Flood Threat to Escalate in Northwest US From Onslaught of Storms

By Kristina Pydynowski, Senior Meteorologist

December 8,2015; 10:58PM,EST

 

 

The threat for significant flooding will heighten across the Pacific Northwest through midweek as the seemingly endless train of storms continues to barrel onshore.
The Northwest is in the midst of an onslaught of Pacific storms. Every day through at least Thursday, a storm will slam the region with heavy rain, mountain snow and strong winds.
Snow levels will rise above the mountain passes early this week before falling back to these levels with the storm that moves in on Wednesday.

Blizzard Conditions at Snoqualmie Pass in Washington

"[The parade of storms] will bring the threat for flooding, especially as the week progresses and the rain continues to fall," AccuWeather Meteorologist Brandon Zapolski said.
Residents should prepare for incidents of flash and urban flooding that will only increase with each system. Sporadic power outages and mudslides are also possible.
The strongest winds into Wednesday will gust to 60 mph (95 km/h) along the coast, immediately to the east of the Cascades and to the lee of the northern Rockies. Gusts can approach hurricane force along the Oregon coast and in part of northwestern Montana.
The persistent pounding this week through Thursday is expected to unload 8-12 inches (200-300 mm) of rain, with more possible locally, on coastal communities from southern Vancouver Island to northern Oregon and the western slopes of the Washington Cascades.

This rainfall is on top of the 2-12 inches (50-300 mm) that fell during the first seven days of December.
In just the 24-hour period ending at 4 p.m. PST, Monday, Dec. 7, 2015, the coast range received 2-8 inches of rain with a general 2-4 inches of rain elsewhere in western Oregon and Washington.
An additional 1 to 3 inches (25 to 75 mm) of rain will fall in Portland, Oregon and Seattle during Tuesday night through Thursday.
Both cities typically receive just shy of 5.50 inches (140 mm) during all of December, not in a span of a week.
Through Dec. 7, Seattle has received 3.30 inches (84 mm) of rain so far this month. Portland has received 5.44 inches (138 mm)

The short-run rivers to the west of the peaks of the Cascades will be very susceptible to flooding in this situation.
The Skokomish River near Potlatch, Washington, has been hovering at major flood stage since Sunday and could approach record stage during the middle of the week.
Other rivers will certainly rise through this week as runoff from the heavy rain drains downstream. With snow levels rising early in the week, melting snow will only heighten the flood risk.
The Johnson Creek near Sycamore, Oregon, reached a record crest of 15.33 feet on Monday night. The old record was 15.30 feet set on Nov. 19, 1996.
"The first in the parade of massive storms pounded the mountain passes of the Cascades with snow this weekend," AccuWeather Meteorologist Dave Samuhel said. "At least a foot (30 cm) of snow fell in the major passes of Washington, snarling traffic."
Whistler Blackcomb Ski Resort in British Columbia reported nearly 16 inches (40 cm) of snow in 24 hours over the weekend.
"The next storm will flood the [Northwest] with warmer air," stated Samuhel. "This is good news and bad news."
RELATED:
AccuWeather Winter Weather Center
Northwest US Interactive Radar
Late-Week Storm to Drop Feet of Snow in California's Sierra Nevada
Snow levels will rise to 8,000 feet (2,400 meters) into Tuesday night.
"The good news [with the rising snow levels] is that road conditions will become wet," Samuhel said. "However, the rain on top of fresh snow will likely lead to flooding and even avalanches."
"The main passes, such as I-90's Snoqualmie Pass, may close at times for avalanche control the next few days," Samuhel said.
A few yards (meters) of snow will bury the high country of the western slopes of the Cascades. This also includes Whistler Ski Resort, where the storm train will be an all-snow event.
Snow levels will fall back to pass level later this week as the storm on Wednesday ushers back in colder air.

The cold press will expand beyond the Northwest later in the week, causing snow levels to crash and snow to return to the rest of the mountains of the Western United States. Significant snow will bury the Sierra with enough snow to disrupt travel aiming for the mountains of the Rockies and Four Corners regions.
"The snow could accumulate a couple of feet in the highest peaks of the central Sierra, the part of the range that needs mountain snow the most for ski resorts and water resources," stated AccuWeather Meteorologist Brian Lada.
Snow showers could even fly around Salt Lake City, but the air will likely not get cold enough for snow around Seattle and Portland.
Despite the cold and storminess pressing southward, the persistent rain in the Pacific Northwest will not translate south to Los Angeles. While steadier rain will wet San Francisco on Thursday, the rain should diminish to spottier showers by the time it reaches Southern California Thursday night into Friday.
"The evolution of the El Niño pattern should direct storms farther south [on the Pacific coast] later in December, but more so during January and February," AccuWeather Meteorologist Ben Noll said.
As the storm drops into the Southwest later this week, additional storms will continue to roll into the Northwest. However, the spacing between the storms will be greater starting this weekend and into next week.