Aside from elevation, Garrett generally receives snow from 3 separate meteorological phenomena:
Garrett County is part of Appalachian mountain chain that is situated perpendicular to the usual jet stream flow, west to east. As air runs up against the Appalachian Mountains, it is forced upward. Any moisture at the surface lifts upward, condenses, and forms low clouds above the ridges. Eventually those clouds will precipitate. The upslope phenomenon dovetails with lake effect snow. While many times upslope snows are due to an arctic frontal passage, bands of lake effect snow from Lake Erie also tend to reach Garrett County, where the same upslope effect occurs.
While lake effect and upslope snows are referred to as mesoscale events (i.e. not generally associated with a large low pressure system), there are several synoptic scale events that give Garrett County snow as well. Note however that even in synoptic scale events, Garrett's elevation and upslope effect will often produce larger amounts of snow than in neighboring lower elevation areas. The first of these systems is the Alberta clipper, nicknamed such because it originates near the Canadian providence of Alberta. These systems are generally associated with a large trough over the eastern United States that allows the clipper to dive southward out of Canada, with an associated polar or artic air mass. While Alberta clippers and other west to east oriented systems usually do not tap into any Gulf of Mexico moisture, they do often maintain enough moisture of their own to produce anywhere from 4 to 8 inches to Garrett County. The recent event that blanketed the area with 7 inches on January 9th was a classic example of an Alberta clipper with associated upslope effect.
The second type of synoptic event is the coastal or nor’easter. These systems, unlike clippers, usually originate further south, often along the gulf coast, and track northeastward up the eastern coastal areas. Also unlike clippers, these systems do often tap into Gulf moisture as well as moisture from the Atlantic Ocean. While Garrett County is usually on the western end of coastal systems, the county often sits just east enough to receive substantial snows from these systems. Coastal systems can become very intense, produce very high winds, and provide memorable snow events in Garrett County. Most of Garrett County's largest snowfall events were associated with coastal-type systems.
To summarize, while some areas in the east are known for their lake effect snows like Erie or Buffalo, and others for their propensity for coastal storms such as Boston or Albany, Garrett County is perfectly situated to benefit from several types of weather systems and thus explains why the County typically averages over 100 inches annually.