Lake Garda: Annual Weather Averages. July is the hottest month in Lake Garda with an average temperature of 29°C (84°F) and the coldest is January at 6°C (43°F) with the most daily sunshine hours at 12 in July. The wettest month is September with an average of 86mm of rain.Loading weather data.uly is the hottest month in Lake Garda with an average temperature of 29°C (84°F) and the coldest is January at 6°C (43°F) with the most daily sunshine hours at 12 in July. The wettest month is September with an average of 86mm of rain. Loading weather data.