Long Range: National pattern reversal runs into resistance
Saturday, September 22, 2018, 9:28 AM - September has been a month of contrasting seasons across the country. For northern and western Canada, temperatures have been more typical of October -- and even November at times. Meanwhile, from Ontario to the Maritimes temperatures have been more typical of summer. Newfoundland has been the only province with near seasonal temperatures.
The map below shows the temperature anomaly pattern for the first 20 days of September. The various shades of blue and green represent colder than normal temperatures while the various shades of orange and red highlight areas that have been warmer than normal.Visit our Complete Guide to Fall 2018 for an in depth look at the Fall Forecast, tips to plan for it and a sneak peek at winter
However, a shift in the temperature pattern is already underway across parts of the country. This weekend the pattern will still be very chilly across the west, especially for the Prairies, but the summer heat will be gone from eastern Canada – well timed for the official start of fall. Temperatures this weekend will actually be colder than seasonal for nearly the entire country as shown on the temperature anomaly map below.
The attempted pattern change, however, will run into a lot of resistance next week. One of the reasons is the unusually strong region of high pressure off the East Coast of the U.S. (often referred to as the Bermuda High). This ridge in the atmosphere which was so prominent during the summer and much of September will continue to work against allowing the trough in the jet stream to progress into the Great Lakes region.
Therefore, from the Great Lakes to the Maritimes, milder weather will quickly surge back north into these regions and we will have several days of near -- and even above -- seasonal temperatures (but not nearly as warm as what we have seen this week).
In addition, B.C. and the Yukon will see a milder pattern develop with near seasonal temperatures and even a couple degrees above seasonal near the coast. The map below shows the temperature anomaly pattern for the middle of next week.
OCTOBER TO START ON CHILLIER NOTE
Late next week an even colder blast of arctic air will descend upon the Prairies and attempt to spread east into Eastern Canada. This round of chilly weather is expected to have more staying power as it spreads into Ontario and Quebec. The map below shows the pattern that we expect for the final weekend of September through the first week of October.
The greatest uncertainty in the forecast as we look ahead to early October is around the edges of the region forecast to be colder than normal (Alberta and from southern Ontario to the Maritimes).
We know that the Bermuda High will continue to resist the pattern change, but how much? This will impact whether the chilly weather continues to come east (allowing milder weather to spread into Alberta) or whether the dip in the jet stream remains west of the Great Lakes, keeping Alberta cold and allowing southern Ontario to the Maritimes to remain milder. Regardless, a large part of Central Canada will see a chilly start to the month of October.