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Weather Network meteorologist Monica Vaswani provides an expert analysis on the major pattern shift ahead this week.

Is the Polar Vortex back? Sort of...

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Sunday, July 13, 2014, 8:19 PM - Well, it’s mid-July and this is typically the hottest time of the year for most in Canada. 

But depending on which side of the country you live in, you might not feel so “summery” in the week ahead. A big change in the weather pattern is coming, one which will bring oppressive heat to some and fall-like temperatures to others. 

A large trough is forecast to set up right over Manitoba and the Great Lakes while a large ridge develops over the western half of the country.

Now, normally a set-up like this would bring cooler temperatures down from the north and into the areas where the jet stream dips or troughs while simultaneously pumping warm air from the south into areas underneath the ridge. So what makes this time around so special? 

Well, the trough digging its way into the mid-United States is associated with an unseasonably cold upper level air mass. So cold that some are even calling it the “July version of the polar vortex”, but we will get to that momentarily. 

Eastern Canada

Late into the weekend, with upper level temperatures in the low single digits, daytime highs in northern Manitoba will barely scrape by with double digits while southern Manitoba sees daytime highs in the high teens. Winnipeg’s normal high is 26oC at this time of year so that puts them almost 10 degrees below their normal.

Once the work week comes about, temperatures in northern Ontario will rival those of southern Manitoba while highs in southern Ontario are in the low 20s. 

Now although these temperatures are nowhere near as cold as the “winter polar vortex” brought, they are still seasonally cool if not cold, and will make mid-July feel more like September. 

As shown above denoted by the colours, upper level temperatures at the 850 mb level (approx. 1,500 m above the surface) are expected to be as much as 4 to 8 degrees below normal for this time of year in Ontario on Tuesday. This cold air mass is originating in the Canadian northwest and it eventually settles right in to the U.S. Midwest and even to parts of the Deep South. 

In fact 850 mb temperatures in some parts of the US will be 10 to 13 degrees cooler than normal, a very notable cold snap for states like Kansas and Nebraska.

Meanwhile, further east, Quebec and the Maritime provinces hold steady with mainly seasonal temperatures but a very muggy week ahead. The amplified pattern of the jet stream enables a southwesterly flow which feeds humid air from the Gulf of Mexico right into Atlantic Canada. 

Newfoundland is no exception: With humidex values expected in the low 30s and daytime highs in the low to mid 20s, they will feel even warmer than normal for this time of year. 

There is one more impact that this large trough or dip in the jet stream will have on Eastern Canada: The jet stream acts as a guide for storms; maneuvering them with every rise and fall. This puts most of the Eastern half of the country in a very active storm track next week so expect periods of rain and likely thunderstorms, especially in Atlantic Canada.

NEXT PAGE: Sweltering Western Canada (and: Is it REALLY a Polar Vortex?)

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