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 Venice (Italy).
Here are the average temperatures.
Sunny Beach - Average temperatures
Min (°C)-103712161817151262
Max (°C)681116212528272419138
Min (°F)303237455461646359544336
Max (°F)434652617077828175665546

Precipitation is not abundant, in fact it's slightly above 500 millimeters (20 inches) per year; the driest months are July and August, when rare thunderstorms occur.
Here is the average precipitation.
Sunny Beach - Average precipitation

Winter, from December to February, is cold: the average temperatures are a few degrees above freezing (0 °C or 32 °F), but at times a freezing wind similar to the Bora can blow. Snowfall is quite frequent, though usually not abundant. At the worst, the temperature can drop to about -15 °C (5 °F). There are also relatively mild and stable periods, sometimes foggy.

Spring, from March to May, is an unstable season: the first mild days, are followed by the return of cold weather, accompanied by wind gusts. Typically, the temperature becomes 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 can be hot and muggy periods, when, however, sea breezes relieve the heat. Daytime temperatures are generally pleasant, although at times there can be heatwaves, usually short-lived, with peaks of 40 °C (104 °F).

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

Sunny Beach

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

The Black Sea, which in winter is very cold, takes a while to heat up, in fact in June it's still cool, around 20 °C (68 °F). The water temperature becomes acceptable for swimming in July, when it reaches 23 °C (73 °F), and in August, when it's about 24 °C (75 °F), and all in all still in September, when it's 22 °C (72 °F).
Here are the average sea temperatures.
Sunny Beach - Sea temperature
Temp (°C)878101520232422181410
Temp (°F)464546505968737572645750

Best Time

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 (59 °F).