It begins far out in the Pacific with swirling rivers of air. Driven by ocean currents and by the rotation of the Earth, cold polar masses mingle with moist air from the subtropics and start to rotate in a counter-clockwise direction. Almost as if alive, the clouds writhe, swell and heave. As the warm air rises, a low-pressure center forms and is pushed westward by prevailing winds.
Eventually, the storm rolls ashore somewhere along the West Coast, then crashes into the formidable barrier of the Sierra Nevada or the Cascades. Fingers of frozen vapor drape across the crest of the ranges, often dropping snow in increments of feet, not inches.
Forecasters along the West Coast have it easy compared to their colleagues in the Rocky Mountains. When low pressure systems and their associated fronts approach of the ocean, it's fairly simple to predict when they'll hit and how much rain and snow will fall.
After the storms pass those first ranges, things get a bit more complicated. The spinning dynamo of clouds that generates precipitation is torn apart by the rugged topography. Instead of a massive front, there are streamers of moisture that still drift eastward. But it's not always clear exactly where they will deliver a secondary punch.
Radar images displayed by Denver-based TV stations often show a big mass of clouds moving across Utah toward Colorado, and the weather maps suggest that the entire state is about to be inundated by widespread snowfall. Even the enthusiastic and knowledgeable forecasters with The Weather Channel don't always acknowledge the complexities of Rocky Mountain snowfall patterns.
Since the storms have lost most of their oomph, snowfall in Summit County depends on the direction of the flow. The air needs to be pushed up against the north- and west-facing slopes in order to wring out the remaining moisture. A juicy storm rolling in from the southwest can dump a few feet of snow in the San Juans and in the Elk Mountains around Crested Butte but leave Summit County high and dry.
Conversely, a narrow band of clouds, even without a well-defined low pressure center, can hammer Summit County's mountains if it's streaming from the northwest to the southeast.
At other times, a low pressure center re-forms just east of the Rockies, on the southeastern plains of Colorado, fueled by warmer moist air from the Gulf of Mexico. These are called upslope storms because the counter-clockwise spin pushes the moist air up the front range of the mountains, where it can snow several feet while the weather is warm and sunny in Frisco.
All these factors are often forgotten when the ski industry hype machine switches into over-drive. During an upslope storm a few weeks ago, an industry PR person appeared on the Weather Channel in a short clip that was filmed near Genesee, where it was snowing heavily. During the segment, there was no mention of the fact that the weather was actually mild and dry just on the other side of the Continental Divide - were the ski areas are.
About the same time, a top resort executive based in Broomfield posted a picture of the snow on his deck to Twitter, eliciting a response from a Breckenridge resident, who pointed out, correctly, that it was warm and sunny in the mountains. The industry leader later said that snow is snow, wherever it is, and that it helps get people excited for the season.
I appreciate the industry's efforts to spur business, but as a weather geek, I'm also uncomfortable with the apparent disingenuousness in some of these efforts, mainly because weather science is a beautiful thing that shouldn't be exploited for commercial gain. Plus, your customers are going to learn the truth soon enough, especially in the era of social media.
I remember standing in a winter forest as a 10-year-old, marveling at the shimmering diamonds in the air, the frost on the branches and the thick layer of clouds in the valley below. A few days later, I went to the library and checked out a book on weather science to learn more about winter snowstorms.
I sometimes wonder whether the ski industry wouldn't benefit more from being completely transparent about weather and snowfall with its customers, but when snow=money, perhaps that's expecting too much.
Bob Berwyn has been reporting from Summit County since 1996 and loves to play in the snow.