* Geographic and topographic maps created from the USGS/ESRI ArcGIS mapping software and database
Variation in elevation within Asheville is sometimes enough to cause differences in precipitation type when air at the surface is marginally cold enough to support snow. Just a few hundred feet difference in elevation will determine whether precipitation is rain or snow or whether or not the snow accumulates. Wind direction also counts as it determines what parts of the city will be more affected by downslope winds from the surrounding higher terrain. The warming and drying action of downslope winds will erode the snow intensity. Cold air trapped in the valley, such as from a cold air damming event, can block these winds from flowing down into the valley. Based on records from the Asheville Regional Aiport about 10 miles south of downtown, the seasonal snowfall mean is 13 inches per year based on 30-year climatology from 1981 to 2010.
Although Asheville is in a valley, it is still part of the elevated terrain of the Appalachian Highlands and experiences some enhancement of snow from upslope winds into the plateau. Some of the moisture is blocked by surrounding mountain ranges. As an example, snow events from northwest wind flow, sometimes incorporating moisture off the Great Lakes, are frequently reduced to just light snow showers or flurries after the moisture passes over the higher terrain to the west.
Cold air damming can result in freezing rain. Cold air damming occurs when a high pressure center over the Northeast or Middle Atlantic states pushes cold air against the mountains. The air is then forced to slide generally southwest, paralleling the Appalachian Mountains, across the Foothills, Piedmont, and Coastal Plain to the east. If the push of cold air against the mountains is strong enough, the cold air can spill into the valleys. As stated in the cold air damming discussion, the winds turn to come from the east to southeast near the base of the mountain slopes before moving into the mountains. If the air in the valleys is below freezing or if the air is dry enough so that evaporating rain can cool the air to below freezing, then freezing rain will occur.
An interesting characteristic about cold air damming events in Asheville is often a south-southeast wind develops at the airport as cold air starts to move into the region. Winds already tend to move into the mountains from the southeast during a cold air damming event. The topography surrounding the airport appears to contribute to a more south component. Also, bear in mind that when winds above the shallow cold pool are from the south or southwest, the winds at the surface may be embedded within a veering layer. The veering layer is where the winds turn from the southeast to the south or southwest, perhaps giving winds at the airport a more south component.
The following sequence of five images shows an example of cold air damming in North Carolina. In the first image, two observations are plotted in the far southwest part of the state. The Asheville Regional Airport is the one where both the temperature in red and the dew point in green are 43 oF. Note in the subsequent images how the winds increase then veer to come from a south direction. The temperature falls even though the sun is up. Clouds and precipitation help to lower the temperature.
An illustration from the Climate Prediction Center (CPC) of the National Weather Service shows the typical influence of El Niño on weather patterns affecting the United States during the winter. Note the strong southern jet stream across the Deep South. Also, note the strong split from the northern jet stream in Canada that brushes the Northeast United States. This depiction of weather patterns is not intended to be exact. The jet streams will vary in position and degree of definition at any given time. The strong southern jet stream results in an active storm track that produces above-average precipitation. Also important is that a greater chance exists for some of the storms to pass far enough south to keep Western North Carolina in the cold sector north of the surface low component of the storms.
Even so, increased snowfall for Asheville is not guaranteed. Cold air needs to get into the valleys for the snow to reach the ground. Higher elevations several thousand feet above Asheville have a better chance to get increased snowfall. They poke up into colder air associated with the upper-level (above the surface) components of the storms. The northern jet stream in Canada needs to drop far enough south to steer cold high pressure systems into the Northeast of the United States. From there, the cold air damming process can funnel cold air south along and east of the mountains.
The northern jet stream is more likely to be in a favorable position when the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) are both in the negative phase. The North Carolina Climate Office at North Carolina State University in Raleigh, NC features a discussion on their webpage titled Global Patterns: Arctic & North Atlantic Oscillations that illustrates the influence of the AO and the NAO on atmospheric pressure and winds.
The positive phase of the AO is characterized by a stronger than average polar vortex, meaning negative pressure anomalies in the arctic, that can be examined looking at geopotential heights of the 500 mb pressure surface. Positive pressure anomalies occur to the south, including over the Eastern United States. The positive phase of the NAO is characterized by negative pressure anomalies centered near Iceland and positive pressure anomalies to the south. The result is a stronger pressure gradient that produces a strong zonal wind flow through a deep layer of the atmosphere. The northern branch of the jet stream tends to stay is far enough north to inhibit cold air from penetrating south into the Northeast United States.
The negative phase conditions of the AO and NAO are the opposite of the positive phase conditions. The pressure systems and pressure gradients are weaker than average. As a result, the northern jet stream in the El Niño illustration is weaker and is likely to be wavier and drop south to allow cold air to move into the Northeast United States more often. The only issue left is the timing. The storms in the southern jet stream would need to move through the Southeast United States at the same time cold air is already in place or is close enough to move south into the region before the storm gets by.
Local Climatological Data - National Weather Service Forecast Office Greenville-Spartanburg, SC
U.S. Climate Data from usclimatedata.com
Climate Prediction Center (CPC) of the National Weather Service
North Carolina Climate Office at North Carolina State University in Raleigh, NC