The autumn equinox (around September 22-23) marks the astronomical transition to fall, when day and night are nearly equal in length at approximately 12 hours each. Following the equinox, the Pacific Northwest loses 2-3 minutes of daylight daily, creating a rapid descent toward winter's short days. This astronomical event coincides with dramatic ecological changes including peak fall color, salmon spawning runs, southbound bird migration, and the first significant fall storms. The equinox triggers important phenological responses throughout Pacific Northwest ecosystems, with plants and animals responding to decreasing day length and lower sun angles. Indigenous peoples throughout the region recognized the equinox as a critical transition point, marking the shift from summer's abundance to winter's challenges. Modern observers can witness this balance point between light and darkness before the inevitable slide toward December's winter solstice.

Chanterelle season reaches peak abundance in western Oregon and Washington—this is the canonical PNW fall mushroom moment in Douglas fir forests from sea level to 3,000 ft. Chum salmon begin appearing in coastal streams, particularly on the Olympic Peninsula and in Puget Sound. Willamette Valley Pinot noir harvest begins. Wenatchee and Yakima Valley apple harvest (Honeycrisp, Fuji, Gala) is at full intensity.