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Snow event caused by wind interacting with Scappoose terrain

by: SPOTLIGHT PHOTO: ROBIN JOHNSON - A broken-down car at the intersection of Havlik Drive and Highway 30 in Scappoose resulted in a non-injury, two-car 'fender-bender' Friday afternoon that, according to Scappoose Police Sgt. Dennis Viereck, was one of several within the city limits. After the car broke down,  Viereck said, two other cars attempting to stop at the intersection slammed into each other. 'People aren't prepared for it,' he said, regarding the three inches of snow that had accumulated in the city.  Viereck added that, as of 1:30 p.m. on Friday, there had been six similar incidents within the city and 12 within the city limits. The weather event that hit south Columbia County Friday, causing school closures in Rainier, St. Helens and Scappoose, multiple traffic accidents, road closures and even one fatality, seems to have been caused by a snow band that formed along the area’s hills to the west, meteorologists say.

Friday’s storm hit Scappoose particularly hard, dumping about 3 to 4 inches of snow. St. Helens was merely dusted with snow, and the little amount that had accumulated in Portland had nearly disappeared by Friday afternoon. The localized “dumping” is typical for such precipitation bands, said Beth Burgess, meteorologist with the National Weather Service’s Portland Weather Forecast Office.

“It was actually just a band that kind of set up right along where Multnomah County goes northwest,” she said. “Convective bands set up and dump over one area. It doesn’t progress like a front. It was a snow band.”

Burgess said snow bands occur when low-level air meets in one area—called a low-level convergence—and forces the air to rise. “If there’s any moisture there, you’ll get a little band,” she added.

Burgess explained that the reason Multnomah County experienced less precipitation Friday was because that county was covered by a large precipitation shield rather than Scappoose’s snow band. “Basically if you get a low level convergence, you’ll get a low level of snow.” she said. “With bands you’ll get locally higher amounts.”

Areas of complex terrain — like the hills west of Scappoose — are much more prone to weather bands, Burgess said. “Wind interacts with the terrain to form a band,” she said. “It’s when the wind is wrapping around at low levels. Air meeting at low levels forces it to rise. In rain it’s more spread out.”

Burgess said bands like the one that hit Scappoose last Friday are typically difficult to predict, which is why meteorologists didn’t foresee much snow coming through the area. “When a band forms, it’s very hard to predict in advance,” she said. “It’s usually about 12 hours or less. There was some indication, we’d have a chance of snow in the Portland area.”

Warmer weather is expected to come through both Columbia and Multnomah counties by this weekend, Burgess said, but not before a potential snow-rain mix causing freezing rain and a glaze of ice. “We believe there will be icing Thursday night [Dec. 12] at low elevations,” she said.

Burgess added the potential glaze of ice would likely be present Friday morning, Dec. 13, at elevations below 1,500 feet where the ground surface is colder. “We’re still uncertain right now in regard to temperature, but we know we’ll have precipitation,” she said.