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An early look at the August long weekend weather

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    Brad Rousseau

    Wednesday, July 31, 2013, 11:01 AM -

    Most of us welcome the long weekend and look forward to getting out whether it be to the cottage, local festivals, or even just visiting with family and friends. No matter what we’re doing though, we all want the same thing: the weather to cooperate!

    For much of the country there will be one unwanted guest, and that comes in the form of the “polar vortex”. Below shows the GFS forecast, one of the many weather models we use, of the upper level winds for Saturday afternoon (first panel) and for Monday afternoon (second panel). We can see that Saturday there is a deep trough that extends from the Arctic down into the Great Lakes and into the northeastern United States (as outlined in the red box). By Monday, we can see a substantial upper level low pressure system (called the “polar vortex”) which extends from Saskatchewan east to Nova Scotia and Newfoundland (as outlined in the red circle).

    Source: http://mag.ncep.noaa.gov/Image.php?image=data%2Fgfs%2F12%2Fgfs_namer_102_500_vort_ht.gif&model=gfs&area=namer&storm=&cycle=12&param=500_vort_ht&fhr=102&group=Model+Guidance&imageSize=L&scrollx=0&scrolly=0

    Source: http://mag.ncep.noaa.gov/Image.php?image=data%2Fgfs%2F12%2Fgfs_namer_102_500_vort_ht.gif&model=gfs&area=namer&storm=&cycle=12¶m=500_vort_ht&fhr=102&group=Model+Guidance&imageSize=L&scrollx=0&scrolly=0

    Source: http://mag.ncep.noaa.gov/Image.php?image=data%2Fgfs%2F12%2Fgfs_namer_150_500_vort_ht.gif&model=gfs&area=namer&storm=&cycle=12&param=500_vort_ht&fhr=150&group=Model+Guidance&imageSize=L&scrollx=0&scrolly=0

    Source: http://mag.ncep.noaa.gov/Image.php?image=data%2Fgfs%2F12%2Fgfs_namer_150_500_vort_ht.gif&model=gfs&area=namer&storm=&cycle=12¶m=500_vort_ht&fhr=150&group=Model+Guidance&imageSize=L&scrollx=0&scrolly=0

    Eastern Canada

    What does this mean in terms of the weather then?

    Well areas that are under the influence of this upper low can expect below seasonal temperatures and generally unsettled conditions. These areas include cottage country, eastern and southern Ontario, as well for the GTA. It’s difficult to get into the specifics with this type of pattern this early in the game. But generally this type of pattern is one that will bring cool, sunny conditions one minute, brief isolated showers the next, and back to sunny conditions again, and having this cycle repeat. This is difficult to forecast this far out as there is no real organized low that will bring large area of precipitation but rather very isolated and scattered showers, a rather annoying pattern. This is what is referred to as “cold core convection” which usually takes place under these large upper lows. But most long range forecast models indicate that by late Sunday and into Monday there will be little available moisture which will limit shower activity. However, below seasonal temperatures will remain in place.

    Associated with this system will be a slow progressing arctic cold front which will push through the St. Lawrence Valley and Ottawa region through Saturday and into the New Brunswick area on Sunday. Ahead of this front will be some instability where some thunderstorm activity will be possible as well as more organized precipitation. The not-so-great news also continues for the Atlantic Provinces (excluding New Brunswick). On the leading edge of the polar vortex, where a stalled front is expect to setup, unsettled conditions along with below seasonal temperatures are also expected for much of the weekend.

    Western Canada

    The unsettled weather looks to continue for the western provinces as well. An upper low is forecast to push into southern British Columbia for Saturday, as shown in the first panel above (outlined in blue). Unsettled and wet conditions are expected Saturday and into Sunday morning for much of southern British Columbia including the Vancouver region and into the Interior as well as for southern Alberta including Calgary and areas south. This could be the first precipitation for the Vancouver area since the end of June.

    Associated with this system will also be below seasonal temperatures. Sunday, the region will continue to remain under the influence of the upper low where below seasonal temperatures are also expected but bulk of the wet conditions looks to remain within the interior and along the Foothills in Alberta. The Edmonton region both Saturday and Sunday looks to escape the bulk of the wet conditions with temperatures at or just slightly below seasonal values.

    By Monday we can see that in the second panel above of the 500 mb flow forecast, a ridge pattern is taking shape which will allow for temperatures to rebound back to near seasonal values for most regions including Vancouver and surrounding region as well as for southern Alberta including Calgary and Edmonton. However, in terms of precipitation there is some variability amongst model guidance with indication that the interior British Columbia as well as the Foothills could see some isolated showers through the afternoon period with a weak disturbance tracking along the British Columbia and United States border (as outlined in blue circle). It would appear as though the Canadian Prairies to northwestern Ontario will sit between the real “meat” of the two systems expected to affect the east and west thus leaving the region relatively quiet in terms of weather. That being said, it does look to remain below seasonal in terms of temperatures for the weekend.

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