Locarno, where it's located

In Locarno, located in the canton of Ticino, on the shores of Lake Maggiore, 195 meters (640 feet) above sea level, the climate is humid temperate. Winters are quite cold, while summers are warm to hot, and characterized by a good amount of sunshine, but also by quite frequent thunderstorms. On the other side of the Maggia River, we find the nearby town of Ascona. Together with Lugano, Locarno is the mildest city in Switzerland.
In the cold half of the year, the wind is generally weak or absent, except when the favonio (or föhn) blows, the warm and dry wind that comes down from the Alps and is able to bring clear skies and very good visibility. On the other hand, during the warm semester, the wind is weak or moderate and follows the breeze regime: during the warmest hours, the breeze blows from the lake towards the mountains (therefore, from the south), while after sunset and at night, it blows from the mountains to the lake (therefore from the north).
Here are the average temperatures.
Locarno - Average temperatures
Min (°C)125812151817141052
Max (°C)791417212427262216117
Min (°F)343641465459646357504136
Max (°F)454857637075817972615245

Rainfall in Locarno is definitely abundant, since it amounts to 1,900 millimeters (75 inches) per year. Winter is the relatively driest season. In spring and autumn, the rains are plentiful in periods of Atlantic depressions, since the southern currents are forced to rise over the Alps and condense. In late spring and summer, thunderstorms in the afternoon or evening are quite frequent (and sometimes intense). During the most intense waves of bad weather, which occur in autumn (but sometimes also in spring), the lake may overflow, flooding the lower areas of the city.
Here is the average precipitation.
Locarno - Average precipitation

Winter, from December to February, is cold enough, but owing to the mitigating influence of the lake, it is milder than in the major cities of Switzerland, and it's also milder than in the Po Valley. Fog is less frequent as well.
There is a moderate amount of rainy days, even though the winter is the least rainy season. Snow generally falls at least once every year, and it can sometimes be abundant, but it tends to melt soon enough because it occurs in conditions of southerly winds. An average of 47 centimeters (18.5 inches) of snow falls each year. The record belongs to the winter of 1977/78, when a total of 242 centimeters (95.3 in) of snow fell.
We can not exclude very cold periods, with night frosts and temperatures around 0 °C (32 °F) or even below in the daytime. However, in these cases, the weather is usually dry. The temperature rarely drops below -5 °C (23 °F), though it can happen in the coldest winters. Here are some values recorded in the past: January 1985: -10 °C (14 °F); February 1991: -8 °C (18 °F); February 2012: -9 °C (16 °F).
However, it is enough to go to the nearby plain of Magadino, east of Locarno, to find much lower temperatures, especially on winter nights, when frosts are frequent, and in the coldest winters, considerably low temperatures are recorded; for example, the temperature plunged to -12 °C (10 °F) in January 1985, to -17 °C (1 °F) in February 1991, and to -16 °C (3 °F) in February 2012.

Summer, from June to August, is warm, and often sunny in the morning. The temperature is generally pleasant or at least bearable, in addition, the lake breeze blows. Sometimes, an Atlantic front, able to bring some cool and rainy days, can affect this area; more often, thunderstorms can break out in the afternoon or in the evening. We can not exclude some hot days, with highs around 33/34 °C (91/93 °F). More rarely, even higher values can be reached: for example in the exceptional heatwave of August 2003, the temperature reached 38 °C (100 °F), while in July 2015, it reached 36 °C (97 °F).

Locarno from above

The amount of sunshine in Locarno is acceptable in winter, when there are sunny days alternating with cloudy periods (in January, there are 4 hours of sunshine per day); in the other seasons, it is not very high, in fact, Atlantic disturbances pass over the area quite frequently in spring and autumn, while in summer, in periods of good weather, cloudiness in the afternoon can form, which can lead to thunderstorms. In July, the sunniest month, there are 8.3 hours of sunshine per day on average (in about 15 hours and 20 minutes of daylight, so the sun can be seen around 65% of the time).
Locarno - Sunshine

Best Time

The best periods to visit Locarno are spring and early summer, in particular, from mid-April to mid-June. September, too, is a good month. However, you have to take into account some rainy days in both periods.
Summer, which is the best time for sunbathing, can be hot, though this rarely happens: the temperature is usually acceptable, especially in June. As mentioned, in summer and in May, thunderstorms can break out in the afternoon or in the evening.

See also the climate of Lake Maggiore.