The danger for significant flooding will only 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 onshore 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.
"[Parade of storms]will bring the threat for flooding, especially as the week progresses and the rain continues to fall," stated AccuWeather Meteorologist Brandon Zapolski.
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 through Tuesday will gust to 60 mph (95 km/h) along the coast and immediately to the east of the Cascades with gusts to hurricane force to the lee of the northern Montana Rockies, such as in Cut Bank. An increase in winds across the Northwest will accompany Wednesday's storm.
The persistent pounding through Thursday will unload 8-12 inches (200-300 mm), locally more, of rain on coastal communities from southern Vancouver Island to northern Oregon and the western slopes of the Washington Cascades.
Around 6 inches (150 mm) is expected in Seattle and Portland. Both cities typically receive just shy of 5.50 inches (140 mm) during all of December not in a span of less than a week.
The rainfall alone from Sunday night to Monday "Could easily reach 3-6 inches (75-150 mm) in western Washington and western Oregon," stated AccuWeather Meteorologist Dave Samuhel.
The Skokomish River near Potlatch, Washington, was already at major flood stage on Sunday.
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 first in the parade of massive storms pounded the mountain passes of the Cascades with snow this weekend," added Samuhel. "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."
Snow levels will rise to around 6,000 feet (1,800 meters) in the Washington Cascades on Monday before soaring to 8,000 feet (2,400 meters) on Tuesday.
"The good news [with the rising snow levels] is that road conditions will become wet," continued Samuhel. "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 need to be closed at times for avalanche control the next few days."
Above snow level, a few yards (meters) of snow will bury the higher 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 heavy 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.
According to AccuWeather Long Range Meteorologist Ben Noll, "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."
As the storm drops into the Southwest later this week, the Northwest will catch a break from the onslaught of storms Friday into Saturday. However, the storm train should pick right back up later in the weekend.