According to the December 19, 2017, U.S. Drought Monitor, moderate to extreme drought covers 26.2% of the contiguous United States, an increase from last week’s 23.9%. Extreme drought coverage decreased slightly from 1.8% last week to 1.3%. Exceptional drought remained absent from the Lower 48 for a 10th consecutive week.
During this U.S. Drought Monitor week, the upper-level circulation tried to maintain a ridge over the western contiguous United States and a trough over the East. But, a string of Pacific weather systems roared through the upper-level flow, distorting the pattern. Several cold fronts and low pressure systems migrated across the country, dropping precipitation across the Northwest, South, and Great Lakes to Northeast. The air masses were not too cold and did not linger long in any region.
The result was much warmer-than-normal temperatures in the central to northern Great Plains; warmer-than-normal temperatures in the Southwest; below-normal temperatures in parts of the West, Gulf of Mexico coast, and Northeast; and weekly temperatures near average elsewhere. But dry weather dominated most of the country, with above-normal precipitation limited to just the Deep South, northern to central Rockies, and parts of the Great Lakes. As a result, drought and abnormal dryness contracted from Texas to the Lower Mississippi Valley, but expanded in several areas from the Southwest to Northeast.
Abnormal dryness and drought are currently affecting over 144 million people across the United States—about 46.2% of the country’s population.