When I've done searches on the net for what kind of winter we will have in North America for any particular year, I get mostly prediction based on the Farmer's Almanac. There's very little in the way of scientific papers that address the physics of meteorology. Perhaps there is a good reason for this. Weather systems on a global scale dwarf even the most powerful computers ability to crunch. Too many variable that replicate themselves to infinity even a week or more out.
So it is then, that virtually anyone (me?) with limited or no experience can post a 'Theory of the Coming Winter', without much in the way of credible certifications. What follows then are a couple of theories that may impact the kind of winter that may occur.
Theory of Progressive Ground Cooling (TPGC)
This theory is the manner in which the ground over a particular region of North America is cooled in the early fall months. In general, as one proceeds from the Canadian border to the south, the surface four to six inches of the soil warms. Snow packs cause a dramatic cooling and can affect large geographical areas. As a region becomes cool, it has less impact on warming the air above it, so that any future incursions of cold air become less affected in their passage to the southern states. Once established, such a 'cool' regions can in turn affect weather pattern and result in reinforcing incursions of subarctic and arctic air from more northern latitudes.
Therefore, one of the first things to look out for would be an early and persistent snowfall in key states in the upper Midwest. Two states I pay particular attention to are North and South Dakota, situated as they are between two elevated ranges of land to the west and east. They form the center of a funnel as it were. In addition, both states will tend to be impacted before the other states to the south and east in the event of an outbreak of cold polar air. When night time temperature get into negative digits with daytime highs never breaking the freezing mark, the stage is set for the rapid conveyance of frigid air from north to south. The end result can be a record cold winter for much of the United states. A record that stretches back through time.
The winter of 1917-18
Meteorologist Preston C. Day (1859-1931) wrote in December 1918, “The severity of the weather experienced during December and January of the winter of 1917-1918 over the greater part of the United States east of the Rocky Mountains, and also over much of Canada and Alaska during the early part of the period, was so unusual as to the length of time the low temperatures persisted, the great area involved, and the degree of cold maintained, that some discussion of the contributing factors, and comparison with similar occurrences of previous years, seems desirable.”
Climatologist Charles F. Brooks (1891-1958) wrote in June 1918, “Even though summer is upon us, it is not difficult to recall that last winter in the United States east of the Rockies was remarkably cold and snowy. The first killing frosts of autumn came early, and nipped crops which had started late and grown slowly in the cold spring and early summer. The South had a real winter, much to the detriment of fruit and truck crops which were caught by frost.”
Brooks continued, “By far the most intense winter conditions occurred in the regions from the Ozarks to New England, where low temperatures brought snow with passing cyclones, and the snow cover in turn cooled the air excessively whenever the sky was clear…In the eastern United States it was not surprising that autumn months which in many regions were the coldest on record, should be followed by a December and a January that defied the memories of the oldest inhabitants. For example, in Ohio, a 64-year record fails to show a colder December, and in New England, January seems to have been the coldest month at least since 1836, if an Amherst record may be considered as representative.”
The cycle of clear skies followed by intense cyclones that Charles Brooks alludes to caught my interest. What begs the question is whether the pattern was coincidental or due to some re-enforcing pattern that escaped the notice of the weather forecasters of the time.
Arctic Oscillation
The second causal effect is what is known as the Arctic Oscillation or AO. This is a little understood tendency for low or high pressure to build over the North Pole during the winter months. When a low builds, the AO is said to have a positive aspect and North America will generally experience a mild winter with little snowfall. When high pressure builds, however, this is known as a negative AO and with it comes very cold, snowy winters across the Ozarks and east coast.
The AO can be thought of as a fluctuation in the shape of the global circulation. The strength of the oscillation, as defined by the AO index, measures the awkwardness of the circulation's geometry by contrasting the weather in polar regions from that observed in more temperate latitudes like ours. Often times when the AO index is negative, frigid air typically found at far northern latitudes has been displaced southward, while at the same time warm air has been moved northward toward the pole. As the graph below shows, last winter was definitely a negative AO season. In fact, the AO index measured during much of last winter was "off the charts" low. It bottomed out at values not reached since record keeping began in the middle of the last century.
The big question that comes to the fore at this juncture in early Fall 2011 will be whether the trend seen last year will persist or even amplify going into December and January of 2012.
