The Jet Stream:
This current of strong wind is the highway on which most storms ride. As far as powder is concerned, the sweet spot is typically just north of the flow - where you'll find the right combo of moisture and cold temps. Being south of the jet can mean milder temperatures and rain.
Low-Pressure Systems can bring heaps of snow because the storms? winds spin like tops. What matters here is 1) the direction of the wind that's behind - or pushing - the system forward, and 2) the orientation of your favorite resort.
Aleutian Storms: These low-pressure systems ride the jet stream southeast out of Siberia (and the Aleutian Islands) before hitting the Lower 48. They're usually cold and dry, sometimes dropping feet - but more often, inches.
Subtropical Pacific Storms that form near the equator soak up more water than their arid siblings up north. That means more - but not always wet - snow, since cold air above the mountains doesn't hold much moisture. Warm, wet air chills and dries as it rises; then the clouds drop light flakes. Just ask anyone at Taos or Wolf Creek.
Pineapple Express: A plume of tropical moisture that forms near Hawaii and spans eastward. When the jet stream dips south, moisture hops aboard and beelines for the western U.S. When you see one coming, head north of the jet, where temps are (with any luck) cold enough to make snow, not rain.
Orographic Lift: The more directly storms hit mountains, the stronger their effect. It's why the San Juans, La Sals, and Sangre de Cristos, for example, get hammered with fluff by otherwise juicy subtropicals.
Relative Humidity: More moisture in the atmosphere means higher relative humidity. As RH increases, there's more wet stuff to wring out over the mountains. If RH is high and a storm is moving in, wax up your fatties.
Gulf Stream effect: Big storms that hit East Coast resorts tend to track from the southwest toward the northeast. But they're tough to predict, in part because it all depends where the jet stream crosses the Gulf stream. The Gulf Stream pulls warm temps north, meaning areas south of the jet mostly get rain - as anyone who's skied in Pennsylvania can tell you.
Nor?easter: This low-pressure system picks up moisture in the North Atlantic, and, when it collides with a cold air mass coming down from Canada, dumps on the likes of Killington. Watch for an Alberta Clipper: It can move from the plains toward the Right Coast and become a Nor'easter, as was the case with the Blizzard of '78.