During the winter season it is quite normal to see temperatures plummet to bone chilling levels across Canada. There are reasons why this happens and the topography of western North America plays an important roles as to why these very cold air masses make their way southward to southern Canada and into the U.S.
When one looks at a topographical map of North America one thing that stands out is the presence of a long string of mountains from Mexico through the western quarter of the U.S. to western Canada and Alaska. These mountain ranges go by different names across these regions but all are a major factor in how the weather affects much of the remainder of Canada and the U.S.
The Canadian Rockies lie through far western Alberta, British Columbia, and through the Yukon towering to over 10,000 feet in many areas. Since weather systems tend to move from west to east these mountains can block and weaken storms as they cross the mountains to interior western Canada. The flat prairies to the east of mountains see a drier climate because of this moisture block. There is also another effect these mountains have on western Canada.
During the winter season it is fairly common to see large polar and arctic high pressure areas develop across Alaska and across northwestern Canada given the proper weather pattern. Snow cover is nearly endless across western and northwestern Canada at this time of year and underneath high pressure we generally see very light winds. The combination of long nights, light winds, and snow cover allows heat to rapidly escape to space. This process is know as radiational cooling and can lead to temperatures falling well below -40F across the Yukon and the Northwest Territories during the winter.
Once one of these arctic highs becomes established the topography of western Canada allows the cold air to slide south and southeastward into the Prairies leading to those periods of very to extremely cold weather for the Prairies. Snow cover aids in this process by producing a surface easier for the cold air to move across as well as helping to maintain the cold. The Rockies to the west help block the much milder Pacific air from modifying the low temperatures but at times a stronger weather system can lead to the well known Chinook winds that blow down off of the mountains sending temperatures shooting upward 30 or 40 degrees and more in a short time.
Arctic air is dense and heavier than milder air thus tends to flow like water through lower areas at first before "filling up" valleys and ravines. This is why the coldest temperatures are commonly found in communities that exist in low areas during arctic outbreaks.
During the next week or so we are likely to see some arctic air pay a visit to the Canadian Prairies. Sometimes if there is not enough snow cover extremely low temperatures can damage winter wheat and rye. At this time there appears to be enough of a snow cover through the region to act as a blanket. Some additional light snow is also a possibility during the next several days adding a little insurance to areas with a thinner snow cover.