An abrupt rise in temperatures will begin on Wednesday after Tuesday brings showers to much of Southeastern Bulgaria and other regions, private Nova TV station's forecast says.
A Code Yellow warning will be in force for the regions of Haskovo, Yambol and Burgas in the country's southeast, with cloudy weather and possibly abundant rainfall also expected elsewhere in Bulgaria.
The conditions might be favourable for hailstorm, meteorologists say.
Maximum temperatures are unlikely to surpass 20-25 degrees Celsius.
On Wednesday, however, a change begins with a gradual increase of maximum values and thermoneters likely to hit up to 40 degrees in the plains.
Temperatures were closer to those in early spring for most of May and the first part of June, but the second part of the month is expected to bring summer weather to most of Bulgaria.