The Transportation Department said the snow made driving difficult on Interstate 5 through Skagit and Whatcom counties. DOT cameras showed snow falling along I-5 in Burlington.

Highway 9 was blocked for nearly two hours near Sedro-Woolley by trees that fell during the snowstorm.

The snow was caused by a convergence zone in which winds from the west pass to the north and south of the Olympic Mountains. They collide -- often over the North Sound -- creating sudden storms.

Forecasters said conditions for more snow will persist through the weekend, especially in the mountains.

The Transportation Department said drivers crossing the Cascades should prepare for winter conditions. The department said it's calling in more maintenance crews to keep Interstate 90 open across the pass.