‘Bomb cyclone’ expected in the Seattle area. Here’s what to know

A particularly strong jet stream over the Pacific Ocean turns off a series of five or six weather systems, including a massive “bomb cyclone” slated to hit the Seattle area on Friday.

According to local weather experts, the system is expected to bring heavy rain and gusty winds of 40 miles per hour.

A remarkable “bomb cyclone” will develop off the PNW coast on Thursday. More about the meteorology behind this storm on the CZ: https://t.co/cFxEInm1PO

– Joe Boomgard-Zagrodnik (@joejoezz) October 18, 2021

“What’s remarkable is how big it is, how deep the center is, and how quickly it transitions from an open wave to a super-intense low pressure system,” said Joe Boomgard-Zagrodnik, an agricultural meteorologist for Washington State University. “That is, it seems to explode out of nowhere.”

This incoming “massive system” has the classic comma shape of a cyclone, a very low pressure center, and “frontal bands protruding hundreds of miles from the center,” he said.

As with most extreme weather events in the Pacific Northwest, he explained on his blog, The Convergence Zone, this series was triggered in the tropics. This series began with the relatively harmless Storm Namtheun, which “helps reinforce an already impressive jet stream”.

The cyclones, which have a low pressure center, spin out of the jet stream, then deepen and eventually disintegrate, he said.

What tropical-origin cyclones lack in size they make up for in moisture, most of which is fed directly into the mid-latitude storms. That means that a lot of water is coming our way.

It won’t be particularly unusual or atypical of our region at this time of year, but it is part of a larger weather pattern that marks the real start of the rainy season.

While the storm expected on Monday could be “more of a troublemaker”, Boomgard-Zagrodnik said for many residents of the Puget Sound area, the weather for the coming week could simply feel like “continuous autumn rains” rather than isolated events.

Tomorrow marks the 18th anniversary of Seattle’s wettest day, falling more than 5 “. While we don’t expect nearly that amount of rainfall, a shift to wetter conditions will affect much of the west in the coming days as the systems arrive #wawx pic.twitter.com/JUP5dWMrKA

– NWS Seattle (@NWSSeattle) October 19, 2021

In some regions further inland, there may be brief rain breaks between the weather systems, but most along the coast do not, said Mary Butwin, a meteorologist with the National Weather Service in Seattle.

There is a possibility of Friday thunderstorms and showers that will last long the weekend before the next round hits, she said.

The weather system expected around Tuesday could bring cooler temperatures, lower snow depths and the possibility of mountain buildup, she said.