How we make our summer forecast: Behind the scenes

Tuesday, May 23, 2017, 8:30 PM - How will a developing El Niño impact our weather this summer? Find out what you can expect for the next several months when The Weather Network releases their summer forecast on Tuesday, May 23 at 9:00 p.m. EDT.

In the meantime, let’s take a behind the scenes look at the science behind the forecast process and the unique global pattern that will impact our summer weather this year.

How do you develop a forecast for an entire season?

A seasonal forecast is very different from the day-to-day forecasts that you check to see if you need an umbrella or a jacket before heading out the door.  Seasonal forecasts cannot address the daily or even weekly details, but we seek to capture the essence of how the summer will be remembered.

The important drivers for weather patterns throughout a season seem abstract because they are things that we don’t directly experience every day, but over a three month period factors like water temperatures in the across the Pacific Ocean or air pressure patterns over Greenland can have a big impact on the type of summer we experience here in Canada.

One of the biggest drivers of Canada’s weather on a seasonal scale from year to year is a cycle of water temperature changes that takes place in the tropical Pacific Ocean, from the coast of South America to the International Dateline. This cycle has two phases, with names that you are probably familiar with if you follow the weather regularly: El Niño and La Niña. Together they make up a larger pattern called the El Niño Southern Oscillation, or ENSO.

SUMMER IS HERE: With El Niño helping shape global patterns what will Canadians expect from summer? Find out with The Weather Network’s 2017 Summer Forecast | FORECAST & MAPS HERE

The strength of these two ENSO phases, and how quickly we swing between them, is one of the most important factors to analyze when creating a seasonal forecast. Although every ENSO event is unique and has its own distinct fingerprint, the current cycle stands out as particularly unusual. And because we haven’t seen this particular pattern before, it leaves us with some big questions about the upcoming summer.

What is so unique about our current weather pattern?

To answer this question we have to roll the clock back a bit, to the winter of 2015-2016. That winter was dominated by a phenomenally strong El Niño event, which at its peak intensity was equal to the “Super El Niño” of 1997-1998. The 2015-2016 El Niño was so strong that its after-effects are still influencing our weather today.

But it’s not just the strength of that El Niño that makes it so unique. Strong to very strong El Niños occur once every decade or so, and they have a well understood effect on North America’s weather. What makes the 2015-2016 event unique is what happened afterwards.

This chart shows all the big swings in the ENSO cycle going back to 1980. El Niño is the warm phase of the cycle, so El Niño events show up as sharp upward spikes on the graph. You’ll notice very prominent peaks in 2015-2016, as well as 1997-1998, ’82-’83, and several other memorable events, with weaker spikes in between.

The downward spikes represent the cool phase of the ENSO cycle known as La Niña.  Some of the stronger La Niña events on record include 1988-1989, 1998-2000, 207-2008, and 2010-2011.

The ENSO cycle tends to swing back and forth between the warm and cold phases like a pendulum.  Typically, after a very strong warm phase we spend an extended period of time, often a couple of years or more, in the cool phase before the next warm phase begins. 

Not this year though, and that’s what makes the next few months so unusual.

After the very strong El Niño of 2015-2016, we saw a very brief, weak La Niña event. It only lasted a few months, and peaked last fall.  Then during this past winter, temperatures began to warm again, as the cycle took an abrupt about-face and began to head back into El Niño territory.

This quick return to a warm pattern so soon after the last very warm event creates unique challenges for this summer’s forecast. Typically the La Niña event following a strong El Niño results in cooler ocean water temperatures across the northern hemisphere. This time, however, those cool ocean temperatures never truly got a chance to set in, and there’s still a considerable amount of lingering warmth on the map instead.

In straightforward terms, that means the normal rules don’t apply to this year’s forecast. 

While the impacts of El Niño are a lot more evident during the winter, history shows us some consistent patterns during summers in which an El Niño was developing.  

However, we also have the have all that lingering warm water to consider, as temperatures hover above normal from the eastern Pacific to the Gulf of Mexico to the Gulf Stream in the western Atlantic.  This pattern tends to impact global weather patterns in a way that is opposite to what we often see with a developing El Niño.  

So, as we head into summer the drivers of our weather pattern will interact with each other in a way that we have not observed as long as reliable records of global temperatures have been kept.

In a typical seasonal forecast, we use a technique called the “analogue method”, where we look back through history to find years that had global patterns that were similar to our current pattern and study the weather that was associated with those patterns. This can give us very useful information on how the upcoming months might play out.

However, in a year this atypical, the forecast process is even more challenging as we seek to determine how the different pieces of the puzzle will fit together and interact with each other. 

Please check back on May 23 for a look at our Summer 2017 forecast. At that time we will release the details on our unique summer pattern, and how it could affect you during the months of June, July, and August.