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Fall has brought rather mild temperatures across much of the country. However, temperatures are about to take a deep dive.

Anomaly over Alaska brings winter to Canada, here's when


Brad Rousseau
Meteorologist

Thursday, December 1, 2016, 9:06 PM - Fall brought mild temperatures across much of the country as an extended period of fall weather kept millions of Canadians outside and in light jackets. In November alone temperatures on average were anywhere from 1oC to nearly 8oC above normal with the core of the warmest air was across the central Prairies where temperatures were pushing 8oC above normal.

But the map below shows a mass of cold air over Alaska and why it is about to bring an end to the extended fall and usher in what we typically expect to see at this time of the year. We are also keeping an eye on Ontario through mid-week, as models suggest a potent storm may track into the Great Lakes basin.


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Now that we are into December, it's inevitable that colder temperatures will be settling in, but the expected pattern change forth coming will put the western half of the country into a deep arctic freeze. The image of Alaska above shows the temperature anomalies over Alaska and the Yukon for the latter half of November into December, where clearly the region has been quite cold. It is this cold air that is expected to flood into B.C. and the Prairies next week.

Through the weekend into next week an upper level ridge is set to develop over eastern Russia and over the Bering Sea with a deep trough digging in over the western half of the continent. As this pattern sets up, it will send the arctic front crashing south with an arctic air mass settling in over the Prairies and B.C.

TIMING: MONDAY

By Monday we are expecting daytime highs into or near the minus teens across much of the Prairies and near or below freezing for much of B.C. The exception will be coastal regions and the Lower Mainland where highs should remain slightly above the freezing mark.

TIMING: TUESDAY

Tuesday the Arctic air entrenches itself further across the Prairies, with daytime highs into the high minus teens and low minus twenties through much of Alberta and northeastern B.C. Much of interior B.C. will be well below the freezing mark, with coastal regions and the Lower Mainland expected to be at or at most a degree or two above freezing.

With the cold air in place across B.C., there will be a couple of systems to keep a close eye on. The first one is a weak little low that is expected to skirt southward along the coast.

SNOW FOR VANCOUVER?

With freezing levels expected to be around sea level, some snow showers will be possible across most of the Lower Mainland, including Vancouver. Some accumulation is possible, but likely not significant, with mainly trace amounts or enough to cover grassy surfaces. However, some of the higher elevations into eastern parts of the Lower Mainland and Burnaby could see a couple of centimetres from this first system.

The second, and what should be a more potent system, will be a large Pacific low.

Timing of this low is still uncertain with the forecast model shown below (American GFS model) expecting it to impact the region Wednesday. Whereas the European and Canadian GLB models (not shown), hold off on this system until late Wednesday night into Thursday morning.

Regardless of timing the take away here is that with the cold arctic air in place, there will be a relatively good chance that Vancouver and the Lower Mainland could see accumulating snow Wednesday night or into Thursday. How much will accumulate is near impossible to tell at this point, but the trend up to now is indicating the potential for some accumulating snow.

As we head into the latter part of next week, the upper trough will continue to dig southeastward across much of the U.S. and into Ontario as shown below.

TIMING: WEDNESDAY IN THE GREAT LAKES

As the trough progresses eastward we are expecting a rather potent fall storm to develop across the south-central U.S. and track into the Great Lakes basin.

Long range models, however, are having a tough time resolving where the low's track will be and the image below demonstrates this well.

All the analyzed Ls in the image shows where each ensemble member thinks the low will be on the evening of Wednesday, Dec. 7, and the shaded areas (the green region) is the mean of all the ensemble members or the average in other words. We can see the individual ensemble members are ranging anywhere from the Atlantic through central Ontario, and even into Hudson Bay with the centre of the low roughly through northeastern/central Ontario.

This low will likely have Ontario run the gauntlet in terms of weather, bringing with it snow, rain, and strong winds.

Where this all sets up is nearly impossible to tell for now and given the range of possible low centre locations, it's easy to tell why. The key take away here is that the expected pattern change will have an impactful fall storm through Ontario mid-to-late next week, which should come as no surprise given we are into late fall. Behind the low as it pushes off into Québec or Atlantic Canada, the cool arctic air will flood into Ontario where daytime highs for most of the province will be at or below freezing late next week into the weekend.

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