Thursday will be warm for the time of year, but a cold front will move in from the west in the afternoon. Heavy rain and thunderstorms are expected in many places and will last through the night into Friday.
Forecasters said on Tuesday that Friday would bring rain and cooler weather, while Wednesday was expected to reach 26C.
See also: Wednesday's weather: up to 26C, up to 19C at the seaside, turning bad on Friday
On Thursday the clouds will break for a short time, then thicken again from the west. Parts of western Bulgaria will see rain. Lows will range from 8C to 16C, with about 10C in Sofia. Highs will range from 20C to 25C, with about 19C in the capital.
A moderate wind will blow from the south-southwest, but it will pick up in eastern Bulgaria and north of the mountains. In the northwest it will be light and come from the north.
It will be cloudier along the Black Sea coast. Early in the morning there will be low cloud and poor visibility in places, with showers mainly in the afternoon. A moderate wind, at times stronger, will blow from the south-southeast. Highs at the coast will range from 18C to 23C. The sea is 17-18C, with waves of 2-3 on the sea scale.
Cloud will cover most of the mountains. Before noon, a short spell of heavy rain with thunder will move in from the west. Rain and snow will fall over the highest peaks of Rila and Pirin. It will be about 15C at 1,200 metres and about 9C at 2,000 metres. The west-southwest wind will be moderate, but it will strengthen in places north of the ridges.
It turns colder on Friday. A strong northwest wind will bring cooler air, and temperatures will drop, most of all in northern Bulgaria. It will be cloudy with rain in many places, and in central and eastern Bulgaria the rain will be heavy in places. There will be thunder in southeastern Bulgaria. Towards evening the clouds will begin to break from the west.
On Saturday the sky will clear across Bulgaria, last of all in the northeast, where showers will continue until morning. The morning will be cold, with temperatures of 1C to 6C, and up to 10-12C in eastern Bulgaria. Fog will cover lowlands and valleys before noon. The northwest wind will ease and turn to the south-southeast, and the day will be a little warmer.
On Sunday there will be fog or low cloud in many places on the plains, and drizzle along the Black Sea coast. Highs will range from 16C to 21C. Rain will start in southwestern Bulgaria in the evening.
Monday will be mostly cloudy, with rain in many places, and temperatures will fall slightly.