Weathercatch: A while longer before true autumn knocks on our door

Now that it’s the last day of August, we’re about to kick off another Labor Day weekend. For many of us in the Inland Northwest, the holiday weekend signals a transition from summer to fall. In a blink it seems, days grow noticeably shorter, kids are back in school and lifeguard chairs go vacant.

But the weather? Not so fast. With temperatures forecast to be in the upper 70s Friday through Sunday and near 70 on Monday, it should feel more like summertime than autumn. And though this weekend marks the brink of a seasonal transition, it doesn’t mean fall weather will arrive abruptly. According to historical patterns, the shift from warm air to crisp autumn air is typically a gradual one – especially during daytime.

Not that we shouldn’t expect some cooler days, like what we saw on Tuesday and Wednesday. But cool days tend to show up more on a drop-in basis than as a steady hanger-on. During September’s first two weeks, high temperatures typically run in the upper 70s to low 80s in the Spokane area, and the overnight lows run in the low- to mid-50s. It’s often the third week that fall weather comes quietly knocking.

An analysis of Spokane-area weather from Sept. 1 to Sept. 21 since the year 2000 shows the normal high temperature declines from 78 degrees to 70 degrees during those three weeks. Meanwhile, the nighttime low dips from 51 degrees to 45. Although temperatures hovering near 70 still feel like late summer, overnight lows in the mid-40s hint of early autumn. So generally, we’re about to embark on a pleasant time of year.

Even so, sometimes we experience intense heat. On Sept. 1, 1888, the high temperature reached 98 degrees in Spokane. Just last year, the temperature topped out at 96 degrees on Sept. 2, and also on Sept. 5, 1988.

On the flip side, occasionally autumn makes a sudden and boisterous appearance in September, hurling unseasonably chilly temperatures toward the region. Imagine the many blackened tomato vines after the overnight low dropped to 32 degrees on Sept. 19 and 20, 1983. While no snow has been recorded between Sept. 1-21, one year it was close. The earliest snow recorded in Spokane is 1.4 inches that fell on Sept. 23, 1926.

Most years, however, the transition from summer to fall is gentle and unhurried, with the most obvious weather changes occurring in October.

Between now and Sept. 21, Spokane will lose a little more than one hour of daylight.

Then, as late September gives way to October, the days will grow progressively shorter, and we’ll experience steeper temperature drops and the steady increase of autumn-colored leaves.

Nic Loyd is a meteorologist in Washington state. Linda Weiford is a writer in Moscow, Idaho, who’s also a weather geek.