Highway 20 has been closed through Memorial Day only once in the past 40 years, the Washington State Department of Transportation says.

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With as much as 45 feet of snow on the highway in some places, the Washington State Department of Transportation (WSDOT) is warning travelers that the North Cascades Highway (Highway 20) could remain closed into June.

The highway typically closes each winter from Diablo to Mazama, and opening dates depend largely on weather.

“We’ve only been closed through Memorial Day once in 40 years, but it could happen this year,” WSDOT maintenance supervisor Don Becker said in a news release.

Communities in the Skagit and Methow valleys eagerly await the passes’ opening each year, said Jeff Adamson, a WSDOT spokesman.

“This is being watched very closely,” he said. “It’s something that will impact the economy.”

Typically, the agency begins removing snow from the roadway in mid-March. This year, avalanche and weather worries have pushed the work into April.

It will take up to eight weeks to clear the highway, WSDOT said in the release. Normally, crews spend four to six weeks clearing the road.

There’s three times as much snow as is usually there, Adamson said. Several avalanche chutes are still full of snow.

“We can’t even put our crews up there to work yet,” he said.

To clear snow, avalanche control crews set off controlled slides with artillery and other tools. Sometimes, a helicopter is needed to drop charges, too. Maintenance crews and contractors then use snowplows, snowblowers, excavators and road graders to remove snow.

Washington State Department of Transportation is working to remove snow slides, some up to 70 ft deep, from North Cascades Highway. Crews are working from both directions of a 7-mile stretch of snow-covered road.

The North Cascades Highway has the most avalanche chutes of any mountain highway in the country, according to WSDOT. Some are more than 2,000 feet long.

When avalanche chutes are emptied, there could be as much as 70 feet of snow on the roadway in some places.

“By the time you’ve gotten to the bottom, it’s got the consistency of concrete,” Adamson said, referring to the snowpack.

WSDOT uses a custom-built snowblower with two diesel engines to clear off the bottom ten feet of “concrete.” It is specially designed for the North Cascades snow, Adamson said.

U.S. Department of Agriculture snow-survey summaries show that all but one area of the state has a snowpack at or above normal levels.

While the rest of the country had a mild winter, the Pacific Northwest experienced uncommon cold and plenty of moisture, too.

Earlier this month, Everett high-school students on a field trip became stranded after an avalanche blocked a part of Highway 20 that is open during the winter.

This story, originally published on March 24, has been corrected.  All but one area of the state has a snowpack at or above normal levels.  The original story incorrectly said all areas have  snowpack above normal levels.