Sun sets on September: Why October may bring brilliant fall colors to Washington

Monday is the final day of September. The calendar then rolls over into October as the days continue to get shorter, the nights will grow longer and the fall colors may get more lively. It feels like fall with early morning temperatures Monday morning dipping into the 40s, and in some places like Olympia, Arlington and Shelton, edging into the upper 30s.

September is going to finish at or just below average for rainfall. For Seattle-Tacoma International Airport (Sea-Tac Airport), this will be the sixth month this year that was drier than average. For the year, the site is approaching five inches below normal.

In contrast, Bellingham has received four inches more than average for rainfall thus far this year and September will finish just above normal. When it comes to precipitation amounts, Olympia in the middle at just under an inch below normal for the year thus far.

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September temperatures in Western Washington were close to normal. When averaging all the high and low temperatures, many locations were just about even with the month’s normal. Sea-Tac Airport will finish the month at an average temperature of 63 degrees, and Olympia near 62 degrees.

The dry weather is expected to continue this week. A couple of weak weather systems moving from west to east into British Columbia will brush Western Washington Tuesday and again late Friday with the primary threat of any rainfall close to the Canadian border, the Olympics, and the northern Cascades.

Otherwise, the region should have plenty of sunshine with fall seasonal areas of morning fog. High temperatures will primarily be in the 60s, though a few places on Tuesday and again Friday could crack the 70-degree mark such the southwest interior and Cascade foothills.

Why do leaves change colors?

With the overall dry weather and cooler nights, it is the time of year for leaves on trees to turn to their vibrant fall colors. Leaves on mountain trees are turning color now and some lowland trees are also starting to turn.

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 warm spring and summer and warm sunny fall days with cool nights similar to this year’s start to fall can potentially produce the most brilliant fall colors in trees.

Earlier weather coverage: The end of summer as we know it. The fall equinox is here

Looking ahead into the coming weekend and next week, the longer range weather outlook offers above average temperatures and relatively dry conditions. With these circumstances, the next few weeks are likely the best time to view fall tree colors along mountain highways. Trees in the western Washington lowlands should also turn to their full fall colors by mid-October as well. And Japanese Maples will reveal brilliant reds before their leaves drop.

Ted Buehner is the KIRO Newsradio meteorologist. You can read more of Ted’s stories here and follow him on X.