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Ask the Meteorologist: The Chetco Effect

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ASK THE METEOROLOGIST

What is the Chetco Effect, and Why Does it Only Happen in Brookings?

If you are a native to Southern Oregon or Northern California, you have probably heard of the Chetco Effect. It is a naturally occurring weather phenomenon that is exclusive to the Brookings Harbor area. It is when warm air is ushered into Brookings because of winds shifting out of the North/Northeast allowing for an offshore flow.

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The conversion of winds from the high pressure ridge and the thermal low force winds over the Coastal Range, creating the Chetco at Brookings.

The basic makeup of this phenomenon is during times when a dominant ridge of high pressure moves into and sits over our area for an extended period of time. Especially in the Spring and Summer, temperatures begin rising quickly throughout the day because of the lack of cloud coverage and overall warm air advecton (the moving of warm air to our region). When air heats up, it begins to rise and when that occurs, low pressure forms. A thermal (heat) low forms over Northern California, and the placement of both the thermal low and high pressure ridge are what forms the offshore flow.

Winds move counter clockwise around low pressure and clockwise around high pressure. The convergence of these winds usually occurs over the west side valleys. Then the winds begin to move southeast towards Brookings. Two events then occur. First, the warm air from the inland locations gets funneled down the Chetco River Valley all the way to Brookings. Also, the winds coming up and over the Coastal Range starts descending the air down the west side of the mountain range. There is a meteorological process called “dry adiabatic heating” that basically means when air lowers down the side of a mountain it tends to warm up. This is the same thing that happens with the “Santa Ana” winds in Southern California. So the combination of these two processes combine to bring the warm air to Brookings.

Also, the winds blowing offshore tend to be stronger than the winds blowing towards the land, therefore any cool air that moves towards the coast, gets pushed away.

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Sometimes, strong enough high pressure will create offshore winds, warming up other areas along the coast.

Brookings is not the only location that sees this warm up. Other areas along the coast can see these downsloping winds. However, the winds have to be just right for this to occur. Therefore, this phenomenon is rare in other locations. Brookings has the right set up, because of coastline orientation and geographical location. Although, North Bend did see a similar situation last April when temperatures rose to the lower 90’s because of offshore flow.