Forecasting Winter Weather
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By Jim Woodmencey
Intently we hold vigil on The Weather Channel, watching as the "Mother-of -all-Storms” is taking shape in the Pacific, spinning like a pinwheel towards the West Coast. Certainly it must be headed our way; how could a storm this big and juicy possibly miss us?
Or is this one of those storms that goes south or falls to pieces and never delivers the promised dump of powder? How can we tell if this is the "big one" we've been waiting for?
The old-timers in Jackson Hole have a saying, "Only fools and newcomers would try to predict the weather here."
Even armed with a degree in meteorology and years of forecasting experience, this is still a tough location to forecast for, mainly due to the complex, mountainous terrain that surrounds us. Nevertheless, I will attempt to give you some insight into forecasting winter storms for this area and what it takes to get that big dump in the Tetons.
Where to Begin?
Just as no two snowflakes are alike, neither are any two snowstorms. But by recognizing certain weather patterns that are more favorable, you can at least hedge your bets on which type of storm will have the best potential.
Begin your analysis by following the jet stream, that giant river of air that zips along at about 100 mph in the upper levels of the atmosphere, somewhere around 30,000 feet. The jet stream is also a de facto dividing line between cold air to the north and warmer air to the south.
During most good snowstorms the jet stream will be located directly over us, or just to the south. But be aware that a shift in the position of the jet by 50 to 100 miles can make a huge difference on who gets the most snow.
The Spin Cycle
Many of those big, curled-up storms that we see spinning on the satellite photos over the Pacific Ocean seem to fall apart or weaken as they move inland, despite the jet stream flow. Why is that?
Think of a toy spinning top on a smooth glass surface (like a storm whirling above the ocean). When you put the spin to it, the top stands up tall and rotates rapidly for a long time, maintaining its energy. Spin that same top on a rough surface, such as pebbled concrete, and watch it wobble, slow down, and fall over in short order. The topography of the western United States has the same effect on the storms, causing them to slow down and lose energy as they encounter rough terrain.
Storms will often regain their spin once they cross the Rockies and enter the Plains, where the surface is smooth and flat again. Those storms bring big snow to places like Lander and Casper, Wyoming, on the east side of the Continental Divide, but they leave Jackson Hole dry.
A Few Storm Scenarios
Southwest Flow: Bigger, juicier looking storms that we see on the satellite and radar coming into the western United States usually approach the Tetons with winds from the southwest. You may hear that Tahoe had four feet of snow the night before, or that it’s snowing to beat-the-band across western Idaho, and that this storm is literally knocking at the backdoor. A big dump is inevitable. Right?
Maybe not, especially if the jet stream is oriented parallel to the Tetons (SW to NE). Often the mountains will deflect a storm in a southwest flow, skipping it northward along the West Side of the range, dumping more snow on the potato fields in Idaho than the ski slopes of Northwest Wyoming.
From the Northwest: A northwesterly flow is more perpendicular to the orientation of the Teton Range and provides the best lifting action for the moisture and clouds. Consequently some of our biggest dumps on the mountain come with a northwest flow. The air is usually colder, resulting in light, dry, snow, that much-coveted "cold smoke." Grand Targhee or Jackson Hole might receive 12 to 18 inches of snow in 24 hours in this situation, while in town, only a dusting will fall.
On the weather maps this northwesterly flow regime looks fairly benign. Clouds are barely detectable on the satellite, radar shows very little precipitation, and the "storm" is generally unimpressive to look at. But the "pow" sometimes appears.
From the West: The best scenario for a big, continuous snowfall in Jackson Hole occurs when a low-pressure system is parked along the Washington/Oregon Coast, and a westerly jet stream flow develops just to the south of that low. This brings moisture in a straight line from the Pacific, across Oregon and Idaho, and aims it right at the Tetons.
This situation can bring fresh powder in great quantities to both the mountains and the valley, day after day. That doesn't happen every year, and call me a fool if you want to, but I can already see it coming for this winter!
Jim Woodmencey is the on-air meteorologist for KZ95 Radio in Jackson and the author of the book Reading Weather. To view weather maps and to get a current powder forecast for the Tetons, go to his web site at
www.mountainweather.com
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