Seattle ties record for amount of 90+ degree days; fall equinox to bring cooler weather

These final days of summer offered more dry weather, sunshine, and — with the exception of Tuesday this week — mild, seasonable temperatures in the 70s for the Puget Sound region.

On Tuesday though, Seattle-Tacoma International Airport (SEA) soared to 91 degrees thanks to warm easterly winds off the Cascades, tying the record high for Sept. 16.

Those easterly winds gave way to low-level onshore winds from the Pacific Ocean, cooling temperatures back down into the 70s for the rest of the week.

But that single day of 91 on Tuesday pushed the number of 90+ degree days to eight for the year, matching the average established since 2015. In the 20th century, Seattle averaged only three 90-degree days per year.

Fall Equinox is Monday

The fall equinox is coming up on Monday, Sept. 22, at 11:16 a.m. The fall equinox is also known as the autumnal equinox, and the following day is the first full day of fall.

This time of year, daylight hours reach their peak, losing nearly 3.5 minutes per day. The last 7 p.m. sunset of the year will occur on Thursday, Sept. 25. During the days around the fall equinox, sunrise and sunset will both be around 7 a.m. and 7 p.m. for a total of about 12 hours of daylight.

In contrast, back in late June, there were close to 16 hours of daylight.

That decreasing daylight momentum will gradually slow as we get closer to the winter solstice in the latter part of December, bottoming out at about eight hours and 20 minutes around the solstice.

Both the spring and fall equinoxes are the only two moments each year when the Sun lies directly above Earth’s equator. Our planet tilts about 23.5 degrees off its axis. As Earth makes its annual rotation around the Sun, for six months the North Pole is tilted toward the Sun for our spring and summer. For the other six months, it is the South Pole that is tilted toward the Sun, resulting in our fall and winter seasons.

Taste of fall this weekend

Speaking of fall, the weather forecast for this weekend will offer a taste of fall as a weakening Pacific weather system works its way onshore.

More cloud cover and some rain are expected, with high temperatures struggling to reach 70 degrees. The bulk of the rainfall is anticipated to fall overnight Saturday into Sunday morning. Although amounts will not be enough to douse ongoing wildfires, they will help cleanse the air of smoke.

Seasonal weather outlook

With La Niña slated to return as the seasons roll into fall and winter, the latest seasonal weather outlook reflects a typical La Niña season. La Niña is the opposite sibling of El Niño, when the ocean waters in the eastern Pacific Ocean, the waters west of Peru, are cooler than average.

El Niño is when those same tropical waters are warmer than average.

La Niña and El Niño make big adjustments in how the North Pacific storm track behaves. For La Niña, Pacific storms spend more time moving onshore into the Pacific Northwest as opposed to El Niño, when the storm track takes a more constant path into California and beyond across the southern tier of the U.S.

El Niño winter seasons for Western Washington tend to be warmer than average and lean toward below-average precipitation and a much poorer mountain snowpack.

La Niña winters are sharply different. They tend to be cooler and wetter than average, and produce a healthy mountain snowpack. The latest seasonal weather outlook reflects that pattern, and if it pans out, more rainfall and mountain snow lie ahead this winter.

The outlook for this fall offers greater chances of wetter than average conditions. By this winter, the rainfall outlook shows no real trend above, below, or near average precipitation.

For temperatures, the new outlook indicates no trend of above, below, or near average conditions this fall, but for winter, the odds tip toward cooler than average temperatures.

This latest seasonal weather outlook offers hope that more significant rainfall lies just ahead this fall, which will help bring the wildfire season to an end. With cooler-than-average temperatures indicated for this winter, that trend also offers hope for a healthy mountain snowpack and even a good chance of at least one lowland snow event in Western Washington.

As the summer season comes to an end, wetter weather is anticipated this fall. If the seasonal weather outlook is on track, that rainfall will help make up the deficit and ease the current drought conditions across the state.

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