With winter just around the corner the various weather-forecasting companies and organizations are starting to come out with their winter forecasts. I will do a complete examination of all the different forecasts in next week’s article, as not all the forecasts are available yet. For this week I thought I would take some time to look at some of the factors or thoughts that go into making a long-range forecast.
The first realization that we have to come to when looking at long-range forecasts is that pretty much anyone can make one. What I mean by this is that we just don’t have enough of an understanding of how to forecast the weather beyond about 10 days, let alone several months in advance. So for the most part, long-range weather forecasting is pretty much guesswork. For some, it is an educated guess, but a guess nonetheless!
If we break down long-range forecasting into two parts, temperature and precipitation, and we look at each of these individually, a long-range forecast can have three possibilities, either above-, below-, or near-average conditions. So, if you simply guess at the forecast you have about a 33 per cent chance of getting the temperature or precipitation forecast correct.
Compare this to our educated guessers and you are not doing too badly. Apart from the almanacs, which have claimed 80 per cent accuracy in their long-range forecasts, all the other forecasters’ accuracy usually falls in the 40 to 50 per cent range. That means more often than not their long-range forecasts are not correct, and as I pointed out, you would probably be better off making your own forecast.
Now on to the big question — why is it that long-range forecasts are so hard to do? To understand this let’s look at some of the main controlling factors that help drive our weather. The first and probably most well-known factor is El Niño or the El Niño/La Niña southern oscillation (ENSO). This is a periodic warming and cooling of the tropical Pacific Ocean. El Niño is the warm phase while La Niña is the cool phase of this oscillation.
Both of these phases affect the atmospheric pressure patterns over the Pacific which in turn influence the weather we experience over North America. For us, an El Niño winter often, but not always, brings milder- and drier-than-average conditions, while La Niña will often bring colder- and wetter-than-average conditions.
Going into this fall an El Niño phase appeared to be developing, but over the last month it has weakened and is now considered to be neutral. Forecasts for the upcoming months continue to show either a very weak El Niño or neutral conditions to continue. This means the Pacific Ocean is likely not going to have any definitive influence on this winter’s weather.
AO and NAO
How about the North Atlantic Oscillation (NAO) and the larger associated Arctic Oscillation (AO)? We talked about these two connected oscillations of pressure patterns over the Arctic and North Atlantic in fairly good detail late last winter. The NAO controls the strength and direction of westerly winds and storm tracks across the North Atlantic. If there is a large difference in the pressure between Iceland and the Azores the NAO is said to be in the positive phase.
This often leads to increased westerly winds and a stronger southwesterly flow of air over eastern North America. This can prevent Arctic air from plunging southward, resulting in milder winters. If the difference in sea-level pressure between Iceland and the Azores is small, the NAO is said to be in the negative phase. Westerly winds tend to be weaker allowing Arctic air to spill southwards into North America more easily.
With the Arctic Oscillation in a positive phase we usually see the upper-level pressure pattern over North America in a zonal configuration, while the negative phase tends to have a much more meridional pattern. Last winter we saw a very negative AO and we can all remember back to last March when we experienced a very meridional flow, with much of North America recording record-shattering heat.
Sea ice factor
So far we have three different factors that influence our weather. Each is hard to predict and each one will have some kind of impact on our weather, it all depends on which will be the more dominant one and how long it will remain dominant.
Now, to make things even more difficult, we have record-breaking Arctic sea ice loss to add to the mix. The extra heat being absorbed by the open Arctic Ocean gets released back into the atmosphere in the fall, and new research is showing that this appears to be influencing large-scale weather patterns. We now have another complicating factor. We are also moving toward the next peak in the sunspot cycle (expected next summer). Research shows that this cycle may have an effect on the Arctic Oscillation and with us now in a very active phase this could also impact the winter weather patterns.
As you can now see, even if you have a background in weather, trying to make heads or tails out of all of these factors is tough. When you feel like taking a shot at coming up with a winter forecast, close your eyes, take out the darts, give them a throw — you just never know what you might come up with!