Issued Friday, Dec. 31 from the National Weather Service. Low Elevation Snowfall, Air Stagnation, and Very Cold Temperatures to Begin 2017. Impacts: ● Light to Moderate snow will coincide with holiday traffic across all major passes. ● Low elevation snowfall could create travel problems across most Westside Valleys. ● Wet roads will become slippery due to very cold temperatures even after the snow ends. ● Air quality likely to become a concern next week in valleys due to limited mixing and light winds. Forecast Confidence – ● High for measurable snow above 2000 feet. ● Moderate fo r snowfall amounts above 2000 feet. ● Moderate for measurable snow below 2000 feet. ● Low for snowfall amounts below 2000 feet. ● High for cold temperatures through much of next week. High for air stagnation issues from Wednesday to Saturday. Timing and Strength – ● Snowfall: ○ Snow will begin near 1500 feet late Saturday. Snow levels will drop to 1000 ft by Sunday; afternoon, and near sea level Sunday night. ○ See attached sno wfall graphic for amounts. ● Temperatures: ○ Low temperatures will plummet beginning on Monday night and continue to be very cold through at least the end of the week. Record low temperatures are likely. ○ If snowfall amounts are near the upper ranges of the forecast, then temperatures could become even colder than forecast. ○ See attached m inimum temperature graphic for an idea of what temperatures could be. Weather Summary – An Arctic airmass will approach southern Oregon and northern California Saturday. This will bring snowfall to major passes since snow levels will start about 1500 ft. Snow levels will drop to valley floors Sunday
morning and near sea level Sunday night, leading to low elevation snowfall on Sunday and Monday. Afterwards, temperatures across the area will plummet beginning Monday night and continue through the end of the week. Record low temperatures are likely. In addition to the cold temperatures, weak winds and mixing may lead to air stagnation and air quality issues next week.
