Northwest upslope snow in the Appalachian Mountains




Map created using nationalatlas.gov

Discussion

Northwest upslope snow is snow that occurs when the primary snow producing mechanism is wind blowing up the northwest side of the Appalachian Highlands. Sometimes the snow occurs with little synoptic scale (large scale) upward vertical motion and sometimes even with downward motion. Northwest upslope snow often occurs in conjunction with snow produced in the wrap around (or back side) of a low pressure system. In these cases, the upslope enhances the synoptic scale snow. Stronger winds and winds that blow more perpendicular to the mountain ridges produce better convergence and lift for the snow. Convergence occurs when air that runs into a mountain barrier slows down and is converged upon by faster moving upsteam air. As a result, lift occurs a little upstream of the mountains and not only when the air rises up the mountain slopes. In the absence of wrap around moisture behind a departing storm system, the primary moisture source for northwest flow events appears to be the Great Lakes of Michigan, Huron, and the west side of Lake Erie.

Any atmospheric processes that create upward motion or decrease the stability of the air will enhance the efficiency of snow production. Instability can develop with surface heating between breaks in the clouds underneath the cold core of an upper level cutoff low pressure system. The heating lowers the stability by increasing the lapse rates within cold upper low. The air then rises up the mountains with less resistance making snow formation more efficient. Sometimes the lift forced by the mountains is strong enough to trigger convection. Now you really get the snow going! Shortwave upper level troughs, embedded within the larger scale upper low pressure system can enhance snow by adding additional lift or destabilization to the air. Low level troughs can provide additional low level convergence.

Heaviest snow occurs on the upslope side of the initial higher mountain ridges along the North Carolina-Tennessee border, far western Virginia, and eastern West Virginia. Lighter snows as expected will fall on the leeward slopes. Higher mountains downwind of the initial ridges also receive heavier snow. A good example is the Black Mountain Range northeast of Asheville, NC. The Black Mountains include Mount Mitchell which is the highest peak in the eastern U.S. at 6684 feet. For reference, Asheville is located near the tip of the southern wind arrow.

On much rarer occasions, valley locations like Asheville can still receive significant snow of a few inches. Such an event involves higher amounts of moisture, surface and aloft, as can occur with wrap around from a departing storm system. The exact wind trajectories through the mountains to the north and west of Asheville help determine how much moisture is left to affect the city. Also, due to varying elevation, wind exposure, and temperature within the city, one location in or near town could receive 1 or 2 inches while another location only gets a dusting. Having lived in Asheville for a Winter, this website author has seen this situation occur.


References

An Overview of Northwest Flow Snow in Western North Carolina - National Weather Service, Greenville-Spartanburg, SC

Northwest Flow Snow in the Southern Appalachians - National Weather Service, Greenville-Spartanburg, SC

Snow Accumulations from Northwest Upslope Flow - Steve Keighton, Science and Operations Officer, Blacksburg, VA

The Influence of the Great Lakes on Northwest Snowfall in the Southern Appalachians - Robbie* Munroe and Doug K. Miller, University of North Carolina at Asheville, Asheville, NC; B. Holloway and Lackmann, North Carolina State University, Raleigh, NC

Perry, B. and C. E. Konrad, 2004: Northwest Flow Snowfall in the Southern Appalachians: Spatial and Synoptic Patterns. Preprints, 61st. Eastern Snow Conference, Portland ME, 179-189.