Portland Weather Patterns

Looking over Portland from Mount Tabor on a cloudy December day. While December and January are seeing normal amounts of rainfall, November was alarmingly dry. Photo by Abe Nelson.

As climate change continues to become more of an urgent problem, we are seeing more and more of its effects. November 2019 was the fifth driest in 75 years for Portland, according to the National Weather Service. Similarly, Salem had its third driest November in 125 years, and Eugene and Astoria had their fifth driest in 125 years. 

KGW’s Matt Zaffino, who has been a meteorologist in Portland for 31 years, says it is very difficult to determine directly how much of Oregon’s changes in weather patterns are a result of climate change. He says that what he has observed most is Portland’s population growth, which makes the effects of extreme weather events, such as the 1996 flood, worse. In terms of climate change, he says he and others in his field have noticed more global changes, including higher average temperatures. He observes weather patterns and temperatures “that we never used to see and we see now.”

This November there were a total of 1.52 inches of rain, over 5 inches below normal precipitation, which is the average precipitation over the last 30 years. To put it into comparison, the record for Portland’s rainfall in November, which was set in 1875, was 10 times the amount of rainfall that we got this November.

According to the EPA, precipitation has increased across the country, 2015 being over four inches above the century average. This is due to the fact that warmer air, which is a result of climate change, is able to contain more water than cooler air. As climate change continues to get worse, areas with more rainfall will get more rain, and areas with less rainfall will continue to get less, making more extreme weather patterns.

This is not to say that climate change is the only driving factor of changes in weather patterns. Natural variation is still the main influence on the weather. Zaffino explains this November was dry as a result of changes in the jetstreams and storm tracks of the United States. This means that the weather normal for Oregon was diverted elsewhere. For example, while Oregon had a drier November, California had above average precipitation. It is impossible to detect how many of these abnormal weather patterns have to do with climate change, but new studies are showing that almost every aspect of the weather is being affected by climate change. For example, a study published by the journal Science Advances in 2018 found that climate change could greatly alter the world’s jetstreams.

While it does seem like Portland’s weather is changing, this past November isn’t going to set the forecast for the rest of the winter. Already, December had more average precipitation levels, with only 1.1 inches less rain than usual. Zaffino expects the rest of the winter to continue near average both in terms of temperature and precipitation.

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