Arctic air diving south out of Canada on Friday and a low pressure system making landfall on the Oregon Coast Saturday will create a dynamic weather pattern through the weekend in the Pacific Northwest. As a result, numerous winter weather alerts from the National Weather Service have been posted for Western Oregon and Southwest Washington with a cornucopia of hazardous weather through Sunday.
Rain fell in parts of the Portland area early Friday morning placing lots of water on roadways and sidewalks just in time for the cold front to arrive. The front comes in two phases – each with a sharp drop in temperature. The first phase comes mid- to late-morning moving south along the Interstate 5 corridor.
The I-5 portion of the cold front reached Longview and Kelso just before 05:00 dropping temperatures from 37°F (3°C) to 30°F (-1°C) in about an hour. Interestingly, as it passed through the Seattle area Thursday afternoon and evening it, plus the Columbia Basin counterpart, joined with the Puget Sound Convergence Zone to create a line of convection all the way from Bainbridge Island to Helena, Montana.
Snow squall warnings were issued in parts of Eastern Washington in association with this enhanced convergence zone. The convergence zone is created by air coming together over the Puget Sound after going around the Olympic Mountains, causing uplift and shower activity in a narrow band.
I have seen the convergence zone extend past Ellensburg in the past but never beyond about Vantage. It seems unlikely that this is possible without a strong cold front aligning with it and eastern portions of this convective line on Thursday probably didn’t have a whole lot of influence from the convergence occurring over the Puget Sound.
Showers are tapering off this morning in the Portland area though light flurries are possible throughout the course of the day. Expect temperatures below freezing by early afternoon with icy travel conditions shortly following. The region won’t need snowfall today for travel to become problematic.
South of Tigard rain and later snow showers are more likely with maybe a half an inch to two inches (1-5 cm) of snow possible in heavier showers between there and about Salem. South of Salem some freezing rain is possible to Corvallis but Eugene should have all rain today.
The second phase of the cold front blasts into Portland from the Columbia Gorge in the late afternoon for another drop in temperature. The I-5 phase is modified to be a bit warmer due to its proximity to the Pacific Ocean and having passed over the Puget Sound. The Columbia Gorge phase, however, is sheltered from these influences. As of 07:00 temperatures in Pasco are 10°F (-12°C) and won’t climb much over the course of the day.
Winds pick up in the east metro with the Columbia Gorge phase of the cold front. The typical east wind zone (Troutdale, Gresham, Camas, etc.) will see gusts to about 40 mph (65 km/h) this evening. Cold air deepens in the Willamette Valley later on.
The low pressure area comes into the mix tonight with precipitation spreading from south to north. An intrusion of warm air in the mid-altitudes is forecast to override cold air coming from the north and east to produce a significant freezing rain event in the Willamette Valley south of about Wilsonville.
It looks like most valley locations, including Salem, will see at least a quarter inch (6 mm), some will get over a half inch (12 mm), and Corvallis and Eugene could have pockets exceeding one inch (25 mm). Tree damage and power outages may be widespread.
The east wind coming out of the Columbia Gorge will intensify as the low pressure center approaches with wind gusts to 40 mph (65 km/h) in most of the Portland metro, to 55 mph (90 km/h) in the standard east wind zone, and in excess of 80 mph (130 km/h) in the gorge itself. Power outages and tree damage are possible here as well.
Frigid temperatures coming from the Columbia Basin will keep Portland quite cold on Saturday. Highs in the city may not reach 20°F (-7°C) for the first time since December 1990. There is a small chance of freezing rain in the Portland area but it appears that the cold air coming from the east will be able to keep precipitation as all snow.
Low elevation snow totals will depend heavily on where the center of the low makes landfall. A landfall near Newport appears to be favored which looks likely to bring 2-6 inches (5-15 cm) of snow to the metro area. It is possible a heavier band of snow sets up somewhere within 30 miles (50 km) of the city for higher snow totals.
Precipitation rates drop quickly on the north end of the system. With a Newport landfall scenario precipitation ends near Castle Rock. If the low lands closer to Lincoln City or Tillamook snow could make it to around Olympia while a track closer to Yachats or Coos Bay could leave Portland without snow. As noted above, the Newport route is favored but out of those possible deviations the southern route has a little higher chance compared to the northern one.
Precipitation tapers off from north to south Saturday night. Cold temperatures are forecast to continue in the Portland metro into the beginning of next week. Highs in the city should remain below freezing through Monday.
Heavy snow is expected in the mountains to add to the big gains observed over the last week. On the slopes of Mount Hood, the snow-water equivalent value has jumped from 30% of the median for this time of year on January 3 to 74% of the median on January 11.
For coverage on cold and snow in the Inland Northwest, see the article I wrote Thursday evening. Fellow WeatherTogether blogger Karl Bonner also wrote about Portland and Willamette Valley winter weather in an article yesterday evening.
The featured image is HRRR modeled radar for 15:00 Saturday via WeatherBell.
