A strong winter storm is forming near the Pacific Northwest, set to unleash damaging winds, snow in the mountains, and heavy rainfall in parts of California, Oregon, and Washington. This could lead to flooding, mudslides, and rockslides across the region.

The storm, referred to as a “bomb cyclone,” is accompanied by an atmospheric river—a band of moisture-rich air that will deliver heavy rain at lower elevations and substantial snow in the higher altitudes, as reported by AccuWeather. The intensifying storm is anticipated to drench the area through Friday and possibly extend into the weekend in certain locales, according to The Weather Channel.

Due to the heightened risk of flooding, NOAA’s Weather Prediction Center has issued an unusual high-risk excessive rainfall outlook for northwest California. The most intense rainfall is expected to shift from Northern California to southwest Oregon, with some regions possibly receiving up to 15 inches of rain, moving northward through western Oregon and Washington. Areas recently affected by wildfires and mountainous roads may be especially vulnerable to mudslides and rockslides, as highlighted by the outlet.

The National Weather Service has also issued winter weather alerts and a blizzard warning covering the Washington Cascades to California’s northern Sierra, as snow levels are expected to rise over the week, according to The Weather Channel. Wind gusts may reach 50 mph, with 12 to 24 inches of snow forecasted to accumulate at elevations above 3,000 feet in the Olympic Mountains, as stated by the weather service. The combination of blizzard conditions and flooding at lower elevations is likely to severely impact travel, as reported by AccuWeather.

Meteorologists expect that coastal areas of Northern California, Oregon, and Washington could experience gusts up to 70 mph later on Tuesday and into early Wednesday, which might lead to falling trees and power outages.

“Numerous flash floods, dangerous travel conditions, power outages, and tree damage are to be expected as the storm hits peak intensity on Wednesday,” the Weather Prediction Center noted on X.

What is a bomb cyclone?

A bomb cyclone is a powerful coastal storm that occurs during the cold season, often referred to as a winter hurricane due to its significant strength.

These storms undergo a rapid intensification process called bombogenesis, where the atmospheric pressure falls sharply, indicating that the storm is gaining strength, according to the National Oceanic and Atmospheric Administration (NOAA).

Bombogenesis occurs when the central pressure of a storm decreases by at least 24 millibars within 24 hours. A millibar is a unit that measures pressure; the lower the pressure, the more powerful the storm tends to be.

Some of the most severe winter storms that have impacted the coasts of the country are classified as bomb cyclones.

The term “bombogenesis” is a blend of cyclogenesis, which describes how a cyclone or storm forms, and bomb, which is self-explanatory.

“This phenomenon can occur when cold air interacts with warm air, such as that above warm ocean waters,” NOAA explains. “The creation of this rapidly intensifying weather system is known as bombogenesis, leading to a bomb cyclone.”

In the 1940s, meteorologists began calling some significant coastal storms “bombs” because they develop “with a ferocity that is rarely if ever observed over land,” said Fred Sanders, a retired MIT professor who popularized the term in a 1980 article in the journal Monthly Weather Review.

What is an atmospheric river?

Commonly referred to as “rivers in the sky,” atmospheric rivers play a crucial role in causing extreme rainfall and snowfall in the Western United States.

These atmospheric rivers operate similarly to surface rivers but can actually transport more water than even the Mississippi River. They have the capacity to flow for thousands of miles.

They account for over 90% of the water vapor that moves to mid-latitudes from tropical regions, as explained by a blog from Kai-Chih Tseng for NOAA. Atmospheric rivers can occur globally but are most frequently found along storm paths near jet streams, Tseng noted.

When examining atmospheric rivers, scientists typically analyze systems that are over 1,000 miles long, less than 620 miles wide, and about 1.8 miles deep. Research indicates that they usually last about 20 hours when positioned over coastal areas.