Weekend Storm to End the Western Storm Parade With More Rain, Wind and Mountain Snow

Following some welcomed mountain snow, one final storm through the weekend finally heralded the temporary end of a Western storm parade that has been ongoing for over a week.

According to the National Weather Service and the Community Collaborative Rain, Hail & Snow Network (CoCoRaHS), at least nine locations across the Pacific Northwest have reported more than two feet (24 inches) of total rainfall so far this month, as of Friday Dec. 11. Much of that has fallen over the last week.

The top rainfall total so far is 28.20 inches at Baring along Washington's western slopes of the Cascades, reported by a CoCoRaHS observer. Parts of coastal northwest Washington have seen more than a foot of rain, while interior locations have seen 4 to 8 inches since last weekend, including Seattle.

Rivers 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, pushed into major flood stage Wednesday, but most will continue to fall into the weekend.

(NWS RIVER FORECASTS: Washington | Oregon)

This heavy rain soaking into unstable slopes has triggered numerous 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)

If that wasn't enough, an EF1 tornado touched down on the south and east sides of Battle Ground, Washington, just before midday Thursday. An estimated 36 homes and two businesses sustained damage from the tornado, packing winds estimated up to about 104 mph, according to the National Weather Service. 

Northstar ski resort in California reported 24 inches of snow in 24 hours from Thursday morning into Friday morning. In Tahoe City, California 12 inches of snow was measured on Friday morning. South Lake Tahoe reported an additional 14 inches of snow from Sunday into Monday morning (Dec. 13-14).

Winds were also gusty in southern California on Friday. Whitaker Peak (elevation 4,120 feet) in the mountains of northern L.A. County, gusted to 79 mph Friday evening.

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 acted as the conductor for this storm parade. The persistent pipeline of moisture has been supplied by what meteorologists sometimes refer to as an atmospheric river. In this case, the plume of moisture impacting the Northwest extended all the way from the western Pacific Ocean near the Philippines. See below for more details on atmospheric river events along the West Coast.

(MORE: Best Weather Images of Pacific Onslaught)

This has been good news for a snowpack that was generally below average for early-mid 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.

Fortunately, a brief period of drier weather is settling into the storm-fatigued West through early this week.

The next storm may not arrive until Wednesday or Thursday.

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.

Thursday's Storm Reports

We mentioned earlier the National Weather Service confirmed an EF-1 tornado near Battle Ground, Washington, where some trees and buildings were damaged.

There were also a pair of large hail reports, one from Gold Beach, Oregon and another from five miles north of Arcata, California. In both instances, quarter-sized hail was reported.

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.

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.

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