Last night, a number of Washington locations received their first frost...but just barely. And there is plenty of interest in the first frost, including from eastern Washington vineyards and those of us with still viable tomato plants.
Here on the western side of the Cascades, freezing temperatures (light blue colors below) were mainly found over the south Sound lowlands away from the water and in some Cascade valleys.
In eastern Washington, light frosts were observed near Ellensburg, Yakima, among other locations.
We do not have unusually cool air over us, but the nights are already getting much longer and much of the region was cloud free over night (see satellite photo below). The surface cooling is caused by radiational cooling to space. Long nights are obviously good for this. Clear skies are good too, since clouds can act as meteorological blankets, absorbing and re-emitting infrared radiation.
The frost risk will lessen over the weekend as the atmosphere warms as high pressure moves in overhead.