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The two forces – a bomb cyclone and an atmospheric river – will come together to deliver heavy rains to the west coast this weekend.

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And, according to the National Weather Service, it’s normal for both systems to occur together.

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related: Atmospheric river is bringing heavy rain to the Bay Area: Here’s the outlook for the weekend

“That’s because when you have a system of low pressure so deep, like it would on Vancouver Island and southwest British Columbia on Sunday night, because of the strong pressure gradient, you’re going to get a lot of wind ahead of the system. “In this case, a strong southwesterly wind over northern California,” said Brian Hurley, principal forecaster for the Weather Prediction Center.

What are the main dangers?

By Saturday night, the Pacific cyclone will rapidly intensify and direct the mighty atmospheric river over the West Coast, bringing subtropical moisture to California.

Water vapor satellite imagery by NOAA(Credit: NOAA)

The main effects associated with the cyclone and atmospheric river will be strong winds and heavy rainfall, mainly from Sunday to Sunday evening.

Winds will average 25 to 40 mph, with gusts of 60-65 mph over northern California into southwest Oregon and higher from coastal areas to the Cascades and western slopes of the Sierra.

Rainfall will average between 3-6 inches, with some isolated areas likely to see higher totals locally.

“In terms of precipitation, precipitation rates of half an inch to an inch or more per hour are expected to move into northern California by Sunday morning, then gradually shift southward into central CA later Sunday and Sunday night. It will happen,” Hurley continued. “This would create a flash flood risk, especially on areas with burn marks that were devastated by recent fires.”

This has prompted the National Weather Service to issue flash flood clocks for some areas.

Hurley noted another risk of high-altitude snowfall on Sunday and Sunday night. Snowfall is expected between 1–2 feet at the higher elevations of the Sierra, with local amounts of up to three feet.

The Storm Prediction Center has issued a risk level of 1 in 5 for severe storms for the Oregon coast on Sunday afternoon and evening, as of Friday evening.

What are ‘bomb cyclone’ and ‘atmospheric river’?

A bomb cyclone occurs when a mid-latitude cyclone intensifies rapidly, dropping at least 24 millibars in 24 hours.

Meanwhile, atmospheric rivers are relatively long, narrow regions in the atmosphere – like rivers in the sky – that carry most of the water vapor outside the troposphere.

related: The science behind atmospheric river and its rain

“These columns of vapor move with the seasons, containing an amount of water vapor equal to the average flow of water at the mouth of the Mississippi River,” Hurley said. “When atmospheric rivers make landfall, they often release this water vapor as rain or snow.”