PORTLAND -- With more fog, rain, and freezing temperatures in the forecast, Portland officials warned the public to look out for frost and black ice throughout the area over the holiday weekend.

Landmark elevations at or above 1,000 feet in Portland include Council Crest, Mount Scott, and the top of West Burnside, said Cheryl Kuck with the Portland Bureau of Transportation.

Click for more information on Portland area elevations

Black ice is treacherous because it is hard to see, Kuck added. It forms when roads are wet and temperatures drop to freezing. Even though the roads look like they are just wet, they can be very slick.

Kuck advised travelers to be especially careful on bridges, overpasses, tunnels and roads that follow along rivers. Those areas tend to freeze first and thaw last.

Related: Heavy snow expected in Cascades

Transportation crews were monitoring conditions to keep streets safe and clean. Anti-icing chemicals are not used when rain is in the forecast, because the rain would just wash it away, Kuck said. But the chemicals can be applied to major roadways when the rain stops.

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The National Weather Service issued a flood warning for the Portland Metro area Friday, as well as a flood warning and a winter weather advisory for the Cascade foothills.

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