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 for the rainfall. 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 much 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.