The West Coast has had a very wet winter as a low-pressure system has stalled off the coast — and no one knows why.

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Heavy rainfall is wreaking havoc along the West Coast, where some cities have received more than twice the amount of rain they’d normally get.

So far this year, Seattle has logged 12.55 inches of rain — 150 percent of what would fall in a normal year, said meteorologist Dana Felton, of the National Weather Service’s Seattle office.

But we’re looking positively arid compared to Sacramento, Calif., where more than 18 inches of rain has fallen since New Year’s Day. That’s more than 284 percent of what they’d normally get.

Other cities receiving abnormally high levels of rain: San Francisco, Portland and Los Angeles, all of which have received more than 200 percent of their normal precipitation.

“Something like this has happened before,’’ Felton said, “It just hasn’t happened in a while.”

The rain has brought floods and landslides to cities along the West Coast.

Last week, 13 vehicles were trapped when a mudslide dumped up to 2 feet of debris along Interstate 90 east of Issaquah. And the West Point Treatment plant in Magnolia, running at half capacity since a pump failed Feb. 9 and caused catastrophic damage, was forced to dump wastewater into Puget Sound when runoff overwhelmed its diminished treatment capacity.

In Northern California, which has recorded its wettest winter on record, rain has caused widespread flooding and an ongoing crisis with the Oroville Dam.

So what’s going on?

Seattle meteorologist Jay Albrecht said a low-pressure system along the West Coast has stuck around longer than normal. No one knows why, he said.

Although there have been a few periods of cold, clear weather, overall, “we’ve had unusually stationary weather patterns,’’ Albrecht said. “Everything’s kind of weird.”

Seattle also is having its coldest winter in decades, according to Felton.

“The average temperature in Seattle is 39,’’ he said. “That will end up being the coldest December to February since 1985.”

Showers are expected to continue for the next few days, bringing snow or a mix of snow and rain to some parts of Western Washington.