JACKSON, Wyo. — This evening, the sun will set on summer one last time. Tomorrow, Sept. 22 is the autumn equinox, which marks the first astronomical day of fall in the northern hemisphere and the first day of spring in the southern hemisphere.
According to the Old Farmer’s Almanac, an equinox describes when the sun’s center crosses the celestial equator—earth’s imaginary extension of the equator line into space. When the sun crosses the equator going from north to south, this marks the autumnal equinox in the northern hemisphere; when it crosses from south to north, this marks the vernal equinox.
Across the world, this happens at the exact same time but the hour and day differ depending on the time zone. In Jackson Hole, it will occur at 7:03 p.m. tomorrow evening.
What this all means is, that after the equinox, days become shorter than nights because the sun continues to rise later and nightfall arrives earlier, until the winter equinox. The arc of the sun will slowly creep south, the leaves will begin to change and the temperatures will begin to dip.
While astronomically fall begins tomorrow, in Jackson Hole, winter could be knocking on the door sooner than later. According to Buckrail’s Meteorologist Alan Smith, Thursday is expected to be cool with highs in the upper 50s in the valley and wet as a cold front moves through the region. Off-and-on showers are expected throughout the day with thunderstorms possible.
Snow is possible from 9,000-10,00 feet, and any accumulations below 10,000 feet will likely be short-lived, while areas in the 10,000-11,000 foot range could pick up a few inches of wet snow.vOnce the showers clear, a hard freeze is likely across the higher elevations.