December in the West
December began with an unusually early and severe cold episode over much of the West, with daily average temperatures within the month ranging from very far below to much above normal. Persistent high pressure developed over the West Coast and helped guide storms towards the Rockies and inland West while keeping the coastal states drier than normal.
A drier than normal December solidified 2013 as the driest calendar year since at least 1895, from the Los Angeles Basin northward to about Eugene Oregon, and in parts of Idaho. Downtown San Francisco received 5.59 in (142 mm), the driest year in the longest climate record in the state (164 years), and only 24% of normal annual precipitation. Further north, Shasta Dam received no December precipitation for the second time in a 71-year record. Precipitation there totaled 16.39 in (416 mm) for 2013, far below the previous record low of 27.99 in (711 mm) set in 1976. In Southern California, downtown Los Angeles received 0.24 in (6 mm) of rainfall this month, only 9% of normal. A scant 3.60 in (91 mm) of rain fell at this location during 2013, the lowest annual precipitation since records began in 1877 (previously 4.08 in / 104 mm in 1947 and 1953). Dry conditions prevailed in Oregon and Washington as well. Medford, Oregon logged its driest December since records began in 1911 with 0.36 in (9 mm) of rainfall, as well as its driest calendar year, with a total of 8.99 in (228 mm). In eastern Washington, Ephrata reported its driest December in a 65-year record with 0.02 in (0.5 mm) precipitation. Snowpack conditions were dismal in the Sierra Nevada and Cascades at month’s end, with snow water equivalent values at <50% of median. Snow water equivalent values in the Intermountain West were at roughly 50-75% of median. The lack of snow impacted ski resorts over the holiday season, many operating only a fraction of their terrain or still closed. Snowpack was near to slightly above normal throughout the Rocky Mountains.
Wetter than normal conditions were observed across central Nevada, in the Northern Rockies, and in eastern Montana. Ely, Nevada recorded 0.99 in (25 mm) precipitation, 168% of normal and the 22nd wettest December in a 121-year record. Precipitation in Billings, Montana totaled 1.63 in (41 mm) for the month, 307% of normal and the wettest December since records began in 1894. Casper, in central Wyoming, saw its 3rd wettest December in a 66-year record with 1.2 in (30 mm) precipitation.
Except for southern California, the West was mostly considerably cooler than normal this month. At Burns, Oregon, the minimum temperature of -30 F (-34 C) on the 8th exceeded all previous values for any month since records began in 1939. The monthly average of 19.3 F (-7.1 C) was also the 4th coldest December at Burns. In Winnemucca, Nevada, temperatures averaged 18.1 F (-7.7 C) for the month, the second coldest December in a 137-year record. The December average temperature in Boise, Idaho was 23.9 F (-4.5 C), 6.8 F (3.8 C) below normal and the 6th coldest December in a record that began in 1940. Salt Lake City, Utah, averaged 24.6 F (-4.1 C) for the month, the 8th coldest December in an 86-year record. Daily mean temperature anomalies ranged from -51 F / -28.3 C on the 7th to +23 F / +12.8 C on the 27th at Havre, and spanned a range from -47 F / -26.1 C to +26 F / +14.4 C on the same two days at Great Falls, both in Montana.
December temperatures averaged slightly above normal in some Southwest locations. Long Beach, California recorded three consecutive daily record highs of 83 F, 83 F (28.3 C), and 84 F (28.9 C) on December 25, 26, 27, respectively, and had a monthly average temperature 1.7 F (1 C) above normal. Records at Long Beach began in 1949. Temperatures at Tucson, Arizona averaged to 53.2 F (11.8 C) for December, 1.3 F (0.7 C) above normal. Only one day with sub-freezing temperatures was observed at Tucson this month, the fewest number of sub-freezing December days since 2000.
Further north, precipitation departures from normal were variable throughout Alaska, though considerably above normal along the North Slope. Barrow saw its wettest December on record with 1.17 in (30 mm), 836% of normal. Temperatures were warmer than normal along the western and northern coasts, and generally cooler than normal in the Interior. In Cold Bay, temperatures averaged to 38 F (3.3 C) for the month, 7.7 F (14.3 C) above normal and the warmest December in a 64-year record. Further south, windward locations on Hawaii’s Big Island, Maui, and Kauai received above normal precipitation. Hilo logged 20.2 in (513 mm) for the month, the 10th wettest December in a 65-year record.
Significant Events for December 2013
**December 15-20: Out-of-season Pfeiffer Fire in Big Sur, California: Extremely dry conditions in California provided highly flammable fuels for a fire that ignited in Big Sur, an area on the coast of central California. The fire charred 917 acres (371 hectares) and destroyed 34 homes. Fires are not uncommon in Big Sur, but typically occur between May and October.
**December 4-13: Sub-freezing temperatures threaten California citrus: California’s San Joaquin Valley experienced multiple days of hard freeze that likely caused significant damage to citrus. Impacts will be evaluated as the fruits develop.
**December (all month): Poor air quality for interior valleys: Strong inversions developed this month in inland valleys such as Salt Lake City, Utah; Boise, Idaho; Reno, Nevada; and California’s Central Valley, trapping air pollutants near the surface. In Salt Lake City, 10 days this month had air quality deemed “unsafe for sensitive groups” (24-hour PM 2.5 between 35.4 ug/m3 and 55.4 ug/m3) and one day of “unhealthy” air quality (24-hour PM 2.5 between 55.5 ug/m3 and 150.4 ug/m3). The 5-year average for Salt Lake City in December is 4 days “unsafe for sensitive groups” and 0 days “unhealthy.”