Friday, July 24, 2009

Cool and rainy summer forecast for Eastern Canada

Air Canada at Toronto Pearson International Airport on July 24, 2009
Photo by Sue Frause
From The Weather Network's Chris St. Clair:
"A few facts: Toronto, Montreal and Winnipeg have been 2-4 degrees below average almost day in, day out since the season began. Halifax nearly doubled its average June rainfall. Days with more cloud cover than sun were common nearly all across Canada ...

The rest of the summer, August at least, will be very close to average for nearly all of the country. The temperature should be where it ought to be but it is likely that cloudier and rainier days might prevail in the east … the weather pattern over North America and the weather we’ve had during the first part of our summer has a lot to do with something called the North Atlantic Oscillation, a pattern uncovered in the 1920’s by Sir Gilbert Walker.

The North Atlantic Oscillation is a variance in the location of a large area of strong and stable high pressure. For the past many weeks it has developed over Greenland and the Labrador Sea. The emergence of the North Atlantic Oscillation has led to a block in the usual, steady west to east migration of unsettled low pressure across our continent.

Simply, the cool rainy weather is stopped once it gets to the Great Lakes Basin because it cannot get past the big, stable high pressure over the western Atlantic. Not until the high pressure, that has manifested itself further east, relaxes will there be a change in the pattern.

While science continues to study the underlying reasons for the temperament and frequency of the oscillation, we can report that it is easing and more typical summer weather is returning to eastern Canada."

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