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PORTLAND -- A weather inversion under way should cause freezing temperatures for the valley into this weekend, but Mt. Hood was seeing springtime skiing conditions.

An inversion refers to warm air that builds, but leaves cold air at the surface in place. The weather pattern makes temperature forecasting uncertain and often leads to poor air quality and areas of persistent low clouds in calm wind areas, KGWMeteorologist Rod Hill said.

Weather Blog: What's an inversion?

During an inversion, temps at higher elevations are higher then those at lower elevations. For example, although Portland stayed right around freezing Tuesday afternoon, Timberline Lodge warmed into the 40s. Temps at the Coast were also in the 40s.

City crews have been treating bridges, steep streets, and known trouble areas with de-icer.

Latest Weather: Weather Cams | Radars | Forecast

Patches of fog were also expected overnight in Portland.

A cold weather shelter in Oregon City was set up at Father's Heart Street Ministry opening at 7 p.m., doors close at 10 p.m. It is adults only and no pets are allowed. A light dinner and breakfast will be provided. Capacity is 30 to 40, the group said.

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