PORTLAND, Ore. – A potent winter storm that is part of a 1,500-mile system pummeling the Pacific Coast is sweeping snow and freezing rain across a swath of the Pacific Northwest – including the Portland area – that has already led to a number of crashes across the area.A significant crash involving 20-30 vehicles occurred in the westbound lanes of Interstate 84 just east of Portland Thursday morning in what deputies described as whiteout conditions, according to the Multnomah County Sheriff’s Office. “Responders are going car by car. There are reports of injuries; no number at this time,” Multnomah County sheriff’s deputies said. The agency originally reported more than 100 vehicles were involved in the crash. But they later corrected the tally to about 20-30 vehicles, saying whiteout conditions made the initial count difficult to distinguish which vehicles were involved in a collision or just stuck in the backup.Elsewhere in Portland’s most populous county, a snow-covered road led to multiple crashes involving more than 20 vehicles, according to the county sheriff’s office. Across the border in Washington, the weather was no better.Multiple semis and vehicles collided in the snow-covered southbound lanes of Interstate 5 in Castle Rock, about 60 miles north of Portland, blocking the main artery connecting Seattle and Portland. “We’re seeing about 10 miles of backups, so avoid the area!” WSDOT officials warned early Thursday afternoon.Officials in Clark County, home to Vancouver, also reported multiple crashes, as snow was causing low visibility and slick roads, according to the Washington State Department of Transportation.WSDOT officials noted that snowfall has caused multiple collisions and vehicles sliding on SR14, Interstate 205 and Interstate 5.Power outages from ice-coated trees and power lines have begun to occur. Nearly 8,500 customers in Washington are without power as of Thursday afternoon, according to FindEnergy.com. In Oregon, almost 1,900 customers are without power.This event is the result of an area of low pressure swirling off the Pacific coast that is connected to an atmospheric river storm bringing torrential rains to California Thursday.The storm is carrying relatively mild air, but cold air is deeply entrenched in western Oregon, which has been in a week-long arctic chill. As the warm rain pushes in over the freezing surface, it’s a classic recipe for freezing rain – and a lot of it.Ice Storm Warnings cover more than 1 million people in the central and southern Willamette Valley, including Eugene and Salem in Oregon, for as much as 0.2-0.4 inches of ice accretion. Freezing rain began falling during the Thursday morning commute, making glazed roads a danger. HOW MUCH ICE IS NEEDED TO KNOCK OUT POWER, DAMAGE TREES?Snow accumulations around Portland are expected to top out around 1-3 inches before the change to freezing rain, which is forecast to accrete to about 0.1-0.2 inches. A Winter Storm Warning is in effect there through late Friday morning.In addition, strong easterly winds could reach 45-60 mph gusts in the eastern Portland suburbs along the cold outflow of the Columbia River Gorge, extending freezing rain threats and dropping wind chills into the teens.WHAT IS THE DIFFERENCE BETWEEN SNOW, SLEET, AND FREEZING RAINThe freezing rain threat lasts through Thursday night and into about midday Friday before the storm pushes off to the east and the region temporarily dries out. Light snow up to about an inch could reach as far north as the southern Puget Sound region south of Seattle before drier air works its way there too.WHAT IS FREEZING RAIN?However, another storm heading into the Northwest for the weekend may present a renewed ice threat in the eastern Portland area near the Columbia Gorge outflow until much milder air sweeps into the Northwest and ends any lowland wintry weather threat.