Tuesday, October 18, 2011

Autumn in the Pacific Northwest

October 18, 2011
1:27 P.M.

Generally, the Pacific Northwest has gradual seasonal changes. Winter very gradually transitions into spring, and apart from a few thundershowers here and there, spring gradually shifts into summer. Summer also shifts gradually... from cool, cloudy days in June to nice sunny days in August. But there is one season in the Pacific Northwest that isn't quite so gradual to change, and this is autumn.

I was reading Scott Sistek's blog post about the warmth we would experience today, and he made an interesting point. The average high for September 18 is 70 degrees. The record high for October 18? 70 degrees. It's amazing what a difference a month can make.

A classic example of the variability of autumn weather was November of 2010. Take a look at the temperatures and weather recorded each day.

It's not too often that you see a high of 74 and a high of 25 spaced 20 days apart. And if you really want to see a crazy month for weather, look at November of 2006.

November 2006 had a lot of temperature variation, but it was very stormy. With an average windspeed of 11 mph and 15.63 inches of precipitation, November 2006 will go down in history as being one of the craziest weather months ever.

On October 12, 1962, the Columbus Day Storm pounded the Pacific Northwest. This storm was the earliest major windstorm to ever strike the Pacific Northwest, and it was the most powerful extratropical cyclone to ever hit the Pacific Northwest in terms of wind speed. I'll go ahead and show you a diagram of the peak gusts from this storm, just because it is amazing to look at.

From Wolf Read's personal website, "The Storm King"

Why is the weather in the autumn so variable around here? It all has to do with temperature differences between the polar regions up north and the tropical regions down south. As the earth receives less sunlight, both the air and water cool off. However, different parts of the earth receive different amounts of sunlight. The equatorial areas are receiving lots of sun right now since we are only a month after the summer solstice, and the north pole is shrouded in continuous night until spring. As a result, the water near the tropics is still very hot, but the polar regions, which have more land and lose heat more quickly, are becoming very cold very quickly.

Mid-latitude cyclones feed off of these differences in temperature, and these are most pronounced during autumn. Additionally, tropical moisture often gets entrained in the westerlies this time of year, providing vast quantities of energy for cyclone formation. The Columbus Day Storm actually started off as Typhoon Freda in the Pacific.

The spring transition is much more gradual because the polar regions, which are cold, heat up relatively quickly compared to the tropical regions. Since there aren't as many changes in temperature throughout the latitudes, there aren't as many powerful extratropical storms for us. Spring is generally dominated by severe thunderstorms and tornadoes because the temperature changes rapidly with height as opposed to latitude. With a high sun angle and an unstable atmosphere, severe storms can flourish. Since the Pacific Northwest generally does not get severe thunderstorms, springs are a pretty gradual transition for us. Take a look at the temperatures recorded at Sea-Tac this past April.

As you can see, these are pretty gradual changes. No big storms, no heat waves, no cold spells. The sun is at a much higher angle over our area in April than it is in November, yet last November saw a 74 degree high, while the warmest day for April was 66 degrees. The coldest high for April was 46 degrees, and the coldest high for November was 25 degrees.

It is very interesting to watch how quickly the weather changes around here in the fall, and I'm hoping for some wild weather swings in the next month or so.

Thanks for reading!

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