Basel is the warmest place in Switzerland north of the Alps. The average temperature is mostly 2-3 degrees above those of other cities. This is nice during winter, but during summer, the temperatures, escpecially in the night, can be rather uncomfortably hot. Basel is situated in the Rhine valley, and the heat is conserved by the warmed up houses and the surrounding hills. 

Best time to visit Basel is the season between Mid May and End of June, Mid August to End of October.  April and May are often wet.

Normally there are only a few days with snow in the streets. A white Christmas is hardly seen. The real Winter is relatively short from Mid January to beginning of March.

Meteo Swiss Official Weather - 5 Day Forecast