The annual leaf-peeping season in Colorado's high country is upon us, as the weather this summer is expected to produce an especially vibrant autumn display.

Aspens turn when nights get longer and the arc of the sun descends closer to the horizon. The prime foliage viewing time in the Rockies is forecast to begin September in the northernmost elevations, and extend well into October as you move southward. Here's are some suggestions for autumn color tours:

Start at Steamboat Resort, Colorado's most northerly ski and snowboard mountain. The lifts are closed now, but aspen groves cover much of the mountain that looms over town. Take a 4x4 ride up the aspen-lined dirt road toward Buffalo Pass and Summit Lake. Hikers get bonus viewing by climbing into the Mount Zirkel Wilderness just north of Steamboat Springs.

Pair Vail Mountain and Beaver Creek together for top-end foliage, especially since both run their gondolas late in the season. Either one gets you up to 10,000 feet and opens up a full vista of yellowing "quakies" in the Vail Valley. Or, drive U.S. 24 between Vail and Beaver Creek to legacy mining town Minturn where the Eagle River rocks the colors.

A fall foliage tour wouldn't be complete without a stop at eponymous Aspen. Splashes of fall yellow dot the Roaring Fork Valley, and a visit to the legendary ski town should include a ride up Maroon Creek to the take the perquisite photo of the aspen-splashed Maroon Bells at the top of the road.

Head to southwest Colorado for a San Juan Range foliage extravaganza. Head over color-drenched Dallas Divide to Telluride, where aspen groves quilt the steep sides of its much-photographed box canyon. Bright canvases of yellows and reds pop out on both side. Hop the free gondola between the town and the resort village for a sky view of the aspen groves. Or take a Jeep tour to go deeper into the forest.

Take the uber-scenic San Juan Skyway to Ouray and over Red Mountain, Molas and Coal Bank passes to Purgatory. All along the way, dense stands of aspens spill down onto the road. At Purgatory, the chairlifts runs into October, rising through aspen groves to a 10,000-foot-high perch. Fall colors paint much of the 270-degree view of the southwestern flank of the San Juan Mountains.

In Colorado, the most definitive way to know that summer is over and winter is not long off comes when the high-country aspen groves put on their brilliant yellow coats.