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Detailed December weather outlook: What's left to come this month?


By Brad Rousseau
Meteorologist
@bradrousseau
Thursday, December 12, 2013, 6:27 AM

The month of November on average was below normal across British Columbia, much of the Prairies, and into northwestern Ontario as shown in the image below by the temperature anomalies. Central and eastern Ontario and the Atlantic provinces, were for the most part, near to slightly above average. There were periods of extremes in these regions, but overall the numbers worked out to be near average.

Temperature anomalies across the country for November

Temperature anomalies across the country for November

Looking forward for the rest of December, the pattern looks to take us on another roller coaster ride with some good bouts of cold to come.

The pattern for British Columbia and through the southern part of Alberta looks to have temperatures below seasonal and gradually trend upward to seasonal or slightly above seasonal through the remainder of the week and into early next week. Central to northern Alberta, across the Prairies, through Ontario, Quebec, and into Atlantic Canada, will fall into a freeze with well-below seasonal temperatures, particularly in the wake of the large low pressure system that will affect the eastern part of the country this weekend. As the wave of the system pushes through, some milder air will move in, but in its wake, temperatures will drop. Keep in mind the numbers in these images do not represent an actual temperature forecast but a departure from the average through the forecast period. In the image below, the departure from average for December 12 to December 17 in represented.

Temperature anomalies for the near term for December 12 through to December 17

Temperature anomalies for the near term for December 12 through to December 17

Moving to the middle of the month (about December 17 to 22) the ensemble forecasts are showing a changeup in the pattern once again. The image below shows a four-member ensemble forecast (four different weather models used for guidance) showing upper air height anomalies which helps us to determine temperatures in the upper atmosphere and then deduce the surface temperatures. Simply put: the blues, greens and greys indicate regions where upper atmospheric troughs takes shape (essentially an upper atmosphere low pressure area). The yellows, reds, and purples show areas where upper atmospheric ridges take shape (an upper atmosphere high pressure area). The thick red line contours approximately where the jet stream would roughly be according to the different outputs. The jet stream helps us determine many components of weather when forecasting including the direction in which storm systems would move, how intense they can get as well as temperatures.

Most of the forecast models show a ridge pattern taking shape for the east, deep trough for the prairies, Alberta, and parts of British Columbia, and a relatively strong ridge for the Gulf of Alaska and Pacific coast. This indicates unsettled conditions through the U.S. Midwest, into the eastern Prairies, northwestern Ontario pushing into central and southern Ontario, as well as along the Pacific coast. The eastern half of the country would stay relatively calm with some uncertainty for Atlantic Canada where a weak trough pattern would maintain some unsettled conditions but likely transitioning to calm near the end of this period.

Upper air height anomaly for December 17 to December 22

Upper air height anomaly for December 17 to December 22

Temperature trend with this pattern is as follows:

Through southern Ontario, temperatures will climb back to more seasonal values and the cold for Atlantic Canada will hold but not be as extreme with temperatures gradually rebounding back.

The trough across the Prairies will once again usher in below seasonal temperatures with Alberta sitting right along the transition. This means Alberta will likely be up and down between bitter cold and milder air.

British Columbia will be near seasonal to slightly above thanks to the ridge that will keep the cool arctic air to the north and east.

Temperature anomaly for December 17 to December 22

Temperature anomaly for December 17 to December 22

Looking at the upper air anomalies again for the end of the month (December 24 to 30), we can see there is not much agreement between the forecast models so there is a level of uncertainty that is on the high side. However, from what is shown, the eastern half of the country (central to southern Ontario, Quebec, Atlantic Canada) as well as central to northern British Columbia look to be in an unsettled pattern for this time period. This means that the days leading up to Christmas could be unsettled (snow, perhaps?).

Height anomalies for December 24 to December 30

Height anomalies for December 24 to December 30

Temperatures through the end of month, as a deep trough settles in across the Parries and the eastern half of the country, look to be below seasonal as frigid air returns with the core of this cold air through the eastern Prairies, northern to central Ontario, and Quebec. Alberta remains along the transition zone of these two air masses which leads to the up and down temperatures trends to continue. The upper ridge pushing into the west will have British Columbia at or above seasonal.

That sounds like a lot to take in, but Canada is becoming snowier and snowier as we continue through December and the question on a lot of people’s minds is, “Are we going to have a white Christmas?” It’s looking more and more likely for many Canadians, but The Weather Network will be releasing a special forecast on December 20 detailing who across the country is guaranteed a White Christmas and who is at risk of having a green (or brown) Christmas.

Temperature anomalies for December 24 to December 30

Temperature anomalies for December 24 to December 30

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