Highlights: December freezes in Florida, expanding drought across the South, and an erratic Western winter wet season highlighted an unusual winter. December was not only cold in the Southeast, but also exceptionally stormy in the West. However, precipitation virtually ceased across much of the West for a 6-week period from early January to mid-February, only to return for the second half of February. The primary impact of Southern drought was deteriorating conditions of pastures and winter wheat. February featured numerous weather extremes. For example, bitter cold was replaced by mild weather across the central and southern Plains and the Mid-South.
Overall, the winter of 2010-11 was cool and dry. The Nation's winter average temperature of 32.3 degrees Fahrenheit was 0.7 degree below the twentieth century mean, and represented the 39th-lowest value during the 116-year period of record. State rankings ranged from the tenth-coldest winter in Florida to the 33rd-warmest December-February period in Nevada. Meanwhile, winter precipitation averaged 5.51 inches (85 percent of the long-term mean). It was the third-driest December-February period on record in Alabama, Louisiana, Mississippi, and North Carolina, but the fourth-wettest winter in South Dakota and the ninth-wettest winter in Montana. Other States reporting a top-ten winter ranking for dryness included Arkansas, Delaware, Tennessee, and Virginia. In South Dakota, it was the wettest winter since 1996-97, when massive spring flooding ensued.
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 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.
January: In a dramatic change from December, little or no precipitation fell in California during January. The water content of the Sierra Nevada snow pack, which rose about 16 inches in December, increased only an inch during January. The dry regime also stretched eastward into the Four Corners States. Farther north, warmth and melting snow accompanied a period of heavy precipitation from the Pacific Northwest to the northern Rockies, leading to some flooding.
Farther east, frequent snowfall blanketed the northern Plains and the upper Midwest, insulating winter grains but hampering rural travel and stressing livestock. In contrast, drought continued to expand and intensify across the southern half of the Plains. From November 28 to January 30, the portion of the winter wheat crop rated in very poor to poor condition increased from 26 to 52 percent in Texas, 8 to 40 percent in Oklahoma, and 25 to 37 percent in Kansas.
Elsewhere, January precipitation was generally below average across the South and East. Exceptions included Florida's peninsula and southern and eastern Texas, which received drought-easing rainfall, and the northern Atlantic region, which experienced record-setting snowfall. Despite frequent January showers across the South, long-term drought remained a concern in many areas.
February: Many parts of the country experienced opposite weather regimes during the first and second halves of the month. For example, the West turned cool and wet in mid-February, following a 6-week period of generally mild, dry weather.
Variable weather conditions also affected the Plains, where two severe cold outbreaks were followed by record-breaking warmth. Of particular concern was winter wheat on the central and southern High Plains, which - in addition to the February temperature swings - has been adversely affected by drought. From November 28 to February 27, the portion of the winter wheat crop rated in very poor to poor condition increased from 26 to 56 percent in Texas, 8 to 42 percent in Oklahoma, and 25 to 40 percent in Kansas.
In contrast, snow continued to accumulate across the northern Plains and upper Midwest, increasing the likelihood of spring flooding. Flooding was a more immediate concern in the central and eastern Corn Belt, where melting snow and late-February downpours pushed many creeks and rivers out of their banks.
Elsewhere, parts of the Mid-South and Southeast experienced some February drought relief, but drought continued to expand and intensify in southern Florida and the western and central Gulf Coast States.
Here's a link to the full report.