October 18, 2009 in Idaho Voices

Why does summer end so abruptly?

Michelle Boss
 

During the past three years of these columns, one of the things that I’ve often pointed out about October, is that it is a transitional month. My husband, also a meteorologist, and I have noticed in the 10-plus years we’ve lived here, that the weather almost always seems to go downhill after about the first week of October.

By downhill, I mean a rather rapid change from the warm and dry weather we experienced in September, to a much colder, unsettled pattern. Though many folks look forward to fall, it seems like the last vestiges of summer end so abruptly.

With 90-degree temperatures here as recently as three weeks ago, it is a shock to the system that we have already heard stories of winter storms from Montana, east to Denver, across the Northern Plains, and even as far south as Kansas. Locally, record low temperatures were shattered throughout last weekend, as temperatures plunged into the single digits in places like Pullman, and in the teens in Coeur d’Alene.

Earlier forecasts from the Climate Prediction Center, called for a warmer and drier than average October for much of the Northwest, influenced in part by the strengthening El Niño. As it stands, temperatures have averaged almost 10 degrees below normal for the first half of the month. Precipitation has been above normal, and it looks like the cool, wet weather will persist through Oct. 21.

The good news – at least for those who would like to enjoy some fall weather before winter is upon us – is that the next string of storm systems look to be Pacific in origin, rather than “arctic.” This should keep the snowflakes at bay at least for now.

Speaking of “arctic” and “Pacific” air masses, my daughter asked a question that many may wonder about when we have a string of chilly, but sunny days. How is it that a day full of sunshine, can be so cold? Despite full sunshine on both Oct. 10 and 11, high temperatures only climbed into the lower 40s, more than 20 degrees below normal. The answer lies in the origin of the air that moved into the region. Meteorologists used the term “arctic” not only to denote that the air was cold, but to describe where the cold air came from.

During that time, winds from the surface up to the mid-levels of the atmosphere, had a northerly component. The air that moved into the Inland Northwest really did have arctic origins. Though the sunshine worked to warm the air around us, the continued flow from the north kept moving new cold air in. On the flip side, despite the fact that clouds moved in on Oct. 12, the air around us began to moderate as the general flow became more westerly, and milder Pacific air moved in to replace the arctic air. Even with very little sunshine (and quite a bit of rain in some places), temperatures were able to climb into the lower to mid-50s.

Michelle Boss can be reached at weatherboss@comcast.net


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