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Winter Settles In
December is the first month of the meteorological winter, although two-thirds of December is still autumn on the calendar. December earns the reputation as 'Dark December'. Not only is daylight at the annual minimum length but cloudiness in many areas is at the annual maximum. In the Great Lakes, parts of the northeast and Pacific Northwest, less than 25% of the possible sunshine is observed.
The long spells of dry, tranquil weather observed earlier in the fall are usually just a memory in December. December continues in most years the stormy pattern which developed in the last part of November. In fact on average, for the nation as a whole, December is the stormiest month of the entire year. Almost every storm can bring snow or now even freezing rain to hamper travelers and holiday shoppers.
Virtually any and sometimes all of the winter climatological storm tracks can be active by December. Pacific storms can enter anywhere along the west coast. Alberta lows move eastward through southern Canada or drop out of Canada across the northern Plains and Great Lakes.
Colorado lows emerge from the west-central Plains and move towards the Great Lakes and St. Lawrence Valley. Oklahoma or Texas Panhandle lows fire up and accelerate either on a northeast trek across the central plains towards the Great Lakes or more eastward towards the Ohio Valley and mid-Atlantic or Northeast. Finally, Gulf of Mexico or east coast storms can make the weather very interesting for the Atlantic and Appalachian states.
The regions with the highest frequency of low pressure this month are the Pacific Northwest, the western Plains, the Great Lakes and the Atlantic coast states. This is the wettest month of the year in the Northwest as the strong Aleutian Low spins storms into the coast on a very regular basis. December is the stormiest and usually the snowiest month near the Great Lakes. Here even after storms pass, the 'Lake Effect Snows' are flying with the air-lake temperature difference very near the annual peak. Along the east coast, December has been notable for some of the greatest snowstorms with a relative maximum frequency during the holiday period.
Behind the storms, polar or even arctic air masses can be brought deep into the southland with bone chilling temperatures and dangerous wind chills. With dry air, long nights and feeble sunshine during the days, the earth readily gives off heat at night and is slow to warm during the day. When the skies are clear and winds light, temperatures can really plummet at night.
This radiational cooling is especially strong when there is snow cover. This is because the snow insulates the atmosphere from the grounds warmth, effectively reflects back what little sunshine is received during the day, and is a very efficient emitter of radiation to space during the night. The result is lower daytime and nighttime readings. Sub-zero cold has been found as far south as Atlanta, Georgia and El Paso, Texas in December.
High pressure systems too can be massive in December. Canada is our primary source with some air masses that are so massive that they occasionally encompass the whole country coast to coast and border-to-border when they move south. Some polar highs can move into the nation from the north Pacific or north Atlantic. These air masses too had their origin over the polar continental regions (Pacific highs from Asia and the Atlantic from North America) but the air masses have been modified over the relatively warm waters. Tropical air masses are rare and usually confined to the southernmost areas. In other areas tropical air appears only in narrow streamers in the warm sector of the stronger storm systems. In the north, this warm air is usually found mostly aloft where it is forced to rise over the colder air in low levels.
The regions with the highest frequency of residing high pressure centers in December are southeast Canada (Quebec province) and the Great Basin. The main tracks for polar high pressure centers are from northwest Canada southeast to Texas or east to southeastern Canada. Polar Pacific or sometimes Polar Canadian air masses will drop into the Great Basin and build as ridging takes place aloft. When this happens, cold nights and mild days can result for prolonged periods. Inversions can develop in the subsiding air within the large scale ridge resulting in low clouds, fog and air pollution in the protected valley and basin areas. To the southwest of the high, the Santa Ana winds can blow bringing warm and dry conditions to southern and sometimes central California.
The subtropical highs that had so dominated the weather during the summer are weak and near their southernmost position. The Atlantic Bermuda High shifts further east to near the Azores. It rarely has any influence on the weather in the United States. The Pacific High however does occasionally build east to bring mild, dry, very pleasant winter conditions to southern California.
The jet stream is nearing its southern most position. Often several jet streams are observed. One on average enters the continent near the Northwest border with British Columbia, lifts across the Rockies then drops to the Ohio Valley and then east to the mid-Atlantic. Usually a second subtropical jet stream shows up, in the mean entering through Baja California then east to central Texas then to the South Carolina. This jet stream helps to develop storms in Texas and the Gulf of Mexico. When these disturbances interact with the northern polar jet stream they can become major winter storms.
When the steering flow across the nation is decidedly zonal (blowing from west to east) mild Pacific air masses can move eastward into the central and eastern parts of the nation. The long nights can mean widespread dense fog that is slow to burn off during the day. Fog also may be found when warm air passes over the cooling near-shore waters near the Gulf coast. Fog can occur also near warm fronts and in the warm sector of storm systems when warmer air moves over snow cover.
Temperature extremes for December: the highest was 100 at La Mesa, CA on December 8, 1938; the lowest -59 at West Yellowstone, MT on December 10, 1924.