go

The best time to go to Sunny Beach, for a beach holiday, is the summer, especially in July and August, when the temperatures are higher. In June, it's often a bit cooler, so it's better to bring a sweatshirt for the evening. Usually the weather is still good in the first half of September, although at night it starts to be a bit cool, with minimum temperatures about 15 °C.