The chart below shows the average annual temperatures and precipitation for Milan. The temperatures in Stresa, and for much of Lago Maggiore, will be similar. It may feel a bit cooler on the lake than in the city, due to the breezes and the water. Travel up into the mountains and it will be cooler still.
The link below will give you the current forecast. Note that you are looking at a map of Switzrland; these forecasts for the Lago Maggiore region are far more accurate than the forecasts on the Italian sites. Piemonte is the region just south of Switzerland, you can spot Lago Maggiore directly in the center.

Domani mattina -- tomorrow morning
Domani pomeriggio -- tomorrow afternoon
Domenica is Sunday, count the rest of the week from there.

And if you need help with the Celsius to Fahrenheit conversion, read my post on that here.