Temperatures in Northwest Oregon and Southwest Washington remained below freezing Sunday after a cold front and low pressure system brought snow and damaging winds to the region. Oregon Public Broadcasting reports that three people died in the Portland metro area as a result of cold temperatures and falling trees. Over 200,000 customers are reported by utilities to have lost power throughout the Pacific Northwest, mostly around Portland and the northern Oregon Coast. As of 19:00 Saturday over 150,000 customers reportedly remain without power.
The Northwest is experiencing a break in between storms but cold air remains trapped in lowland valleys. Highs will fail to reach freezing again on Monday but sunshine is expected to help melt some of the snow and sleet that fell on Saturday. Winds will be blustery Monday into Tuesday morning with a wind advisory for Portland for wind gusts to 50 mph (80 km/h), highest east of I-205 and north of about Burnside. Clouds increase Tuesday morning with the next low pressure system approaching.
The National Weather Service has issued a winter storm watch for the Portland metro and WIllamette Valley from 10:00 Tuesday to 04:00 Wednesday. Up to an inch (2 cm) of snow is possible but the main story is the possibility of 0.3 inches (7 mm) of ice accumulation. Ice accumulations are likely to create significant travel problems Tuesday evening and overnight into Wednesday but the lowlands should transition to rain before sunrise.
Freezing rain is a common way to end Pacific Northwest cold spells. Once cold air settles in the non-coastal lowlands, it becomes trapped because cold air is denser than warm air causing it to sink in the valleys to form a temperature inversion. From about November to February, the sun angle at this latitude is too low to heat the ground enough to warm the air and clear the inversion.
Because of this we often require something like a Pacific storm to clear the cold air out. The first phase of a Pacific storm, however, is typically a warm front bringing in an increase in temperatures in the mid-levels of the atmosphere. Warm air rises, so in this scenario the warm air skims across the top of the cold air stuck in the valley. Snow falling from above melts in this warm layer but freezes on contact when landing on a frozen surface.
Precipitation is likely to begin in the late afternoon on Tuesday. Rain is forecast on the coast and in the mid-elevations of the Coast Range and Cascades but will probably be freezing rain along Interstate 5. There’s about a 50/50 chance of the freezing rain beginning in Portland before the standard evening commute. It should continue overnight but transition to rain before sunrise Wednesday. Ice accumulations are also possible in the Puget Sound Region.
On top of travel concerns, accumulating ice will add strain to trees and energy infrastructure damaged in the weekend winter storm. Ironically the incoming cold front will bring warming temperatures to Portland and Seattle since the cold sector of the incoming low pressure system will be able to mix out lingering cold from last weekend. Model guidance has consistently shown the low pressure system making landfall on the Olympic Peninsula and continuing eastward.
Wednesday afternoon and beyond, stereotypical Pacific Northwest weather is forecast for Portland. Highs generally in the mid-40s (7°C) will be accompanied by frequent rain brought in by a train of Pacific storms. A rubust plume of moisture sourced from the subtropical Pacific aims at the Pacific Northwest for several days during the second half of this week. Both Portland and Seattle may observe over two inches (50 mm) of rain through the end of the day Sunday, January 22.
Snow levels rise rapidly behind Tuesday’s warm front, reaching about 6,000 feet (1,800 m) before dropping again after the cold front. Snow levels rise again Thursday and modeling favors them remaining elevated beyond that as the flow pattern turns more southerly.
Snowpack has improved rapidly this month but remains below average for this time of year. The SNOTEL site at 5,370 feet (1,640 m) elevation on Mount Hood reported a snow-water equivalent about 79% of the median for January 13. This is up from 33% of the median on January 6. Assuming weather models are reasonably accurate, the Cascades will lose lower elevation snow over the next week, possibly remain about the same in the mid-elevations, and continue gaining snow above pass level.
Widespread flooding is not forecast though rivers and streams should rise this week as low-elevation snow melts off under heavy rain. Some small stream flooding is possible.
1 Comment