December 2012 Synoptic Summary
|Highest Temperature||48°F at Adak in the central Aleutians on the 12th|
|Lowest Temperature||-60°F southwest of Fort Yukon in the northeastern Interior on the 16th and 17th|
|Highest Average||39.0°F at Metlakatla in southern Southeast Alaska|
|Lowest Average||-30.5°F at Birch Creek in the northeastern Interior|
|Most Precipitation||24.31 inches at Little Port Walter in southern Southeast Alaska|
|Most snowfall reported||74.8 inches at Annex Creek in northern Southeast Alaska|
|Most snow on the ground||80 inches at the top of the Eaglecrest Ski Resort at elevation 2,600 feet near Juneau in northern Southeast Alaska on the 17th and 31st|
The pattern of circulation over Alaska in December was quite similar to that of November. Vigorous low pressure systems were in the Gulf of Alaska and high pressure was over the Arctic for most of December. There was an important difference: three low pressure systems emerged from the North Pacific and moved overland into southwest Alaska.
The first appeared as a 950 millibar (28.05 inches) low in the eastern Aleutians on December 8. Gale to storm force winds and heavy seas swept over the eastern Bering Sea all day. The winds gradually weakened as the low moved over Southwest Alaska on December 9, and dissipated on December 10. Light snow fell over the northeast Interior, the southwestern mainland, the Cook Inlet regions and Prince William Sound, finally ending a period of very little or no snowfall at all.
The second low moved from the Bering Sea to Southwest Alaska on December 11 and held over the Yukon Delta on December 12. It then moved east and dissipated over the southeastern mainland on December 13. Extensive and generally heavy snow fell over the northeastern Interior and the southwestern mainland. A number of new daily snowfall records were set. The influx of moisture from the southern Bering Sea and the eastern Aleutians made a profound, though rather brief and welcome change. Winter sports and recreation out of doors finally proceeded normally.
The heavy snow in the Interior caused blizzard conditions on area roads from December 11 through 14. At the height of the snowstorm, there were whiteout conditions on Fairbanks area roads. From December 18 through 31, the National Park Service issued advisories of high avalanche danger in Denali National Park and in the Chugach Mountains southeast of Anchorage.
The third and final low in the series moved northeastward over the Bering Sea on December 16 and reached Norton Sound on the morning of November 17. It dissipated north of Barrow on December 18. Colder air then swept into western Alaska. Thereafter, the pattern of ongoing Gulf of Alaska lows resumed.
The onset of southerly flow over the mainland at the end of November brought about a dramatic rise in temperatures over the southern mainland and the Interior beginning on December 29. This change was not at all enough to bring up monthly temperatures very much. It was accompanied by onshore flow over Southeast Alaska. Some new daily precipitation records were set at northern Southeast Alaska stations on December 30 and 31.
In addition, the inflow of warm air from the mid latitudes of the North Pacific provided the energy to create a storm in the Gulf of Alaska on December 31. The low pressure system brought in by this flow pattern strengthened rapidly on New Years Eve, and reached the Kenai Peninsula that evening. Strong southeast winds swept into Turnagin Arm, southeast of Anchorage, and damaged a tram at the Alyeska Ski Resort. Such a mishap had never before occurred there. There were unofficial reports of winds over 100 mph at some Kenai Peninsula mountain automated weather stations.
The temperature pattern in December was quite similar to the pattern in November. An area of December temperatures mostly from 2°F to 4°F below normal encircled Alaska's coastal regions, and well below normal temperatures remained over the southeastern Interior and the Copper River Basin. Temperatures in Fairbanks were so cold in the first few days of December that the annual ice carving competition was almost delayed.
Although the cold weather in November and December was not severe, the duration of the cold spell was unusually long. One reflection of this and the thin snow cover was the depth of the frost layer. On December 20, the frost depth in Fairbanks was measured 30 inches below the ground surface. At the same time of year, the frost depth in 2011 was 15 inches and in 2010 12-13 inches.
River and lake ice around Fairbanks continued to thicken in the cold weather in most of December. The ice thickness increased from 13 percent to 93 percent during December.
The precipitation pattern was different from Novembers. A large area of above normal precipitation extending from Southwest Alaska to the northeastern Interior reflected the path of the second low on December 11 through 13. Precipitation was well below normal in Northwest Alaska, where the wind had blown much of the snow away, leaving large areas of barren, nearly snow free land for all of November and December. The snow cover in the central Brooks Range was so thin that a predator control operation was delayed. It is not possible to track animals in so little snow, especially when it has been packed down by wind.
