Snowfall: from the Appalachians to the Andes
by Susan King
From October through April in the central and southern Appalachian Mountains, this forecast is all too familiar.
But what is it about northwesterly winds that makes them the likely bearers of severe winter weather, particularly in the Appalachian Mountains?
Baker Perry, an assistant professor of geography at Appalachian State University, has been looking for the answer to that question for the last seven years.
His fascination with the subject began much earlier, however.
"My interest in snowfall patterns in mountains began as a child growing up in the Bolivian Andes and later, in Haywood County in the Southern Appalachians where I was constantly amazed at how the northern areas of the county along the Tennessee border could receive heavy snow, six or more inches, during periods of northwest flow, while much of the rest of the county would have only snow flurries," Perry said. "I was also fascinated by the stories my grandfather, the late Dr. H.B. Perry Jr., told about February and March 1960, when 84 inches of snow fell in Boone, and snow fell nearly every day.
"The high-impact snowstorms April 2-5, 1987, May 6-9, 1992, and March 12-14, 1993 (Blizzard of '93), each with snowfall totals of several feet, had a profound impact on me and sensitized me to the tremendous impacts that major snowstorms can have in this region. As I began to pursue graduate study in climatology, I realized that there was limited scientific understanding of the atmospheric processes and spatial patterns of snowfall in the Southern Appalachians, particularly the very common northwest flow snowstorms. And at that time, no research had yet been conducted on the subject. Therefore, I chose to write my dissertation on northwest flow snowfall," Perry said.
Northwest flow snowfall usually develops when NW winds force a shallow, moist layer of air from the Ohio River valley up against the Southern Appalachians. However, extreme storms, such as the Blizzard of '93, can also occur with NW flow, allowing snowfall to really pile up. In recent years, 80 to 90 percent of snowfall in the Southern Appalachians has been associated with NW flow.
Perry's research has shown that when air trajectories come from the Great Lakes, snowfall may be enhanced due to the additional moisture.
The layer of moisture in clouds that produces snowfall is very shallow, compared with the moisture layer in clouds that produces thunderstorms. How significant snowfall can develop in such a shallow layer of moisture remains a source of uncertainty for Perry and his colleagues.
Perry's research has demonstrated that snowfall patterns are strongly influenced by topography, with those areas along the windward northwest slopes and at higher elevations typically receiving the most snowfall.
Snow crystals called graupel (pronounced "gropple") occur during many of the snow storms. Also called soft hail or snow pellets, graupel is precipitation that forms when super-cooled droplets of water collect and freeze on a falling snowflake, thus encasing it with rime ice. Rime is a coating of ice that freezes almost instantly on a colder surface that is exposed to super-cooled water droplets present in clouds.
The implications of graupel are huge, particularly for travelers, because when graupel occurs, road conditions often deteriorate rapidly.
Perry has helped establish a number of collaborative endeavors, including the Northwest Flow Discussion Group, a forum in which atmospheric scientists from across the UNC system interact with National Weather Service offices with forecasting responsibilities from Georgia to Maryland.
The group meets monthly by teleconference to discuss recent/upcoming NW flow snowstorms, research updates, local modeling efforts, and other notable developments pertinent to the NW snowfall phenomenon.
"These activities have ultimately improved scientific understanding, led to better coordination among the National Weather Service forecast offices, and improved snowfall forecasts with longer lead times. They have also been an important linkage between the academic and operational forecasting communities," said Perry.
Perry's snowfall research also has direct connections to current investigations underway in the Andes that link the atmospheric processes of snowfall development to climate indicators preserved in tropical ice cores. "This knowledge gives us a much more detailed understanding of how climates have changed in the past and how they may change in the future," Perry said.
Perry is also a founding member of Appalachian Atmospheric Interdisciplinary Research, which operates additional atmospheric research stations on the Appalachian campus and at Poga Mountain, Roan Mountain, Beech Mountain (in cooperation with Beech Mountain Resort), Grandfather Mountain (in cooperation with Grandfather Mountain), and Hardin Park and Bethel elementary schools.
To learn more about his research go to http://geo.appstate.edu/faculty-staff/faculty/baker-perry or http://appalair.appstate.edu.