Environment Canada's preliminary 3 month summer outlook was issued May 1st, garnering some enthusiastic media headlines across the country about how EC was proclaiming a hot dry summer across the country. Stock up on the sunscreen and invest in that inground pool! This summer will be a scorcher according to the headlines. Unfortunately, this was more media hype than reality when taking a closer look at what EC's summer outlooks were really showing..
The image to the left is the outlook map that caused all the excitement. It shows most of Canada glowing red.. like some great fireball had landed on the country and oozed red hot lava all over the country. Only the coasts would be spared from this glowing fireball of a summer, and an inexplicable patch of coolness over southern Hudson Bay. Well, if the map is mostly red, it must mean a hot summer, right? Well, not really.
Let's take a look at the more revealing probabilistic forecasts from the very same model output.
This chart breaks down the odds of a warmer than normal, near normal, and cooler than normal summer for country, in increments of 10% probabilities. It is a far more revealing product than the grossly simplified image above and tells us a lot more information. For example, this map shows that the far north has the greatest chance of seeing an above normal summer, while it's not as likely over the south. It also shows a greater chance of a "near normal" summer over Eastern Canada, and cooler than normal over the West coast. Everywhere else, it's a tossup. This is a very different interpretation from the original map which showed virtually all of Canada above normal. This more detailed chart is known as the probabilistic forecast, and is a far more valid product than the oversimplified and misleading deterministic product that is so misinterpreted by the media and public. Simply put, the deterministic map is a poor reflection of the probabilistic output from the climate models, and should not be taken at face value without consulting the probabilistic forecast for additional interpretation.
What about precipitation? Again, here's the simplified deterministic map given to the media..
Blue areas denote drier than normal, and red denotes wetter than normal (shouldn't the colour scheme be the other way around when talking about precipitation?) Note that the Prairies and NW Ontario are shown as wetter than normal, while the West Coast and the Arctic are drier than normal, as well as parts of Ontario. The probabilistic precipitation forecast is much more informative (as well as having a more logical colour scheme) showing near normal pcpn over much of the country, drier than normal over the Arctic, and wetter than normal over BC and Alberta. Again, a vastly different story than what the oversimplified deterministic forecast here shows. It should be noted however, that summer precipitation forecasts have little or no skill, (less than 35% accuracy as national average) which means they're pretty much useless over much of the country.
Put it all together, and the forecasts are actually pointing to near normal temperatures and precipitation for much of the Prairies.. hardly the hot dry summer being advertised.
So how are these long range seasonal forecasts produced? Seasonal 3 month outlooks are produced by the Canadian Meteorological Center's supercomputer in Montreal by running an ensemble of 4 global climate models initialized by weather patterns over the last 10 days of the month. The models run simulated weather patterns for the next 90-120 days, and an average temperature map for the country over that time period is produced. This temperature map is compared to "normal" temperatures for the country for that 3 month period, and the outlook map shows areas of warmer than normal , colder than normal or near normal temperatures according to the model ensemble. This outlook is totally model driven with no human intervention or augmentation whatsoever. Since it initializes with the last 10 days of weather patterns, long range outlooks are heavily weighted towards climate anomalies over the past couple of weeks. It does not take into account expected changes in ENSO patterns or other major climatic influences and as a result, the seasonal outlooks predictive skill is very low.
In contrast, seasonal outlooks from NOAA's Climate Prediction Center (CPC) incorporate expertise and input from human forecasters who have experience in long range climate patterns. As a result, CPC seasonal forecasts tend to be more realistic and more accurate. Even here though, predictive skill with seasonal outlooks is limited, with the best skill noted during major climate episodes like El Nino or La Nina. For what its worth, CPC is predicting a cooler and wetter summer over the Prairies this year (see image above), at least through June, with a trend towards warmer and drier conditions towards the latter half of summer.
So, still think we're going to have a hot dry summer?