Precipitation was below normal over the southeastern mainland, most of Southeast Alaska and the Bering Sea region. Reduced northeasterly flow over the Arctic resulted in significantly less precipitation than in November.
The weather at sea was rough during December, even more so than is typical. Gale force winds blew somewhere over Alaskan waters on all but one day in December – on December 22 – when a weak high pressure area briefly settled over the Bering Sea.
On December 28, a huge drilling rig, the Kulluk, ran into a Gulf of Alaska storm while being towed from Dutch Harbor to Seattle. The Kulluk is a 266-foot diameter, cone-shaped vessel weighing more than 50 million pounds with a 160-foot high derrick in the middle. Storm force winds and seas as high as 35 feet shook the Kulluk and broke the towlines between the vessel and several powerful tugs. On the afternoon of December 29, the 18-person crew was safely lifted by helicopter from the ship in gale force winds and heavy seas. As the New Year began, it was not clear what would happen.
As happened in November, a number of large oceangoing freight ships sought refuge from December's storms in protected waters. There were other close calls elsewhere in Alaskan waters. On December 30, 25 to 35 mile per hour offshore winds carried a man on an ice floe away from land at Nome. He was rescued by helicopter about a mile from shore.
On December 1, sea ice covered all of Alaska's Arctic waters and waters within 50 to 200 miles of the Bering Sea coast from Bristol Bay to the east half of Saint Lawrence Island. All of the Saint Lawrence Island waters were icebound by December 10. On December 31, the ice in the Bering Sea had advanced to 40 miles south of Saint Matthew Island but had made little further advance into Bristol Bay. The Pribilof Island waters remained ice-free. At the end of December, the ice cover in the Bering Sea was similar to its extent at the end of 2011.
The ice in Cook Inlet advanced past Kalgin Island by December 7 and by December 28 had advanced to Cape Starichkof on the east shore and to Cape Douglas on the west side. Ice began forming in the waters surrounding Augustine Island on December 21. After south winds and rising temperatures began on December 29, the ice cover held, except on the east side of the Inlet, where the ice edge retreated up to Ninilchik. Due to the cold weather in most of December, ice cover in Cook Inlet at the end of December was greater than it was at the same time in 2011.
Sea surface temperatures were from 2°F to 6°F colder than normal in the Gulf of Alaska, and as much as 8°F colder than normal over the continental shelf in the Bering Sea at the start of December, and remained in a similar range through December 31.
Although it seems unlikely that wild fires could ever occur in Alaska during winter, it has indeed happened. At the beginning of December, firefighters in Palmer, north of Anchorage, were getting a 150 acre wildfire under control. The fire started on November 29 in dry, windy weather. The ground was snow free and dry. Sparks from a vehicle that crashed and flipped lit the fire, and there was nothing to stop the flames until the wind diminished. Firefighters on scene estimated peak wind gusts at 80 miles a hour. Several homes in the area were threatened, but spared by strong containment efforts. This incident was similar to a fire near Anvik in Southwest Alaska, set on New Years Eve in 2000. Dry, nearly snow free ground provided the fuel, and sparks from a snow machine striking a stone lit what became a 5-acre grass fire.
Until the temperatures rose in the closing days of December, strong temperature inversions persisted over many Interior valleys. Temperatures in the hills were sometimes 25°F warmer than temperatures on the valley floors just 300 feet below. Concentrations of fine particulate matter around Fairbanks reached 300 micrograms per cubic meter on December 20.
Alaskan volcanoes remained quiet in December. The Cleveland and Little Sitkin volcanoes, located in Aleutians, and the Iliamna Volcano on the west shore of Cook Inlet showed very weak disturbance.
Several earthquakes occurred in Alaska during December. The first was a 5.8 magnitude tremor centered 25 miles west of Anchorage on the afternoon of December 3. The quake was felt in Anchorage and as far away as Willow and Homer. Damage was limited to a few items knocked off shelves. On December 14, a magnitude 5.9 quake occurred 75 miles southwest of Shemya at the west end of the Aleutian Islands. A small magnitude 3.4 earth tremor occurred 40 miles west northwest of Fairbanks on the evening of December 18; it was felt in Fairbanks. Finally, a 4.6 magnitude earthquake occurred on Christmas Eve 40 miles northwest of Valdez.