Weathercatch: Graupel, the oddest winter precipitation, fell in our region

Mother Nature recently treated parts of the Inland Northwest to an unusual show called graupel. And talk about an oddball.

When we think about winter precipitation types in our region, snow, sleet and freezing rain typically come to mind. But every now and then another kind makes an appearance.

Graupel, also known as snow pellets or tapioca snow, resembles pea-sized Styrofoam balls. Though it’s fairly common in high elevation areas, not so much here. So when it made fleeting appearances in our region during the weekend of Feb. 14-16, plenty of folks were mystified over what was falling from the sky. Spongy, milky-colored pellets tumbled to the ground in sporadic intervals in locales that included Spokane, Sprague, Moscow-Pullman and Coeur d’Alene.

This precipitation type – which, by the way, is pronounced GRAH-pull – is a small, mushy, cold-season version of hail. Compared to sleet, it’s larger, more opaque and more grainy. Squeeze graupel between your fingertips and it will crumble. Not so with hailstones, sleet or freezing rain.

And you know what’s inside that crumbly crust? Snowflakes. Graupel is formed when snowflakes encounter super-cooled water droplets in subfreezing clouds. In a process called rime accretion, the water droplets flash freeze around individual snowflakes to create granular balls of white.

That weekend in mid-February, atmospheric conditions were just right for generating graupel. We had very cold air at the cloud level with milder air near the ground that created dynamic updrafts, or upward vertical motion. This, in turn, jettisoned the tiny water droplets directly into the path of newly formed snowflakes.

Simply put, we had cold air above, instability in the atmosphere below and precipitation. Late winter into early spring is when these conditions are most likely to merge as the sun becomes stronger but the air aloft is still fairly cold.

The next time you see a graupel shower, consider it an off-chance gift from Mother Nature. After all, it’s a precipitation type that makes few appearances here in the Inland Northwest.