The US has every climate imaginable. The Southwest (Arizona, Nevada) is desert: extremely hot (40°C+) in summer, mild in winter. The Pacific Coast is temperate year-round. The Southeast is subtropical: hot, humid summers and mild winters with hurricane risk June–November. The Northeast has four distinct seasons: cold winters, warm summers, spectacular autumn foliage.
The Midwest and Great Plains have extreme weather: very cold winters, hot summers, and spring tornado season. Hawaii and southern Florida are tropical year-round. For most domestic tourism, the best weather windows are May–June and September–October, which also avoid peak summer crowds at national parks.