Temperatures are forecast to rise in the canton of Valais from highs of 26 degrees on Monday to above 30C in the canton of Valais on Thursday, where the mercury is expected to jump to 33C on Friday, MeteoSwiss, the national weather office said.
Elsewhere in the country, by the end of the week it should reach 31C from one end of the country to the other in Geneva, Basel and Chur (in the canton of Graubünden), the office said on Monday.
Zurich is expected to see a high of 30C, while Bern can expect a maximum reading of 29C on Friday, MeteoSwiss said.
The warm weather is expected to persist through the weekend although thunderstorms are possible in some regions.
The return to summery conditions follows an erratic spring with temperatures higher than average in May (by 0.7 degrees), April (up 1.4 degrees) and March (1.2 degrees), MeteoSwiss said.
However, there was less sunshine and more rain, the office said.
Sion, the capital of Valais, recorded the highest rainfall in 150 years.
The canton also experienced extremes in the middle of the month, recording a high of 30 degrees on May 13th, followed a day later by a high of 15 degrees while snow fell at an altitude as low as 550 metres.