Weather models indicate La Nina, the weather pattern that typically brings cooler temperatures to Western Canada, is declining in strength. That means it shouldn’t affect the weather in western North America this summer.
“They all seem to weaken in the spring, according to the historical records we have,” said Mike Halpert, a meteorologist with the Climate Prediction Centre for the U.S. National Weather Service.
“As far as the forecasts we make, we’re not factoring La Nina in our forecasts for the summertime.”
Yet there is a chance that cooler weather could hang around for the next month because the Pacific Ocean may be warming in the tropics, but the equator is a long way from the Prairies.
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“The atmosphere usually lags those kinds of (ocean) changes,” said Halpert.
The lingering La Nina, which began last August, has dragged down temperatures for the last two months.
“It’s generally been cooler for the spring,” said Bob Cormier, an Environment Canada meteorologist. “Not record setting in any kind of way, but in a general sense.”
In Manitoba for example, Cormier said the region from the Red River Valley to the Saskatchewan border was two or three degrees below normal in March.
There is a possibility that cooler trend could continue, because the 2007-08 La Nina is stronger than the last one, which occurred in 2001.
The strength of a La Nina is measured by comparing the surface temperature in the equatorial Pacific to the historical average. This winter it was 1.5 C below the average surface temperature, which is in the moderate to strong category.
On top of La Nina, a weather pattern in the north Pacific may have contributed to cooler spring temperatures.
“The Aleutian low … we monitor that quite closely, because the strength of that particular low determines, to a great extent, what happens in Western Canada,” said Amir Shabbar, a research climatologist with Environment Canada.
What’s happening now, he said, is that the Aleutian low is weaker than normal, which brings more northerly winds and Arctic air to the Prairies.
“These two (La Nina and the Aleutian low) have been reinforcing each other this winter and spring. Consequently, the temperature has been below normal,” said Shabbar.
Since this is the first La Nina in seven years, climatologists are curious to see how it plays out.
“What becomes less certain is does it weaken and become near normal? Or does it weaken and hold in the weak status? That’s what we’re trying to figure out right now,” said Halpert.
Should La Nina fizzle out rapidly this spring, Shabbar said meteorologists will rely on other factors, like ice cover and wind patterns, for their seasonal forecasts for May and June.
Environment Canada is expecting below normal temperatures for May, from the West Coast to the middle of Saskatchewan. Normal May temperatures are predicted for eastern Saskatchewan and Manitoba.
