During December, two large-scale atmospheric phenomena strongly influenced weather patterns across the United States - La Niña and a blocking high-pressure system over the northern Atlantic Ocean. The result was stormy weather in the western and north-central United States, along with drier-than-normal conditions from the central and southern Plains into the Southeast. In addition, the North Atlantic block displaced cold air southward, locking frigid air into place across the Southeast. In contrast, mild weather accompanied the western storminess.
Western storms were most intense from central and southern California to the western slopes of the central Rockies. In those areas, heavy precipitation bolstered high-elevation snow packs and improved water-supply prospects, but also caused flash flooding and mudslides.
Meanwhile, little precipitation fell from southern sections of Arizona and New Mexico to the central and southern Plains. Between November 28 and January 2, the portion of the winter wheat crop rated in very poor to poor condition climbed from 25 to 33 percent in Kansas and from 8 to 19 percent in Oklahoma. On the northern Plains, however, a well-established snow cover helped to protect winter wheat from periodic weather extremes.
Farther east, record-setting snowfall accumulated in the upper Midwest, while cold but relatively benign weather covered the central and eastern Corn Belt. The upper Midwestern snow and cold maintained stress on livestock and hampered rural travel. The Northeast also experienced several episodes of bad weather, with a post-holiday storm causing major travel disruptions.
Elsewhere, multiple freezes struck Florida's winter agricultural region, causing extensive damage to vegetables and requiring growers to employ a variety measures in an effort to protect citrus, sugarcane, strawberries, ornamentals, and nursery crops. December temperatures were the lowest on record in dozens of communities in Florida and elsewhere in the Southeast, eclipsing standards that had been mostly set in 1935, 1963, or 1989.