Dry weather this week offers perfect window for fall color viewing in Washington

The fall season is here. This season is the time of year when tree leaves change colors, offering a collection of vibrant autumn hues.

Why do leaves change colors?

The change from green leaf colors is driven primarily by the shrinking hours of daylight and the growing length of nights. Depending on the variety of tree, biochemical processes in leaves end the production of green chlorophyll, unmasking two other chemicals already within leaves — carotenoids and anthocyanins — and revealing their colors.

Weather conditions also play a role in the brilliance of leaf colors, with temperature and moisture the primary influences. The amount of moisture in the soil also impacts autumn colors.

Like weather, soil moisture varies from year to year. These two highly variable factors result in no autumn season tree colors and timing being alike. A dry year like the region has had this year can on occasion delay the onset of fall colors by a few weeks.

In contrast, a warm, wet spring, a warm summer, and warm sunny fall days with cool nights can produce the most brilliant fall colors in trees.

Timing

Higher elevation trees usually begin the change in leaf colors before lower elevation trees do the same, usually within a matter of days or a few weeks.

In the Cascades and Olympic Mountains, the leaves on trees are already starting to turn to their vibrant fall colors. As the calendar rolls over into October, more tree leaves in the lowlands of western Washington will follow suit.

Weather into this weekend

As the fall season gets underway this week, more dry weather with a mix of morning clouds and fog, and afternoon sunshine is expected. Temperatures are expected to be quite seasonable, with highs ranging from the mid-60s to the mid-70s in the usually warmer spots.

By late this week, though, a weakening Pacific weather system is forecast to move ashore for more clouds and some much-needed rainfall. Conditions are expected to dry out by Sunday.

Where to view

For those who enjoy viewing emerging autumn colors, mountain trees lead off the transition to fall colors. The two Mt. Rainier highway passes, Cayuse and Chinook, are great places to view these vibrant colors through the weekend.

Another full-day route is taking the Cascade Loop National Scenic Byway that involves Stevens Pass Highway and the North Cascades Highway. The I-90 corridor over Snoqualmie Pass and the side roads in and around Cle Elum also offer colorful opportunities.

Some lowland western Washington trees are already turning color. Moving into October, more varieties of these trees will display the colors of their leaves.

If you enjoy colorful autumn tree leaves, don’t delay. The fall rainy and windy season will get started at some point, likely in October, with trees being stripped of their leaves. In the meantime, enjoy what nature has to offer with autumn colors, including Japanese Maples and their brilliant red color.

Ted Buehner is the KIRO Newsradio meteorologist. Follow him on X and Bluesky. Read more of his stories here.