Pacific Northwest Storm Parade to Bring More Rain, Wind and Mountain Snow Through the Weekend

December 9,2015
A parade of Pacific storms that began last week will continue to impact the Northwest into the coming weekend, though their nature will be changing from those producing flooding rain, to those with lower snow levels more typical of mid-December.
According to the National Weather Service, about 10 locations in southwest Washington and northwest Oregon have reported a foot or more of total rainfall in the three days ending 5 a.m. PST Wednesday. The top rainfall total so far is 18 inches at Lee's Camp in Oregon's northwest coastal range. Parts of coastal northwest Washington have seen 4 to 10 inches, while interior locations have seen 2 to 5 inches since this past weekend.
Rivers have topped flood stage at over two dozen river gauges in western Washington and northwest Oregon. Stretches of the Snoqalmie, Snohomish, Cowlitz and Chehalis Rivers, among others, are either forecast, or have already pushed into major flood stage Wednesday, but will fall by later Wednesday or Thursday.
(NWS RIVER FORECASTS: Washington | Oregon)

Flood Alerts

This heavy rain soaking into unstable slopes has already triggered at least nine landslides, including one that shut down eastbound U.S. 30 on the northwest side of the Portland metro area near the St. Johns Bridge, another in Edmonds, Washington, forcing the shutdown of Sounder and Amtrak service between Seattle and Everett, and one that trapped a resident near Kalama, Washington.
(NEWS: Pacific Northwest Flooding)
A classic November-December setup featuring a powerful jet stream stretching from eastern Asia across the Pacific for 5,000 miles to the Pacific Northwest is acting as the conductor for this storm parade. The persistent pipeline of moisture is being supplied by what meteorologists sometimes refer to as an atmospheric river. In this case, the plume of moisture impacting the Northwest extends all the way from the western Pacific Ocean near the Philippines. See below for more details on atmospheric river events along the West Coast.

Western Radar

Current Storm (Wednesday)

As colder air filters in, snow levels will fall to 3,500 to 4,500 feet Wednesday.
(MORE: Up to 15 Feet of Snow on Mount Rainier)
The heaviest additional rainfall will target mainly western Oregon and southwest Washington early Wednesday, before rain tapers to showers briefly Wednesday afternoon.
The risk of additional landslides will also be enhanced in the region due to the recent heavy rain weighing on unstable slopes.
If that all weren't enough, strong winds may lead to downed trees and power outages across the Pacific Northwest.
Tuesday night, winds up to 60 mph ahead of the Pacific cold front downed trees and knocked out power in Tillamook County, Oregon. About 5,000 customers were without power in the northern Willamette Valley, mainly in Clackamas County, Oregon, as well.
The strongest winds will affect the Cascades through early Wednesday, shifting to east of the Cascades, the northern Great Basin, and northern Rockies/High Plains into later Wednesday and Wednesday night.
While the winds are not expected to be as strong as last month's destructive windstorm, power outages and downed trees/limbs are possible in these areas.
Numerous high wind watches and warnings have already been posted from the Northwest into the Great Basin, northern Rockies and High Plains.

High Wind Watches, Warnings, Advisories

As of Tuesday evening, the NWS reported that winds had gusted to 125 mph near Mount Hood and to 97 mph at Marys Peak.
(INTERACTIVE: Wind Alerts)

Wednesday Night-Thursday Storm; Another Storm This Weekend?

Another strong storm system will approach the Northwest coast Wednesday night and push inland Thursday with even more rain, mountain snow and strong winds.

This Week's Forecast

Some of the heaviest rain with this storm may target parts of southern Oregon and northern California. Showers will spread as far south as the valleys and coast of central California. Some showers may even reach southern California by Thursday night.
(FORECAST: Seattle | Portland | San Francisco)
Of course, this precipitation will fall as some much-needed snow for California's Sierra Nevada. The National Weather Service has issued winter storm warnings for the Sierra, where up to 2 feet of snow is possible in the highest terrain. Travel will be significantly impacted by not only heavy snow but also strong winds through passes in the Sierra, including on Interstate 80 over Donner Summit. Eventually, some accumulating snow will be possible at elevations as low as 3,000 to 4,000 feet as colder air arrives.
Snow will also spread throughout the Rockies Thursday into Friday as the southward dip in the jet stream associated with this storm moves farther inland.
Yet another system should impact the Northwest the Pacific Northwest Saturday, then slide southeast into California and the Great Basin Sunday.
This weekend system should put an end to the Pacific storm parade, for the time being, as the pattern shifts to a colder, drier scenario into next week.
Taken altogether, the heaviest additional rainfall through this weekend will likely be in southwest Oregon and northwest California, where totals in excess of 5 inches of rain are expected.

