According to the weather forecaster Meteo Swiss, the temperature could reach 38 degrees in Sion on Thursday.
High temperatures are also expected in Basel (37), Geneva (37), Zurich (36) and Lausanne (36) on Thursday.
The weather service has classified the rest of the weak as a heatwave and issued a degree 3 warning, described as a significant hazard. The health effects include circulatory complaints and physical discomfort.
Temperatures are expected to rise today and remain high until Friday. On Saturday, temperatures are expected to be much lower. Predicted highs on Saturday are: Basel (29), Geneva (29), Zurich (29), Lausanne (27) and Sion (30).