Weathercatch: Autumn’s here, so where’s the frost?

A word keeps popping up that many of us haven’t heard or even thought about in a while: frost.

The National Weather Service Spokane accurately predicted that after a warm start to October, a dip in temperatures on Tuesday and Wednesday of this week could lead to pockets of frost overnight.

Spokane, Coeur d’Alene and other larger communities didn’t get cold enough to experience fall’s first frost, but other areas did.

Frost was reported in Deer Park, Colville, Republic, Kettle Falls and near Cheney, where lows dipped closer to freezing.

Frost develops near the ground when water molecules in the air freeze on contact with a cold, solid surface and form ice crystals . The surface can be anything from potted petunias and garden vegetables to a car windshield and irrigation pipes. Typically, frost forms overnight when skies are mostly clear and winds light.

Spokane usually sees its first frost on Oct. 7, which means we’re running behind schedule. With the coldest low temperature of 42 degrees so far, the mercury needs to tumble another 10 degrees or more.

Last October, Spokane’s first fall frost arrived on Oct. 11, when the overnight low hit 29 degrees. October 2020 started out warm like this month and the first frost came on Oct. 15 – followed by a major snowstorm eight days later. Needless to say, that year’s growing season came to an abrupt end.

The city’s earliest frost recorded was on Sept. 10, 1895, and the latest frost is early November, which occasionally does happen. Generally, however, there’s a 90% chance that frost will have occurred at Spokane International Airport by Oct. 31.

With early morning inversions becoming stronger, a cold air mass combined with clear skies and light winds are the right conditions for frost to form, even if afternoon highs are in the 60s.

For now, the ridge of high pressure responsible for our warm autumn weather is rebounding after the recent two-day cooldown. So we’re talking about daytime temperatures reaching into the 70s. While overnight frost may well occur again in outlying areas, it’s not likely to appear in the Spokane area any time soon.

Nic Loyd is a meteorologist in Washington state. Linda Weiford is a writer in Moscow, Idaho, who’s also a weather geek. Contact: ldweiford@gmail.com.