"A deep fetch of subtropical moisture noses into the region starting Monday night and lingers through midweek," they wrote. "This warm, moist conveyor belt will overrun a very cold air mass currently in place, bringing the potential for several hours of freezing rain, sleet and high mountain snow. Significant ice accumulations will be possible within the Cascade valleys, western Columbia Basin and northern mountain valleys."
Ice accumulations are expected to be from 0.15 to 0.30 inches, with some localized areas receiving up to half an inch of ice from both freezing rain and sleet. Periods of light to moderate rain and sleet mixed with snow is expected to begin falling tonight, and become steady with moderate intensity through Tuesday afternoon, when precipitation is expected to turn to mostly rain. The chance of rain continues through Wednesday, but the possibility of ice will be considerably diminished as temperatures warm into the mid 30s to low 40s..
At the expected accumulation of ice, roads will be treacherous and, helped by south winds of 5 to 15 miles per hour with gusts to 25 miles per hour, trees and power lines could be damaged, and power outages are possible, if not likely.