The jet stream pattern across North America will have a high degree of amplitude through the weekend, which usually means that there can be extended periods of extremes from one coast to the other.
The image below shows the overall pattern this weekend. Two strong ridges of high pressure aloft, one anchored over the Canadian Rockies and the other just off Newfoundland, will keep those regions unusually warm. The big difference between the two is that the air in the west is very dry, while the air will remain humid across Atlantic Canada.
This high pressure ridge over western Canada will also trap more smoke and haze through the weekend, leading to poor air quality in some valley locations, especially in BC.
With the block in place, most of the Pacific storms are directed into Alaska with wind and heavy rainfall.
Between the two high pressure ridges, we have a tough, or dip in the jet stream, which is directed into Ontario and the Midwest U.S. The eastern half of this trough will be dominated by showers this weekend, while the western side will turn chilly. There may even be some wet snow over northwestern Ontario, eastern Manitoba and northern Minnesota this weekend.
It looks like this pattern finally breaks down in about a week, leading to a more typical west to east flow of the jet stream which should bring back the cooler and wetter weather into BC.
August and Summer 2012 Recap Summer 2012 temperature anomalies in degrees C., courtesy of NASA GISS
To no surprise, the summer of 2012 was dominated by warmth across much of Canada. As usual, precipitation was much more variable, but for the summer as a whole it did end up as a wet one for a large part of the Prairies, while dryness dominated in eastern Canada.
The Env. Canada image below shows the August 2012 temperature anomalies.
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Dry and hotter pattern in store for southern B.C. into next week, while the east turns cooler and more unsettled.
Potential for a significant storm later Sunday into Monday from Ontario to Quebec with heavy rain, severe thunderstorms, wind and snow.
Late-season snow for the western Prairies later this week and a look at the weekly long range pattern.