U.S. Drought Monitor Update for December 7, 2021

According to the December 7, 2021, U.S. Drought Monitor, moderate to exceptional drought covers 46.5% of the United States including Puerto Rico, an increase from last week’s 44.8%. The worst drought categories (extreme to exceptional drought) remained the same at 14.9%. 

A strong high pressure ridge in the upper atmosphere was anchored over the western contiguous United States. Pacific weather systems moving in the jet stream flow brushed the Pacific Northwest with precipitation, but then they were weakened and deflected into Canada by the western ridge. As they moved past the ridge, they strengthened and brought precipitation to the northern Plains, Great Lakes, and parts of the Northeast. Cold fronts associated with their surface lows tapped Gulf of Mexico moisture to generate precipitation across the Mississippi and Ohio Valleys. 

Weekly precipitation amounts were above normal from Montana to Michigan, from the Mid-Mississippi Valley to Ohio, and in parts of New England and the Lower Mississippi Valley. But most of the contiguous United States was drier than normal for the week, with little to no precipitation falling across much of the West, Plains, and Southeast. The upper-level ridge contributed to warmer-than-normal temperatures across almost all of the contiguous U.S.

Meanwhile, a Kona Low brought heavy rain to parts of Hawaii, ending a multi-month dry streak. The continued dryness expanded or intensified drought from the High Plains of Colorado to the southern Plains and Lower Mississippi Valley, and across much of the Southeast to Mid-Atlantic states, as well as Puerto Rico. Only a few places in the Pacific Northwest and Puerto Rico saw drought contraction. Overall, expansion exceeded contraction, with the nationwide moderate to exceptional drought area increasing this week.

Abnormal dryness and drought are currently affecting over 147 million people across the United States including Puerto Rico—about 47.3% of the population.