When does fall begin in Seattle? Understanding the autumnal equinox

Now that the calendar has officially flipped to September and the fall Starbucks menu is in full swing, many residents in Washington are anticipating the arrival of fall

But when exactly does fall begin in Seattle?

According to the Farmer's Almanac, this year's autumnal equinox, marking the start of fall, occurs on Sunday, Sept. 22, at 8:44 a.m. EDT. The equinox, which happens simultaneously worldwide, signifies the point when day and night are roughly equal in length due to the sun crossing the celestial equator.

Keep reading to learn more about the autumnal equinox and what you can expect in Seattle this year.

Understanding the autumnal equinox

The autumnal equinox is an astronomical event marking the transition from summer to fall. During the equinox, the sun crosses the celestial equator from north to south, which leads to shorter days and longer nights in the Northern Hemisphere. This shift continues until the winter solstice when daylight starts to increase again. In 2024, the winter solstice will arrive on Dec. 21 at 4:20 a.m. EDT.

The term "equinox" is derived from the Latin words "aequus" (equal) and "nox" (night), reflecting the roughly equal duration of day and night. For those in the Southern Hemisphere, the equinox marks the beginning of spring.

The harvest moon connection

One notable celestial event associated with the autumnal equinox is the Harvest Moon — the full moon closest to the equinox. This moon rises around sunset for several consecutive nights, traditionally providing farmers with extra light to complete their harvests. This phenomenon results from the moon’s orbit aligning with Earth's tilt, causing the moon to rise just 20 to 30 minutes later each night during this period.

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According to the Farmer’s Almanac, the tradition of avoiding white after summer dates back to the 1950s when the affluent adhered to it as a sign of "refinement." Those who followed this rule were typically viewed more favorably.

Seattle's weather outlook for early fall

As fall approaches, Seattle is experiencing a notable warm-up following its coolest August since 2011, with an average temperature of 66.5 degrees at Sea-Tac Airport. This week, temperatures are expected to rise significantly, with highs reaching the upper 80s by Thursday and Friday, potentially tying record temperatures.

The warm and sunny conditions contrast with the typical cooling associated with the start of fall. FOX 13 Seattle forecasters predict elevated fire danger due to the unseasonably warm weather, and are advising residents to stay hydrated and seek air-conditioned spaces during the warm up.

Despite the upcoming heat, the long-range forecast indicates a shift to wetter-than-normal conditions for September in western Washington, although these predictions remain subject to change.

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