It's going to be warmer and drier than normal for a large chunk of Western Canada in December, which doesn't bode well for farmers and possibly skiers and people who rely on stormy winter weather for business.
'For about the first two weeks of December we're seeing an upper ridge developing in the West, which will shield much of British Columbia, Alberta, Saskatchewan and parts of Manitoba from moisture-laden lows coming in off the Pacific,' says Ron Bianchi, Head of Meteorology at The Weather Network.
'These Pacific lows will instead be pushed down into California and up into Yukon, Northwest Territories and Nunavut, resulting in wetter conditions for them.'
The upper ridge will also keep cold, Arctic air from pushing southward into the West, paving the way for warmer Pacific air from south of the border to flood and dominate the region.
Other systems will be working hard too to keep the West warmer and drier than normal in December, namely El Niño and the warmer than normal waters in the Gulf of Alaska. Because of these systems, any air blowing
in off the Pacific will be milder than normal.
One system that isn't performing its normal function, however, is something Bianchi calls 'the cross polar connection.' This system's atypical behaviour is another reason why the West will be warmer and drier than normal.
'When Siberian highs cross over the top of the polar cap and head downward into North America, we call this the cross polar connection. When this occurs we usually see the development of some really cold air flowing into North America,' says Bianchi.
'But currently, there is no cross polar connection happening, which is why we aren't seeing any extremely cold temperatures or major winter storms.'
The forecast of warmer and drier than normal conditions doesn't bode well for Prairie farmers, who need the snowpack to moisten and insulate ground soil and to prevent the top layers from being blown away. Skiers will likely escape the worst of the situation, says Bianchi, as temperatures will still be cool enough in the mountains to make artificial snow.
Meantime, what can Eastern Canada expect weather-wise in December? Most places will average out near normal in terms of both temperature and precipitation. But the month won't start off that way. For about the first two weeks, most places will be cooler than normal, as the same upper trough that has been spiraling cool, Arctic air through Eastern Canada for several weeks will continue to do so.
This trough will also help generate some lake squalls to the lee of the Great Lakes in early December, making highway travel dicey at times.
Newfoundland and Labrador is about the only province where some places will average out wetter than normal in December, due to their close proximity to the storm track. Nova Scotia is forecast to record near normal amounts of precipitation, although Bianchi admits the province could end up slightly wetter than normal, but only if the storm track shifts from its current forecasted position.
'There are still some warm waters off the Carolinas, which could be good breeding grounds for some coastal storms,' he says. 'But we're not expecting any major storms to develop in the first part of December. We believe the Maritimes will see normal storm activity for about the first two weeks.'
And how about the middle and end of December? What can Canadians expect during these periods of the month?
Around week 3, the entire country is expected to shift to a 'zonal flow,' which is a straight, west to east air flow. Bianchi says the country normally quiets down weather-wise during a zonal flow, as there are no big dips in the jet stream where strong storms can develop.
'For storm systems to be really ripping along you need big dips in the jet stream, where warm air meets cool air,' he says. 'But storm systems become weak when there's a zonal flow because everything sort of smoothes out.'
With El Niño underway in the Pacific, the zonal flow is also expected to sweep mild, Pacific air across the entire country mid-month.
As for the end of December, that's when the weather will take a turn for the dramatic. Around week 4, a shot of cool air is expected to sweep across the entire country starting in the West. Remember that 'cross polar connection' we explained earlier? Well the end of December is when it will start performing normally.
'Around week 4, we will start to see the zonal flow breaking down and the jet stream once again starting to buckle, making way for stormy conditions,' says Bianchi. 'This is also when we could start seeing some Siberian highs crossing over the top of the polar cap and heading downwards into North America.'
What this means for Western Canada is a drop to cooler, more seasonal temperatures, after getting away with a few weeks of warmer than normal weather. Eastern Canada will also see a return to more seasonal values, after enjoying a period of milder air courtesy of the zonal flow.
Atlantic Canada and the Maritimes could see an increase in storm activity because of this shot of cool air and the jet stream starting to buckle. 'We will be watching these regions closely for increased storm activity, which could signal the start of a true winter pattern,' says Bianchi.
When all is said and done, however, this late shot of cool air won't be enough to throw off the temperature forecasts for Western and Eastern Canada. The West is still expected to average out warmer than normal for the entire month of December, while Eastern Canada will average out near normal.
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