Near the Earth’s equator, solar heating is intense year round. Converging trade winds and abundant water vapor all combine to produce a persistent belt of daily showers known as the Intertropical Convergence Zone.
Large-scale shifting of the weight of the atmosphere between mid- and high latitudes creates climate patterns known as the Arctic and North Atlantic Oscillations. These patterns have a big influence on winter weather in the Eastern U.S.
Most of the southwestern United States is having a very dry winter. The northern parts of Arizona and New Mexico were clipped by a few drenching storms exiting California in December, but dry conditions are more the norm.
Weather in the Southeast this fall and winter is keeping up with the dry part of the typical La Niña pattern. Precipitation across most of the Southeast was "below" or "much below" normal for October-December.