What’s up with upslope? The snow is always on the other side of the mountain – but why?
The Appalachians are oriented southwest to northeast. After a frontal passage, winds usually shift to west northwesterly or northwest - switching on the upslope machine.
When winds become perpendicular to the mountainside, they move up the slope of the mountain – hence upslope. Wind angles determine the intensity of the upslope event. Ninety-degree angles (NW winds) produce the most ideal conditions for upslope snow. However, WNW and northerly winds will also produce smaller upslope events.
Air that is moving horizontally along the mountainside now has a vertical component and will be pushed up the western slopes. As air is forced up the mountain, it cools. Any moisture in the air will condense into clouds and potentially precipitation. In the wintertime, snow showers are likely, with rain showers likely throughout the rest of the year.
There are several moisture sources for upslope precipitation. The most common moisture source is the Great Lakes. Winds carry moisture from the Great Lakes to the Appalachians, which move up the mountainside. Sometimes moisture is available in the current air mass to produce upslope precipitation.
Other times, an upper level disturbance will cause enough lift in the atmosphere to bring isolated snow showers and flurries to the Valley.