Bomb cyclone whips Northwest with ferocious winds as it moves inland

Alex Sosnowski

A potent storm that barreled into the Northwest from the Pacific Ocean on Wednesday will continue to bring a dose of heavy snow and rain to the Pacific coast of the United States.

The storm system intensified quickly enough Wednesday to qualify as a bomb cyclone as the central barometric pressure dropped 0.74 of an inch of mercury in 12 hours. In order for a storm to meet the bomb cyclone criteria, barometric pressure must plummet at least 0.71 of an inch, and do so within a 24-hour period.

The National Weather Service in Seattle said winds approached hurricane force (74 mph or greater) on Destruction Island late Wednesday when winds gusted to 72 mph. The island is located about 3 miles off the coast of Washington.

A new round of snow is in store for much of Washington, including some coastal areas as the storm pushes inland prior to the end of the week.

Up to an inch of snow fell around the Seattle area during Wednesday afternoon. Portland, Oregon, received a mix of snow, sleet and rain from the storm with little or no accumulation.

Up to a few inches of snow is forecast for The Dalles, Oregon, and Spokane, Washington, into Thursday night. Travel over the passes in the Washington Cascades will be a challenge, with up to a foot of snow predicted to accumulate.

The greatest amount of snow will focus over the Oregon Cascades, the northern Sierra Nevada and the Coast Ranges from southwestern Oregon to northwestern California. These areas are forecast to receive a general 1-2 feet of snow from the storm with an AccuWeather Local StormMax™ of 30 inches over the high country.

Freezing levels will dip low enough with ample moisture for heavy snow to fall over Siskiyou Summit along Interstate 5 in southern Oregon and Donner Pass along I-80 in California. AccuWeather meteorologists urge motorists to be prepared for slow and dangerous wintry travel conditions with the risk of road closures for a time.

Snow is also expected to fall farther south over California, including areas that experience snow only a couple of times each winter.

"A little will fall on the ridge tops north and east of San Francisco with the storm," AccuWeather Senior Meteorologist Dan Kottlowski said. "A few flakes can occur at the onset of the rain, but most of the small snow accumulation is likely at the tail end of the storm as colder air moves in later Thursday and Thursday night."

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Meanwhile, several inches of snow are also in store for the mountains in Southern California north and east of Los Angeles and east of San Diego. Motorists venturing over Tejon Pass, known as the Grapevine, could face a couple of inches of snow with slippery travel, the likelihood of delays and perhaps the risk of stopped traffic from Thursday to Thursday evening.

As snow falls over the mountains, wet weather produced by the storm is likely to be overall beneficial.

"This will be a drenching storm and has the potential to bring the most rain from a single storm so far this season for San Francisco," Kottlowski said.

From July 1, 2019 to Jan. 16, 2020, rainfall has been about 60% of average with 5.35 inches compared to a normal of 9.54 inches.

"The first 15 days of January alone typically bring about 2 inches of rain to San Francisco, and prior to this storm, there has only been just over 0.25 of an inch so far this month," Kottlowski said.

The rain should help to soak the brush and soil around the region with minimal risk of mudslides even though the rain will be heavy enough to cause urban flooding. Slick travel conditions and delays are a given with any storm that affects the region.

Heavy rain pummeled the San Francisco area during morning rush hour on Thursday with multiple incidents of street and highway flooding.

This radar image was captured at 9:26 a.m. PST Thursday, Jan. 16, 2020. A wall of rain and mountain snow was advancing southeastward across the northern part of the state.

Soaking rain is also forecast for the lower elevations and coastal areas of Los Angeles and San Diego with an average of 0.50 of an inch to 1 inch of rain and locally higher amounts.

A powerful winter storm impacts the U.S. West Coast on Thursday, Jan. 16, 2020. (NOAA/GOES-West)

The potent storm will also generate gusty winds that could add to travel slowdowns along with other issues along the Pacific coast.

"It looks like most of the strong winds will be on the front side of the storm from the west and southwest into Thursday," Kottlowski said.

Gusts through the passes and over the ridges can average 40-50 mph in Central and Southern California into Thursday night. Gusts along the Northern and Central California coast will tend to average 30-40 mph with a period of rough seas, large breaking waves and minor coastal flooding into Thursday night as well.

In the wake of the storm into Thursday night, another storm is forecast to roll into the Northwest this weekend. However, with less cold air around, little, if any, snow is likely to fall in Seattle. Snow and wintry travel delays would still be a concern for the major passes in the Cascades. Precipitation from the storm this weekend is likely to stay north of Donner Pass and may not be significant around Siskiyou Summit.

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