For skiers, sledders, and riders in Montana there is no better place to experience copious amounts of dry snow than the Bridger Mountains. The low water content of this snow is so renowned that it has been dubbed 'The Cold Smoke'. That means the snow is so light and airy that it's easy to breathe in accidentally, just like the incessant smoke during Montana's infamous summer fire seasons. The difference however is all in the fun factor, skiing deep powder and feeling the tingle in your throat and the cool burn in your lungs is an unforgettable experience for anyone on their first outing.
The chances of encountering world class snow are well known by locals and visitors alike, what's less understood are the atmospheric conditions that are necessary to form heaps of the white fluffy stuff. By understanding for yourself how simple these conditions are to predict you'll be able to better plan when and how to use your sick days - cough, cough.
To better understand this process myself I placed some phone calls to National Weather Service meteorologists in Billings and Great Falls. Megan VanDenHeuvel, a meteorologist and storm warning coordinator in Great Falls is a skier at heart and knows the Bridger Mountains very well. In fact, she's something of an expert on the weather that makes us smile. Megan took time out of her busy Christmas schedule to answer of few of my questions as a pilot trying to better understand the local flying conditions, and as a skier wanting to know how to predict those epic snowfall events. The ensuing conversation was a review of basic weather theory mixed with a few nuggets of local wisdom.
Air generally cools as it rises and as the temperature drops any available moisture begins to squeeze out as cloud droplets, excess moisture then forms precipitation. This type of moisture release at at the intersection of Bridger Canyon and Brackett Creek Road intersection just downhill from Battle Ridge is particularly intense. Before pointing me in the direction of scholarly research journals she presented a simple idea. She says, "Think of the Northern Bridgers as a large funnel. When a strong northwest or northerly flow occurs, the air funnels south between Bridger Ridge and Battle Ridge, it then ascends because of the terrain." While westerly flows carrying lots of Pacific moisture do tend to produce snowfall on the east side of the Bridgers these conditions pale in comparison to the northerly flows which come down from Canada.
Because this is a complex system with many moving parts I immediately thought of the classic venturi - you know, the aerodynamic principle that defines pressure and temperature drops across a constriction and how those changes also relate to the speed of the fluid passing through the constriction. Megan confirmed that the venturi effect is at work here. The upwardly mobile air mass is squeezed between the ground and the atmosphere above which forces it to experience a pressure and temperature drop combined with a velocity increase. Pilots who fly on the east side of the Bridgers refer to this area southeast of Ross Pass as, "The nasty spot" due in part to the upper limit of the rising air and it's interaction with smooth air which causes mean turbulence.
So there you have it, the Bridger Mountains represent the perfect terrain trap for moist weather traveling south from Canada. When the air nears the town of Sedan, MT it begins to travel uphill and into a narrowing valley. In doing so it gets squeezed and trapped, having to travel faster and faster (the epic winds just north of Bridger Bowl), all the while dumping it's load of moisture as Cold Smoke worth it's weight in gold. By the time that air mass reaches the mouth of Bridger Canyon near Jackson Creek Rd it's largely subdued, it carries cooler, drier air, which is moving slowly. Next stop, Livingston - we'll talk about that ball of wax another day.
In Megans paper A Study of Heavy Snowfall Pattern Recognition for Bozeman, MT she points out what many of us know well, "Forecasting winter weather in the northern Rockies is a consistent challenge, given the diverse climates and topography across the region. Over southwest Montana, sparse observational and radar data combined with steep terrain create particular difficulties near Bozeman, one of the largest population areas in Montana." Accurately forecasting mountain weather is an exacting and difficult science. After talking with Megan I have a better idea of how to spot the will it or won't it of weather in the Bridger Mountains.
It's easy to spot these different systems for yourself by looking at surface prog charts which are published multiple times a day by the Aviation Weather Center, a tenant office of NOAA and the NWS. As you get your feet wet in weather data watch for formation of the 'Yellowstone Low', the center of low pressure will be hovering over Yellowstone Lake and the plateau. Since areas of low pressure rotate counter-clockwise in the northern hemisphere this brings the prevailing wind from the east towards the Bridger Mountains - less snowfall. Conversely you'll notice interesting combinations of low pressure moving southeast from British Columbia. As these come down the chute from Canada, they'll be fast moving, energetic systems which are perfectly poised to dump big on Bridger.
A Study of Heavy Snowfall Pattern Recognition for Bozeman, MT; Megan VanDenHeuvel (Syner) - Warning Coordinator Meteorologist, National Weather Service, Great Falls, MT