NORTHWEST UPSLOPE SNOW IN THE APPALACHIAN MOUNTAINS

Northwest Upslope Snow in the Appalachian Mountains

* Geographic and topographic maps created from the USGS/ESRI ArcGIS mapping software and database

Discussion

Northwest upslope snow occurs when air flows up the northwest side of the Appalachian Highlands and enough moisture exists in the air for clouds and precipitation to form. More technically the lifting process is called orographic lifting. Areas of precipitation that already exist will intensify as the move into the mountains. Northwest upslope snow often occurs in conjunction with snow produced by synoptic scale (large scale) lift in the wrap-around (or backside) of a departing low pressure system. In these cases, the upslope is usually more of an enhancer that the primary cause of the 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 air farther upstream catches up to it. As a result, lifting also occurs a little upstream of the mountains and not just when the air rises up the 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 process that creates upward vertical motion or decreases the stability of the air will enhance the efficiency of the orographic lift and consequent snow production. Instability can develop with surface heating between breaks in the clouds underneath the cold core of an upper-level low pressure system. Air can then rise up the mountains with less resistance making snow formation more efficient. With the instability present, the lift due to the mountains can trigger convection that produces bursts of heavy snow! Any smaller short wave disturbances within a larger upper-level low pressure system or any surface troughs to focus low-level convergence will enhance the intensity of the snow event.

Typically the 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 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 southernmost wind arrow on the topographic map at the top of the page.

On a few occasions, valley locations like Asheville, NC have received significant snow of several inches. Such events involve higher amounts of moisture as can occur with wrap around from a low pressure system moving off to the east. The exact wind trajectory across the mountains north and west of Asheville strongly influences how much moisture is left to affect the city. Snow could accumulate 2 inches in one part of town and only a dusting accumulate in another.

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.