Finally, some real rainfall for the Pacific Northwest

One thing that really stands out this winter is the lack of heavy rain in western Washington, especially since the start of the new year. A person has barely needed an umbrella in the past few months.

To illustrate, below is a chart of daily rainfall for the past six months at SeaTac. Some days are wet (up to 1-1.5 inches) in December and then taps are idle in 2023, with the wettest days only reaching about 0.5 inches. I cycled to work pretty much every day.

The central problem was the lack of warm, humid atmospheric river events moving into the Northwest. Instead, streams of moisture entered California. Interestingly, throughout this winter, San Francisco has received Seattle’s normal rainfall and vice versa (see below).

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A big change in all of this will happen this weekend. A strong atmospheric river with significant humidity will head into the Northwest on Sunday and Monday (see atmospheric humidity forecast below for 11 a.m. Sunday).

This stream of moisture will be supported by strong winds from the southwest, resulting in moderate to strong winds moisture flux (humidity times wind), which is a measure of how much water vapor is pushed into the topography of our region (see below).

As you can imagine, the impact of all this incoming moisture will be significant rainfall in the area, as shown by the total predicted through 5 p.m. Monday (see below). 3-7 inches in the mountains and a substantial strip south of Puget Sound.

As shown by a close look at the predicted rainfall, there will be a dark area northeast of the Olympics, with Sequim, Port Angeles and Victoria relatively dry. SeaTac Airport will get wet. Very wet.

The event will bring significant warming up high, and by the end of Sunday snow levels should rise to at least 6500 ft. The mountains will be challenging, first with heavy snow then heavy rain. Rivers will rise rapidly around the area.

Time to find your umbrella!