Average august had some extremes

Two days of 90 degree heat and several days of rain due to convergence zone conditions averaged out to near normal.

Total rainfall and temperatures in August were very close to the long-term Coupeville averages. However, August served up some unusual weather. August began with five consecutive foggy mornings, and ended with winter-like wind and rain. Sandwiched in the middle of the month we had typical sunny, warm days that we expect in August, which included two days of real heat (90 and 91 on my thermometer on August 14 and 15).

While Coupeville got two consecutive hot days, Seattle set a record with 3 consecutive days of 90 degrees and above, once in July and again in August. So, I decided to check the 97-year Coupeville weather data for information on the number of consecutive days at 90 degrees or higher.

Since 1950 Coupeville has had only one occurrence in those 61 years of 3 consecutive days above 90 degrees. The official Coupeville NWS station on Terry Road (Dave and Delores Engle) measured 92, 94, and 91 degrees on July 27, 28, and 29 in 2009. Clearly, 90 degree temperatures are rare in Coupeville and only last a few days. Cool marine air is near and soon comes to our rescue.

The other unusual feature of August weather was the occurrence of no less than three convergence zone storms from August 22 through 31. Convergence zone storms most often occur between Everett and Seattle due to the convergence of winds from the north and south after flowing around the Olympic Mountains. In late August the winds shifted and placed the convergence zone further north and closer to Whidbey Island.

The first convergence zone on Sunday August 22 brought rain to South Whidbey, as Freeland and Langley got from 0.25 to 0.50 inches. Central Whidbey escaped most of the rain, however, and I measured only 0.01 inches near Crockett Lake. The second storm formed on Thursday August 26, and stretched from the Strait of Juan de Fuca to Everett, Mt Vernon, and Bellingham. Between 9AM and 11AM that morning my rain gauge captured 0.36 inches, with Everett at about 0.50 inches and up to one inch over Snohomish County.

The August 31 convergence zone was the worst of the three because it was much larger and enhanced by a dose of moist tropical air. Strong winds, heavy rain, and cool temperatures (maximum of only 57 degrees) made for a nasty day. Rainfall over Central Whidbey was about 0.60 inches. The nastiness was even worse to the east of us, however, as Paine Field got 1.72 inches of rain, and several observers east of Everett measured more than 2 inches.

You might be wondering about the expected weather for the rest of September and the coming fall months. Of course, the sun is setting earlier and night time temperatures are beginning to cool, but September weather is often very good. The long-term Coupeville records show that it is not unusual to have 10 to 14 days with maximum temperatures above 70 degrees. And average rainfall is 1.26 inches, which is only slightly more than the average August at 0.92 inches.

Weather forecasts extending out more that 10 days have a reputation of not being accurate. In recent years, however, forecasting accuracy has improved based on understanding of the link between ocean temperatures and the atmosphere. Last winter’s warm temperatures (record warmth in January) in the Pacific Northwest were predicted by the NWS Climate Prediction Center, based mainly on El Nino warm ocean temperatures in the Equatorial Pacific Ocean. Well, as of July 2010 those ocean temperatures are now colder than normal, which is called a La Nina condition. The forecast is for wetter and colder conditions than normal for the Pacific Northwest from now until April/May 2011. See the link below to the Climate Prediction Center for more information on La Nina.

The gentle smile of summer weather is transitioning to a demonstration of the powerful teeth behind the smile. Enjoy the September gentle smile while it’s here.

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