Additional Rain and Snow Through Sunday

In addition, 2 feet or more of snow should fall in the Cascades, Siskiyous and Sierra through this weekend, with some locally heavy snow also piling up over the Bitterroots and Tetons.
This is good news for a Cascades snowpack that is well below average for early December, from a snow-water equivalent perspective.
This is a pattern that was noticeably, largely absent last fall, winter and spring along the West Coast. Blocking high pressure aloft diverted the jet stream away from the U.S. West Coast last season, leading to the record-paltry Cascades and Sierra spring snowpack.
For the latest information on this complex setup, regularly check back to weather.com for updates.

More About Atmospheric Rivers

As mentioned earlier, the weather systems impacting the West Coast this week are tapping into what meteorologists call an atmospheric river. This term is used to describe a long, narrow plume piping deep moisture from the tropics into the mid-latitudes. One type of atmospheric river you may have heard of is the "Pineapple Express," a pronounced plume tapping moisture from the Hawaiian Islands to the U.S. West Coast.
Here are some more facts about atmospheric rivers:

• According to NOAA's Earth System Research Laboratory (ESRL), a strong atmospheric river can transport as water vapor up to 15 times the average flow of liquid water at the mouth of the Mississippi River.
• If an atmospheric river stalls over a particular area, significant flooding can be the result.  In fact, a study by Ralph et al. (2006) found atmospheric rivers responsible for every flood of northern California's Russian River in a seven-year period.
• They're also important for western water supply considerations. According to NOAA/ESRL, 30-50 percent of the average annual precipitation in the West Coast states typically occurs in just a few atmospheric river events.

Recap: Monday's Soaking

Late Monday morning, some roads were closed due to urban flooding in Tillamook County, Oregon, according to local emergency management. In addition, a few landslides were also reported. There were also reports of flooded intersections near the Portland International Airport.
In Washington, U.S. route 12 in Yakima county near Oak Creek was closed Monday morning due to a rock slide.
Significant urban and small stream flooding was also reported in the Portland, Oregon, and the Vancouver, Washington, areas Monday.
Portland, Oregon set a number of rainfall records on Monday, Dec. 7 including tying the record for the wettest calendar day in records dating to 1940 as 2.69 inches of rain fell. A 24-hour rainfall record for December was also set when 3.22 inches of rain was measured from 3 p.m. Dec. 6 to 3 p.m. Dec. 7.
Rainfall totals of up to 7.5 inches were reported in northwestern Oregon, near Lees Camp. Falls City in the central Willamette Valley measured 6 inches of rainfall from this system.
In Washington, the Seattle-Tacoma International Airport recorded 1.08 inches of rain which set a daily rainfall record for Dec. 7.
The top rainfall total since this wet weather pattern began is 11.44 inches as of Tuesday morning in Humptulips, Washington, which is located near the Olympic Mountains.
Strong winds have also arrived with a gust of 92 mph reported at Marys Peak in west-central Oregon early Monday morning and a gust of 141 mph was measured at Mount Hood on Monday afternoon.
In addition, freezing rain was observed in southern Idaho near Holbrook where 0.38 inches of freezing rain was reported with slick roads in the area.

Recap: Wind Reports Thursday, Saturday

On Thursday, Mount Lincoln in the Sierra of California reported a gust to 106 mph at 2:20 p.m. PST, with sustained winds as high as 74 mph earlier in the afternoon. Squaw Peak in southern Oregon recorded a wind gust of 107 mph just after 8:30 a.m. PST, with sustained winds of 80 mph an hour earlier.

A look at reported wind gusts on Thursday across the area.

Typically windy spots along the Oregon coast also reported strong wind gusts, with Mount Hebo clocking a 72-mph gust just after 9 a.m. PST Thursday.
The greater Reno area had reported just over 5,000 power outages around midday Thursday, most of the power had been restored by Thursday evening. A semi truck flipped over about 5 miles east of town, where local authorities believed high winds were the cause.
Winds were strong enough to partially blow off a metal roof from a structure near Montague, California. In Brookings, Oregon, the National Weather Service relayed reports of down fences, blown in window panes and other minor damage.
The next round of wind on Saturday was much less intense than what was observed two days prior. Although winds gusted to 78 mph atop Crystal Mountain in western Washington Saturday morning, most other wind gusts remained below 60 mph across the region.

Recap: Heavy Icing Tuesday Night, Wednesday (Dec. 1-2)

The first in the series of impactful weather systems arrived in the region this past Tuesday night into early Wednesday.
Freezing rain accumulations of a half inch thick were reported near Skamania, Washington. In Oregon, Troutdale saw ice accumulations of up to one quarter inch thick.
Ice and snow in eastern Oregon from the storm forced school cancelations in Hermiston, Pendleton, Umatilla, Boardman, Milton-Freewater and elsewhere, The Associated Press reported. Icy roads in the Columbia River Gorge also closed or delayed schools on both sides of the Oregon-Washington line.
MORE: Northwest Powerful Wind Storm, Nov. 18, 2015