December in the West
Much of the West saw a continuation of the above normal temperatures that have dominated this calendar year. Positive anomalies were seen throughout the month until the final week, when clear skies, persistent snow cover, and inversions produced cold temperatures that moderated the monthly average to near-normal values at many valley and basin locations. Precipitation was abundant throughout much of the West as an active pattern in the North Pacific brought a series of storms and moderate to heavy precipitation.
Following a warm November, the Inland Northwest continued to be anomalously warm in December, with temperatures 3-6 F (1-3 C) above normal. Temperatures for the 2012 calendar year as a whole were above average for this area, especially throughout Montana. Billings saw its warmest year in a 78-year record with an average annual temperature of 51 F (10.5 C), and Missoula recorded its 4th warmest year at 45.3 F (7.4 C). To the south, Riverton, Wyoming saw its second warmest year since records began in 1940 at 48.7 F (9.3 C). In California’s Central Valley, Fresno tied for the 7th warmest December on record, which helped to seal 2012 as the warmest calendar year at an average temperature of 66.7 F (19.3 C). Records in Fresno began in 1948. In the desert Southwest, Tucson, Arizona had only a slightly warmer than average December, but still clinched the title of warmest calendar year in an 82-year record at an average 71.4 F (21.9 C). Phoenix, Arizona recorded its 2nd warmest year with an average annual temperature of 76.7 F (24.8 C). In contrast, central Alaska continued to follow a trend of cooler than normal temperatures that have persisted through the fall. Fairbanks was 13.2 F (7.3 C) cooler than normal this month at an average -17.3 F (-27.4 C), the 14th coldest December in a 63-year record. As Arctic sea ice extent increases after September’s all time low, the positive temperature anomalies present along Alaska’s North Slope throughout summer and fall finally gave way to cooler than normal December averages.
A heavy precipitation event that began in the last week of November in the Northwest carried over into the first few days of December. Eight-day precipitation totals for November 28-December 5 exceeded 20 in (508 mm) at Honeydew and Brandy Creek in Northern California. Over 9 in (229 mm) of precipitation was recorded at several other northern California locations, and minor flooding occurred along creeks and rivers. Just before the Christmas holiday, another series of storms brought heavy precipitation to the Pacific Coast and Intermountain West. Storm totals for isolated locations in Northern California exceeded 10 in (254 mm) of rainfall, and locations along the Sierra Nevada crest received over 5 ft (152.4 cm) of snowfall. Another weaker shot of snowfall over the last few days of the month brought Sierra Nevada snowpack to nearly 140% of normal by December 31. In contrast, on the same date last year, the Sierra snowpack stood at 14-28% of normal. These storms also brought 15.8 in (40.1 cm) of snow to Elko, in eastern Nevada, making for the 8th snowiest December there since records began in 1888 with a total snowfall of 22.1 in (56.1cm). The Pacific Northwest received above normal precipitation in December as well, sealing several top-ten wettest year titles for 2012. Seattle-Tacoma Airport in Washington saw its 7th wettest year in a 65-year record with 48.26 in (1225 mm), 130% of normal. In eastern Washington, Spokane recorded its 12th wettest year in a record that began in 1881. In western Oregon, Medford saw its 7th wettest year on record and 146% of normal annual precipitation. Further inland, though wet conditions prevailed for December, 2012 as a whole was a dry. In Billings, Montana, 2012 was driest in a 78-year record at only 7.13 in (181 mm) of precipitation, 52% of normal. Persistent dry conditions led to an extended fire season in Montana that burned over 1 million acres in 2012. Wyoming also saw a dry 2012 with locations throughout the state recording top 10 driest years. In the Pacific, dry conditions continued in Hawaii. Honolulu received only 0.01 in (0.25 mm) rainfall this month, the location’s driest December in a record that began in 1949. Precipitation during the last three months has been below 50% of normal throughout the state with the exception of the west side of Kauai and Hilo on the windward side of the Big Island.
Significant Events for December 2012
December (all month): Fern Lake Fire, Rocky Mountain National Park, Colorado: The Fern Lake fire began on October 9 and was in inaccessible terrain until winds over 70 mph (112 kph) November 30-December 1 doubled the size of the fire and forced evacuations of outlying areas of Estes Park, Colorado. On December 8 and 9, cold temperatures forced firefighters off the fire. The fire is only 88% contained. At year’s end, the number of fires in the United States in 2012 was well below the 10-year average, while the acreage burned ranked third amongst the last 10 years.
December 24: Sierra Avalanches: A rain event followed by over 3 ft (91.4 cm) of snow December 22-24 in the Sierra created unstable snow packs. Several avalanches occurred both in the backcountry and at Lake Tahoe area resorts. Two people were killed and two injured in avalanches at ski resorts on the 24th.
December 13-14: King Tide Flooding in Coastal California: Astronomical high tides combined with a large swell and moderate precipitation caused minor flooding in low-lying coastal areas of California. Damages were minimal.