Winter weather advisory for Portland metro as compact low moves through

A compact low pressure system embedded within the regional flow pattern is forecast to track near the Portland metro to produce heavy precipitation Tuesday night into early Wednesday morning. This has prompted the National Weather Service to issue a winter weather advisory from 22:00 Tuesday to 06:00 Wednesday.

In the advisory, the National Weather Service calls for one to three inches (2-8 cm) of snow above 500 feet (150 m) elevation. Below that level, some snow is possible but accumulations are forecast to be light. In general, the 500 foot (150 m) elevation contour captures the West Hills and the upper slopes of Boring Lava Field vents throughout the city and suburbs.

The amount of snow will be heavily influenced by the track of this low pressure system. Small deviations in the track will cause big changes in observed snowfall as snow amounts at the valley floor will be mainly confined to the north side of the low.

Modeling has consistently favored the low center passing near Woodburn. If it ends up even 20 miles (30 km) north of this, everywhere in the metro except perhaps the West Hills will just see cold rain. On the other hand, 20 miles (30 km) south would produce a few inches of snow down to the Columbia River.

Under the favored low track, areas south of Tigard, including Salem, can expect cold rain. Meanwhile a narrow band with snow totals above three inches (8 cm) appears possible between Battle Ground and Longview, including St. Helens.

Accumulating snowfall will be inhibited by warm soil temperatures. As of 15:00 Tuesday, the AgWeatherNet site at WSU Vancouver is recording a temperature of 46°F (8°C) at a depth of two inches (5 cm). Soil temperatures will cool overnight, but may remain above freezing. Still, bridges are not insulated by the ground and may be especially treacherous.

Snow is forecast to transition to rain before sunrise Wednesday meaning that most commuters will only see snow if they arrive at their office before about 07:00. Of course, this transition will occur later at higher elevations and likely not at all above 1500 feet (300 m).

A winter storm warning remains in effect for the Cascades and Cascade foothills between 1500 and 3000 feet (300 and 900 m) elevation where one to two feet (30-60 cm) of snow is forecast through Wednesday afternoon. Above 3000 feet (900 m), a blizzard warning for the combination of heavy snow and strong winds is in effect. Both alerts end at 16:00 Wednesday.

Models continue to show some sort of winter storm event for the Pacific Northwest Friday into Saturday. Winter storm watches were posted for the Columbia Basin on Tuesday afternoon, but around Portland the jury is still out on what this will look like. Heavy rain, heavy snow, and significant freezing rain are all possible. Hopefully the forecast will become better resolved in the coming days.

The featured image is 18z HRRR modeled radar for 22:00 Tuesday via WeatherBell.