The United States offers a wide variety of climates from frozen tundra to steamy tropical and from dry desert to soggy rain forest and everywhere in between. In the 50 states, the extreme temperatures have ranged well over 200°F from -80°F (Prospect Creek, Alaska ) to 134°F (Greenland Ranch in Death Valley, CA). Rainfall has varied from virtually nothing in the driest years in the southwestern desert region to 460 inches at Mt. Waialeale on the Island of Kauai in Hawaii.
The wettest regions are those in the Pacific Northwest and the southeastern United States as well as the coastal and adjoining mountainous areas of Hawaii and Alaska. In the Northwestern United States and the Alaska coastal regions, it is the steady barrage of winter season storms that brings the heaviest rains (and mountain snows). In the southeastern United States, strong storms and cold fronts bring rains in winter and then in the warm season it is the daily thundershowers and occasional tropical systems that keep it wet. In Hawaii, it is the moist and steady trade winds that are responsible. The desert regions and the large expanse of semi-arid climates in the Great Basin get that way because of the shielding effect of the western mountains which remove a good deal of the Pacific moisture as systems ride the jet stream from west to east across the country.
One of the most important factors in the annual cycle is the position and strength of these storms and the polar front that separates warm air from cold. In winter the storm tracks and the polar front is suppressed well to the south, often reaching the Gulf Coast. Storms move along the polar front bringing snow and ice to their north. Although the cold air can occasionally reach the southern border states, much of the time, the deep south from southern California and Arizona to Florida stays relatively mild. This area is a haven for tourists seeking a quick escape from harsh winter conditions to the north.
In summer, the polar front is much weaker and retreats north, often into Canada. Tropical air spreads to the north and brings the heat and humidity and the scattered daytime thundershowers of summer. Low pressure moving along the front can organize these thunderstorms into squall lines. There is often a reversal of tourist destinations in the summer, as many southerners flock to the northern areas to escape the sweltering heat and humidity over the south.
In the transition seasons, the polar front migrates (north in the spring, south in the fall). In the spring, the still strong storms can feed on the reappearing tropical moisture and instability, and spawn severe weather. In the late summer and fall, the excess heat of the tropics manifests itself in the form of tropical storms and hurricanes that threaten coastal areas with wind and storm surge and bring flooding rains inland.