Weather Blog: Week Ahead and Arctic Oscillations
The week ahead is not looking as stormy as the past two weeks, however, continue to expect cold temperatures in many areas and still some chances for snow in the Great Lakes, New England and the Upper Plains States. Some pretty good rains are expected in the south central states of eastern and near the Gulf Coast. Temperatures will remain chilly over most of the lower 48 states.
Despite the slow start to winter this season, the snow and cold is adding up. Below is the snow cover report as of December 27, 2012. Snow cover has really increased. Compare to the same date a year ago below.
Compare to the same date a year ago below.
As you can see, snow cover to date is ahead of the same time as last year. We are still expecting January to be a cold month of most of the USA, especially the central and northern areas. There are hints of cold air build up in the higher latitudes getting released after the 10th of the month as a strongly negative Arctic Oscillation has been in place for the past several week.
The Arctic Oscillation
From time to time, readers of this blog will be presented with topics that will touch on some of the weather phenomenon that meteorologists track carefully to assist in making long range forecasts.
For this week’s blog we are going to discuss what is known as the “Arctic Oscillation”.
While not as well known or discussed as much as El Nino or La Nina, the Arctic Oscillation is quickly becoming well known among meteorologists and others who track weather and climate.
You won’t hear about it during the nightly news during the local weather segment or even on your favorite cable tv weather program, however, you will read about how the Arctic Oscillation is behaving on this blog during the winter season.
Unlike El Nino and La Nina which are weather phenomenon based in the Pacific Ocean (warmer or colder than normal ocean waters), the Arctic Oscillation is all about what is going on in the polar regions.
Arctic Oscillation is an atmospheric circulation pattern in which the atmospheric pressure over the polar regions varies in opposition with that over middle latitudes (about 45 degrees North) on time scales ranging from weeks to decades.
It comes in two phases, a positive phase and a negative phase. In the positive phase, higher pressure at mid latitudes drives ocean storms farther north, and changes in the circulation pattern bring wetter weather to Alaska as well as drier conditions to the western United. In the positive phase, frigid winter air does not extend as far into the middle of North America as it would during the negative phase of the oscillation. This keeps much of the United States east of the Rocky Mountains warmer than normal. A negative phase of the Arctic Oscillation can lead to major cold outbreaks in the lower 48. This was especially true in February 2010 when severe cold hit most of the nation. Weather patterns in the negative phase are in general "opposite" to those of the positive phase, as illustrated below.
Effects of the Positive Phase Effects of the Negative Phase
of the Arctic Oscillation of the Arctic Oscillation
(Figures courtesy of J. Wallace, University of Washington)
To whittle this down to layman terms, whenever we discuss a negative phase of the Arctic Oscillation during the winter, expect a higher probability of colder temperatures and winter weather to develop over time. A positive phase can mean warmer, less “winter like” weather conditions. Interestingly enough, many Arctic cold outbreaks across the USA happen when a strongly negative phase turns positive. This is because cold air can build during a negative phase and be released and pushed south during the phase change. See illustration below.
Credit: National Climatic Data Center, National Oceanic and Atmospheric Administration.
During the next two months, there are strong indications that Arctic Oscillation may be strongly negative at times, so keep the mittens handy, especially during the second of January and early February.
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