Note: This Synoptic Discussion describes recent weather events and climate anomalies in relation to the phenomena that cause the weather. These phenomena include the jet stream, fronts and low pressure systems that bring precipitation, high pressure systems that bring dry weather, and the mechanisms which control these features — such as El Niño, La Niña, and other oceanic and atmospheric drivers (PNA, NAO, AO, and others). The report may contain more technical language than other components of the State of the Climate series.
Like last month, a vigorous weather pattern dominated the contiguous United States during December 2011. Several strong upper-level low pressure systems dug into the Southwest then trekked toward the Great Lakes, dropping a wintry blanket of snow that covered nearly 39 percent of the country by December 6 and 31 percent on December 24. Cold air behind the upper-level lows brought below-normal December temperatures to the West and Southern Plains, while warm air advection ahead of them resulted in above-normal temperatures across the East and Midwest to the Northern Plains. This storm track kept moisture away from the West Coast, Interior West, Northern Plains, and Coastal Southeast, where December precipitation totals were below normal, and resulted in several tornadoes in the Southeast later in the month.
The above-normal precipitation brought welcomed relief to the drought areas of the Southwest and Southern Plains. The extreme to exceptional drought area fell from 83 percent at the end of November to 67 percent at the end of December in Texas, and from 63 percent to 23 percent in New Mexico, according to the January 3, 2012, U.S. Drought Monitor (USDM). The dry start for the water year (October 1 to present) for the Far West, however, counterbalanced the improvement in the Southwest, with Oregon having the eighth driest October-December in the 1895-2011 record and California ranking eleventh driest. Seven states in the West and Northern Plains had the tenth driest, or drier, December on record. A large area of abnormally dry to moderate drought conditions expanded from Washington, across Oregon, to California and Nevada. In spite of the dryness, wildfires for December 2011 were near average, nationally. The moderate to exceptional drought footprint for the contiguous U.S. closed the year at about 32 percent, slightly higher than the end of November.
Warmer-than-average air funneled into the country ahead of, and in between, the upper-level low pressure systems. Over 600 daily high temperature records and more than 1000 record warm daily low temperatures were set across the country during the month, with 15 states from the Northeast to the Northern Plains having the tenth warmest, or warmer, December in the 1895-2011 record. But the snow cover and cold temperatures associated with the upper-level low pressure systems brought over 300 daily low temperature records and about 500 record cold daily high temperatures to the West and Southern Plains, with New Mexico having its fourth coldest December.
When averaged together, the mixture of temperature and precipitation extremes gave the U.S. the 34th warmest and 46th driest December in the 117-year record. Averaging extremes tends to cancel them out. But when extremes are combined cumulatively, like in the U.S. Climate Extremes Index (CEI), they may tell a different story. The national CEI for December 2011 ranked 21st highest, a little above the long-term average. But for the Northeast region, the December 2011 CEI ranked third highest, due in large part to unusually warm maximum and minimum temperatures and lingering moist soils from heavy rains earlier in the year.
Cold fronts and low pressure systems moving in the storm track flow are influenced by the broadscale atmospheric circulation. Four such large-scale atmospheric circulation drivers were potentially influential during December:
- Ocean temperatures and atmospheric circulation anomalies indicated that the equatorial Pacific continued in a La Niña state. La Niña this time of year (October-December) is associated with temperature and precipitation anomalies across the U.S. — temperatures are typically warmer than normal from the Mississippi River to the Interior West, with drier than normal conditions dominating most of the country, and wet conditions limited to the Pacific Northwest and northern California.
- The Pacific/North American (PNA) pattern rose from near neutral at the beginning of the month to slightly positive by the end of the month. A positive PNA this time of year (between October and January on the teleconnection maps) typically is associated with cooler-than-normal temperatures from the Southern Plains to Northeast and warmer-than-normal temperatures in the Northwest, with drier-than-normal conditions in the Northwest and across much of the country from the Mississippi River to the Northeast.
- The Arctic Oscillation (AO) pattern was positive at the beginning of December and again later in the month. A positive AO this time of year (October-December) is typically associated with warmer-than-normal temperatures in the Northern and Central Plains to the Northeast, dryness in the Gulf Coast and Lower Mississippi Valley states, the Upper Mississippi Valley to Great Lakes states, and Northeast, and wetter-than-normal conditions in the coastal Northwest. The October-December averaged upper-level circulation anomalies for a positive AO consist of a below normal pattern over the Arctic and above normal pattern across much of the U.S.
- The North Atlantic Oscillation (NAO) pattern was consistently positive throughout the month. A positive NAO during this time of year (between October and January on the teleconnection maps) is typically associated with drier-than-normal conditions in the West while temperatures trend warmer than normal from the Plains to East Coast.
The December 2011 averaged upper-level circulation anomalies are consistent with a positive AO pattern across the northern and eastern United States. The pattern of observed temperature anomalies for December 2011 and the last three months (October-December) corresponds to the La Niña and AO patterns in the Northern Plains to Great Lakes and NAO from the Northern Plains to the East Coast. The December 2011 and October-December 2011 precipitation patterns are a reasonable match for the La Niña pattern across the mid-Gulf to Southeast coasts, the PNA and NAO patterns for part of the West, and the AO pattern for the mid-Gulf Coast and Upper Mississippi Valley. However, the unusually wet December across the Southwest and Southern to Central Plains does not correlate with any of these four atmospheric patterns, but instead is more reminiscent of a springtime El Niño influence. The Ohio Valley had above-normal precipitation during December and October-December 2011. Below-normal precipitation, not above-normal precipitation, in the Ohio Valley is associated with La Niña and a positive AO, NAO, and PNA in October-December. Wet conditions here are more likely in the late winter to early spring with La Niña and positive AO, and during the spring with a neutral AO.