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Coming Nov. 20: How will La Niña affect winter in Canada?


Michael Carter and Dr. Doug Gillham
Meteorologists

Thursday, November 16, 2017, 2:15 PM - In less than a week Weather Network’s meteorology team will release their updated winter forecast for the months of December, January and February with a sneak peek ahead to the month of March. This forecast will be released on Monday, November 20th. In the meantime, here’s a look at some of the key questions that we have been asked as we have been finalizing our 2017-2018 Winter Forecast.


Visit our Complete Guide to Winter 2017/18 for a preview of the Winter Forecast, tips to survive it and much more.


How will La Niña Affect Winter Across Canada?

La Niña has a reputation for focusing the coldest and most active winter weather across Western Canada with more variable and often milder weather from the Great Lakes to Atlantic Canada.

Credit: NOAA

Credit: NOAA

However, one should not make assumptions about the upcoming winter simply based on the development of La Niña.

Perhaps an analogy from sports will be helpful here. When a star player is the starting pitcher, quarterback or goalie for your favorite team, the potential for a particular outcome (a win) is much higher when that player participates versus when they are sitting on the bench. However, no two games are alike, and having star player on your team does not guarantee the same outcome every time. There are many other factors (players) that determine the final outcome.

Likewise, the developing La Niña will be a major driver for the weather patterns that we see this winter, but it is just one of the many variables that we are looking at in developing our winter forecast. Water temperatures in the Gulf of Alaska, across the Atlantic Ocean and the potential for blocking patterns to develop in the atmosphere over the arctic are also key considerations in developing a winter forecast.

Will this Winter be a Repeat of Last Winter?

This is a common question heading into winter because this time last year we also had a developing La Niña.  However, no two La Niña events are alike and as such no two winters associated with La Niña will be identical.  La Niña events have varying intensities and this year’s La Niña event looks to be stronger than what we saw last year.

There are some other critical differences to last year’s La Niña event, and these differences will likely have an impact on the dominant weather patterns across Canada.

The two maps below show our current ocean temperature patterns (blue highlights areas that are cooler than normal) in comparison to last November.  The long rectangle near the Equator highlights the region where we see the cooling in ocean water temperatures associated with La Niña.  In addition, ocean water temperatures in the North Pacific are much warmer relative to normal than they were last year.  This will likely have an impact on our winter pattern as well.

Given the significant differences in the ocean temperature patterns between this year versus last year, we expect that winter will have some noteworthy differences and theses differences will be highlighted in our winter forecast. 

It is worth noting that this November has already been a stark contrast to last November which was exceptionally warm across nearly all of North America.  This year has featured colder than normal temperatures across western and central Canada.

Is it Really Possible to Forecast the Weather Months in Advance?

Yes, but it’s important to understand the differences between our day-to-day weather forecasts and these seasonal forecasts. Because of the nature of seasonal forecasting, we cannot provide the level of detail that you expect to see in a daily forecast. So if you want to know if it will snow on Christmas, or if you should make outdoor plans on Valentine’s Day, you’ll have to wait for those specifics.

However, we can often get a very good idea of how the big picture of a season will look – the overall patterns, and how they relate to climatological normal – well in advance. This type of seasonal forecasting can provide useful information, and it’s a common approach in other areas outside of weather.

Take sports for example – at the beginning of a given season, it would be very difficult to predict the outcome of any single game far in advance. This would be like trying to make a daily forecast months ahead of time. But often we can predict with reasonable certainty which teams are likely to be better than average and to win more games than they lose, or vice versa.

You can think of a seasonal weather outlook in the same terms. It provides information on which areas are likely to be warmer more often than colder, or wetter more often than drier, in relation to a normal year. But of course for every rule there will be exceptions. An area that is forecast to have “Above Normal” temperatures for the season will still see cold days, just like a sports team having a great season will still take occasional losses.

How do you develop the Winter Forecast?

The Weather Network’s meteorologists take a three-pronged approach when developing seasonal forecasts. Each of these methods gives us information about how the upcoming season is likely to play out, and using all three in concert with each other gives us the best understanding of where the overall weather patterns are headed.

The first forecast method we use is the analysis of teleconnections – or global weather drivers. Earth’s atmosphere and oceans form one big interconnected system, and things that happen in one part of the system can have big influences elsewhere. The classic example of a teleconnection is the El Nino cycle. Water temperatures over the equatorial Pacific affect trade wind patterns, which influences the jet stream pattern over North America.

El Niño and La Niña are one example, but there are many others. Water temperatures in the Gulf of Alaska, or air pressure patterns over Greenland, or the tracks of eastern Pacific typhoons can all influence North America’s weather. Understanding these influences is a major part of developing a seasonal forecast.

The second method we use is the analysis of analogues – or past years with similar characteristics to current conditions. This is based on the general principle that if something happened in the past, it is likely to happen again given similar circumstances.




Our forecasters look through over 65 years of past weather data, to identify years with similar global weather drivers to the current year. These are our analogues. Once we find a good set of analogues, we can run the tape forward to see how those winters played out, and see if there are any common patterns that we can identify. Essentially with the analogue method we are using the past to provide some cluse as to what the future may hold.

The third method we use is the analysis of forecast models. Seasonal forecast models are similar to the ones that we use to make your daily forecasts, but they are specifically tuned to help identify long term patterns and trends. Weather agencies around the globe develop and run their own seasonal forecast models, and we take the results of all of these models into consideration when developing our forecasts.

The models aren’t perfect, and they don’t always agree, but by using them together with the information we gain from the other two forecast methods, we can filter out the solutions that seem the most likely. When model forecasts show good consensus with the teleconnections and analogues, it gives us higher confidence that they are correct.

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