What is a ‘bomb cyclone?’ The weather term is back in the spotlight

In the wake of some brutal storms that struck the Southeast United States in the last couple of months — Hurricanes Helene and Milton — some people in Western Washington asked, “Do we get hurricanes here?”

The short answer is no, but this area does get hurricane-force winds via bomb cyclones.

What is a bomb cyclone?

Waters in the Northern Pacific Ocean are much cooler than 80 degrees. Yet strong, dangerous wind storms can and do develop. In fact, they frequently happen across the Northern Pacific during the fall and winter seasons. One such intense storm moved north into the Bering Sea last month — on Oct. 3

The storms that intensify rapidly are called meteorological bombs or the new term some have adopted – bomb cyclones.

One can happen when a cold air mass collides with a warm air mass, which is something that can occur over ocean waters, the National Oceanic and Atmospheric Administration (NOAA) stated, according to The Associated Press.

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The measurement needed to determine whether a cyclone can be classified as a bomb cyclone can be tricky, but it largely concerns a swift drop in pressure. Atmospheric pressure is measured in millibars by the National Weather Service. If a storm decreases 24 millibars or more in 24 hours or less, it can be considered a bomb cyclone, Stephen Baron, a forecaster with the weather service in Gray, Maine, said to The Associated Press.

“I would say rapid intensification of hurricanes is one of the more common times we see it,” Baron added. “We do see it with Nor’easters occasionally.”

Those in Western Washington often do not hear of these storms — until they approach the area.

Why is it happening on the West Coast?

The National Weather Service Weather Prediction Center has issued excessive rainfall risks starting Tuesday and running through Friday because of the powerful storm expected in northern California and the Pacific Northwest. The storm is arriving as the region experiences an atmospheric river, which is a long plume of moisture, over the Pacific Ocean.

The Weather Prediction Center said the storm intensified swiftly enough that it’s considered a bomb cyclone.

Bomb cyclones can happen in many places, and aren’t unique to the West Coast. They can occur in several parts of the world’s oceans, including the Northwest Pacific and North Atlantic.

What conditions could it bring?

Cliff Mass, a professor of atmospheric sciences at the University of Washington, said to KIRO Newsradio that he expects strong winds along the Washington coast and Vancouver Island on Tuesday, with gusts up to 60-80 mph. Downed trees and power outages are likely, he said.

“There may be a significant impact here in Western Washington because this very deep low offshore will create a large difference in pressure across the Cascades,” he warned.

During an appearance on “The John Curley Show” on KIRO Newsradio Monday, Mass explained that those on the coast can expect to see those winds pick up in a bit later in the day.

“The big, big event during the late afternoon (Tuesday), the winds will really pick up along the coast,” Mass said.

This storm is expected to bring severe rainfall to some areas. That could lead to flash flooding as well as winter storms in different parts of the West Coast depending on elevation.

High wind watches are also expected in some parts of the West Coast.

Travel is expected to be hazardous, and power outages are expected. There could also be significant damages to trees and infrastructure.

When else has it happened?

Bomb cyclones have been associated with major weather events all over the country in recent years. Hurricane Milton, which made landfall in Florida last month as a Category 3 hurricane, was a recent example of a bomb cyclone, Baron said.

A bomb cyclone in 2018, which helped popularize the term on social media, brought snow to the Southeast and winds that were close to hurricane force. Another in 2022 brought extreme weather and bitter cold to much of the country.

The last strong, widespread, damaging wind storm to strike Western Washington with winds well above 70 mph was the Hanukkah Eve Wind Storm of December 2006. Winds along the coast peaked near 145 mph. More than 1.5 million people lost electric power; some went without power for over a week.

Other memorable big, strong, impactful wind storms in recent decades include the 1993 Inauguration Day Storm that packed winds up to 65 mph, the 1979 Hood Canal Storm that had winds of 80 mph bringing down the floating bridge, the December 12, 1995 storm with winds up to 80 mph and the November 14, 1981 wind storm which had gusts up to 75 mph.

Yet the grand-daddy of them all, the wind storm all other wind storms are compared to, is the 1962 Columbus Day Storm: The strongest non-tropical wind storm to ever hit the lower 48 in American history. Winds along the Oregon and Washington coast had gusts up to 150 mph, and western interior winds from Eugene, Oregon to Vancouver, British Columbia, were in excess of 100 mph.

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The storm along the West Coast claimed 46 lives, hundreds were injured, thousands of buildings destroyed, power outages to millions from the San Francisco Bay Area to British Columbia, and it blew down 15 billion board feet of timber from the coast to Western Montana, enough lumber to build a million homes.

The Washington State Climatologist’s Office estimated the region gets a strong, damaging windstorm with winds up to 60 mph about every 10 to 20 years. Wind storms with hurricane-force winds occur far less frequently, yet they do impact the region with downed trees, and power and communication outages for extended periods of time. In the wake of the Columbus Day Storm, many did not have power restored for more than two weeks.

Back in 1962 with the Columbus Day Storm, utility crews from all over the nation moved into the region to help restore power. The effort was exceptionally challenging since even those big BPA transmission towers carrying hydroelectric power from east of the Cascades were toppled to the ground. Some did not get power restored until November.

In the Hanukkah Eve Wind Storm of December 2006, Puget Sound Energy (PSE) noted they lost 70% of their infrastructure, including the loss of power lines and power poles by the hundreds.

Contributing: Ted Buehner and Charlie Harger, KIRO Newsradio; The Associated Press; Steve Coogan, MyNorthwest