Weathercatch: Anniversary of rare snow rollers today; too cold for a repeat

<p><p>We know Mother Nature makes cool things during winter. Hoar frost, icicles, rime and sundogs, to name a few. But 13 years ago today, she left behind a field full of one of her rarer wintertime meteorological creations, 20 miles west of Spokane.</p></p><p><p>It was a surreal sight on Dec. 30, 2008, when dozens of large, balls of snow dotted a snowfield near Medical Lake. It was almost as if Mother Nature had displayed her own version of sushi rolls. But instead, they were snow rollers.</p></p><p><p>Nearby resident Ron Trees spotted the snow rollers that day, snapped photographs and forwarded them to the National Weather Service of Spokane. The agency then posted them on a web page highlighting significant or unusual weather events that occurred during December 2008.</p></p><p><p>Snow rollers are rare because it takes very precise weather conditions to create them. Wind, snow and moisture must come together in a just-so confluence of conditions. The snow must be moist, not powdery – the same consistency necessary for humans to build a good snowman. It also needs to be fairly deep. And conditions need to be windy, perhaps 15 to 35 mph or so. The wind is the force that picks up a chunk of snow on the ground and rolls it along, enabling it to acquire more snow and grow larger. The wind should blow hard enough to keep the cylinder rolling but not so much that it falls apart.</p></p><p><p>All of which led to that rare occurrence 13 years ago. Mid-December of that year, it was cold and snowy in the Medical Lake area and other parts of the Inland Northwest. In Spokane, nearly 2 feet of snow fell in 48 hours on Dec. 17-18. Unlike this year, temperatures warmed up after Christmas. Then, on Dec. 29, it snowed again – wet stuff atop a sizable layer of older snow. Next, on Dec. 30, the sun appeared and the high temperature of 30 degrees was optimal for making the freshly-fallen snow stick, but cool enough to keep it from melting. Winds gusts at an AgWeatherNet station near Medical Lake clocked over 20 mph, with sustained winds at 15 mph. Those winds pushed small chunks of the wetter snow across the open field where they increased in size as they collected more snow, similar to a child pushing snowballs across the ground to make a snowman.</p></p><p><p>Once the snow rollers grew too heavy, the wind couldn’t push them any farther and they stopped. The result was a crop of large fanciful shapes strewn across the snowy terrain.</p></p><p><p>Snow rollers occasionally appear in our region because we have the conditions to support them. We get wet snow, temperatures near the freezing mark, and wind levels necessary to scoot them along the ground. Furthermore, we have plenty of wide open areas where they won’t get snagged by a tree, boulder or fence post.</p></p><p><p>But don’t expect to see any snow rollers form under our current weather conditions. With high temperatures only reaching into the teens and lows hovering around zero, most snow is too dry and powdery. As long as you bundle up, it’s better suited for making tracks with skis.</p></p>