With the prevailing jet stream moving across the Arctic or north Pacific Ocean and then onto the North American continent via the Pacific Northwest, it is no wonder that this air arrives laden with the moisture and cold it has picked up along the way. Often the result is orogenic precipitation (mountain rain), and in the winter, mountain snow. This high altitude air mass is a good predictor of forthcoming weather. This jet stream model produced by San Francisco State University extrapolates current jet stream wind and pressure gradients into a prediction of tomorrows jet stream. Animation control by localsnow.org. To animate click start.