Sequim, Washington, lies in a rainshadow (Blue Hole) where the annual rainfall has averaged around 16 inches per year over the past few years. This is about the same amount of precipitation as San Diego or Los Angeles. It is so dry in Sequim that irrigation is necessary to raise crops and keep the area green. In fact, Sequim's major festival is the annual Irrigation Festival held in May to commemorate the opening of the first irrigation ditches over 100 years ago. Yet, less than 50 miles away, on a bend in the Wynoche River, the rainfall is the heaviest in the United States, averaging over 150 inches annually.
Why does this happen?
When warm marine air from the Pacific Ocean reaches the Olympics, it is forced upwards and cooled. The cold air can't hold as much moisture as warm air and it rains. But, it is the west side of the mountains that gets the drenching. That cold dry air then starts to flow down the eastern side of the range. As it descends, it warms itself by compression. This newly warmed air travels a considerable distance before it again becomes heavy with moisture, where it converges with other moisture laden air flowing inland from around the southern end of the Olympics. These bodies of precipitation usually converge off the lower end and eastern side of Puget Sound.
Sequim, because of its sheltered position close to the north end of the Olympics, is favored with dry air. The town gets a good flow of warmed mountain air, but before it has taken on much moisture. It is also under the influence of other air currents, which disperse clouds and prevent fog from forming. Only about three or four days of fog are expected annually. In addition, Sequim is almost completely protected from the onslaught of southwesterly storms that often blast the upper Puget Sound region.
When warm marine air from the Pacific Ocean reaches the Olympics, it is forced upwards and cooled. The cold air can't hold as much moisture as warm air and it rains. But, it is the west side of the mountains that gets the drenching. That cold dry air then starts to flow down the eastern side of the range. As it descends, it warms itself by compression. This newly warmed air travels a considerable distance before it again becomes heavy with moisture, where it converges with other moisture laden air flowing inland from around the southern end of the Olympics. These bodies of precipitation usually converge off the lower end and eastern side of Puget Sound.
Sequim, because of its sheltered position close to the north end of the Olympics, is favored with dry air. The town gets a good flow of warmed mountain air, but before it has taken on much moisture. It is also under the influence of other air currents, which disperse clouds and prevent fog from forming. Only about three or four days of fog are expected annually. In addition, Sequim is almost completely protected from the onslaught of southwesterly storms that often blast the upper Puget Sound region.
Rain maps and rainfall records, kept since the early part of the nineteenth century, indicate that one can expect an increase in rainfall of about 1.6 inches per mile as you travel west from Sequim, and an increase of about 1 inch traveling in the opposite direction. The "Blue Hole" extends about six miles west and seven miles east from Sequim, and from the Olympic foothills to the Strait of Juan de Fuca.
November through January are traditionally the wettest months in Sequim, averaging from about three inches in December to a little less than two inches in January. The area's driest period is from July to September.
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