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

An especially strong jet stream over the Pacific Ocean is spinning off a series of five or six weather systems, including a massive “bomb cyclone” that is expected to arrive in the Seattle area on Friday.

The system is expected to bring heavy rain and gusty 40 mph winds, according to local weather experts.

“What is remarkable is how big it is in scale, how deep the center is and the speed with which it goes from an open wave to a super-intense low-pressure system,” said Joe Boomgard-Zagrodnik, an agricultural meteorologist for Washington State University. “Meaning it will seem to explode out of nowhere.”

This incoming “massive system” has the classic comma shape of a cyclone, a very low-pressure middle and “frontal bands spiraling out 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 set off in the tropics. This series began with the relatively benign storm Namtheun that is “helping supercharge what was already an impressive jet stream.”

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

What cyclones originating in the tropics lack in size, they make up for in moisture, he said, most of which will be fed directly into the mid-latitude storm track. That means a lot of water is headed our way.

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

While the storm that’s expected to hit Monday could be “more of a troublemaker,” said Boomgard-Zagrodnik, for many residents of the Puget Sound region, the coming week’s weather could feel simply like “continuous fall rain,” rather than separate events.

Some regions further inland may experience a short break in the rain between weather systems, but most along the coast will not, said Mary Butwin, a meteorologist with the National Weather Service in Seattle.

There’s a chance of thunderstorms Friday and showers that will linger through the weekend before the next round hits, she said.

The weather system expected around Tuesday could bring cooler temperatures, lower snow levels and a chance of accumulation in the mountains, she said.