Temperatures will start to rise early, with morning lows not expected to drop below 20 degrees in Ljubljana and many parts of south-western Slovenia. Elsewhere, the lows will range from 15 to 18 degrees.
In the afternoon, temperatures are forecast to exceed 30 degrees everywhere. In central parts, highs will hit 33 degrees, while they will hit 34 degrees in the west around the city of Nova Gorica.
Isolated afternoon thunderstorms are possible in the highland areas of north-western Slovenia.
The hot spell will persist through the weekend, with temperatures on Sunday expected to rise into the mid-30s.