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One of the most intense extratropical cyclones ever to strike the Pacific Northwest drew an equally historic amount of moisture onto the West Coast of North America on October 24-25, 2021. The storm off the coast of Washington—with a central pressure of 942.5 millibars, equivalent to a category 4 hurricane—was the second extreme low-pressure storm in the North Pacific in a week. Both exhibited pressure drops of more than 24 millibars in 24 hours, making them “bomb cyclones.”

The two storm fronts directed streams of moisture from north of Hawaii toward the West Coast in long, narrow bands of moisture known as atmospheric rivers. Atmospheric rivers account for up to 50 percent of all rain and snow that falls in the western United States.

The parade of storms brought high winds and extreme precipitation that doused wildfires and brought some relief from extreme drought to Central and Northern California. But the events also caused power outages, flooding, landslides, and mud and debris flows that washed out roads.

The animation above shows a model of the movement of total precipitable water vapor over the eastern Pacific Ocean from October 10-25, 2021. Green areas represent the highest amounts of moisture. Note the fingers of white to light green that repeatedly flow into the Pacific Northwest of North America. Note also how areas in the tropics are often saturated with moisture that can eventually curl up into middle latitudes.

Total precipitable water vapor is a measurement of the amount of water in a column of the atmosphere if all the water vapor in that column were condensed into liquid. The animation was derived from a NASA product known as the Goddard Earth Observing System Data Assimilation System (GEOS DAS), which uses satellite data and mathematical representations of physical processes to calculate what is happening in the atmosphere. Not all precipitable water vapor actually falls as rain. However, it is a useful indicator of regions where rainfall could be excessive.

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