The climate in Bulgaria basically has two climate systems. Inland we find a land climate, in the sea area a sea climate.
The western part of Bulgaria has a land climate. This type of climate is known for periods of dry heat in the summer and dry cold in the winter. And this is exactly what you can expect in this part of Bulgaria. Summer temperatures can reach up to a soaring 35 degrees Celsius for weeks on end. The real winters Bulgaria offer periods of heavy snow with temperatures reaching as low as -20 degrees Celsius. Now this is in combination with little humidity and generally moderate wind.
On the sea side there is more moderate and with more wind and humidity. Temperatures do not meet the same extremes as inland. This does not mean summers are not warm and winters not cold. On the contrary, summers are excellent at the sea, but beware of sun burn. The winters tend to get nasty due to the humidity in this area.
Summers are great for parks and terraces, while in the winter a good coat will do the trick. Skiing conditions in winter are generally good. Try Vitosha mountain or Bansko!
Bulgaria also has plenty humidity though, even a lot. Heavy showers occur mostly during spring and fall. In cities streets will be flooded while in some country side areas complete inundations take place.
Beware that in winter traveling conditions can get warnings ranging from green to yellow, to orange or red. With a red alarm traveling is basically not possible. In cities snowplows will clear the roads, in the country side this is not always possible. It is advised to check the forecast frequently in these periods.
A good site to find updated weather information is: http://weather.digsys.bg/