Significant Warming Induced by Downslope Winds near the Smoky Mountains

by

David M. Gaffin


Introduction

On Saturday afternoon, January 2, 1999, unexpected warm temperatures were reported across the central East Tennessee Valley, evidenced by the Knoxville McGhee-Tyson Airport's high temperature of 590 F. Nearby sites such as Knoxville Downtown Island Airport and the Gatlinburg-Pigeon Forge Airport, reported highs of 610 F and 600F, respectively (Fig. 1). What made this event unusual was that surrounding sites in East Tennessee reported highs only in the mid 40s (450 F at Chattanooga, 480 F at Oak Ridge, and 460 F) at Tri-Cities), and temperatures were in the lower 30s on the eastern slopes of the southern Appalachians (for example, 310F at Asheville). Other sites in the central East Tennessee Valley recorded high temperatures in the 50s (Athens 540 F, Morristown 520 F), while other sites farther northeast, such as Greeneville and Elizabethton, reported highs of only 450 F and 420 F, respectively. The warmer temperatures evidently occurred in a narrow band downwind of the Smoky Mountains.

The cause of this narrow band of warm temperatures, some 10 to 150F warmer than surrounding sites, was a strong downsloping south-southeast wind off the Smoky Mountains which warmed adiabatically as it descended. This phenomenon is normally observed downwind of mountain ranges, but is typically associated with dramatic temperature rises near large mountain ranges, such as chinook winds near the Rockies and the Santa Ana winds near the mountains of southern California. However, as will be seen in this study, downslope winds can also occur downwind of the Smokies and bring significant (and sometimes unanticipated) temperature rises to localized areas.


Downslope wind phenomena

Downslope winds have been frequently documented near mountain ranges worldwide. According to Houghton (1985), if stably stratified air in the lower troposphere crosses a mountain ridge and is forced above its equilibrium level, it will become negatively buoyant and will accelerate downward toward the valley on the lee of the mountain ridge. Also, the downward flow is enhanced when the surface pressure gradient on the leeward side is directed away from the mountain range. As the air rapidly descends, adiabatic warming is induced by the compression.

Significant temperature contrasts due to downslope winds are rare near the Smoky Mountains, since the Smokies are a relatively small mountain range with peaks around 6500 feet and elevation rises of only 5000 feet above the surrounding landscape (Fig. 2). Previous studies of downsloping winds near the Smoky Mountains (Shumway 1993) discussed rapid temperature rises only on the order of 50 F.


Synoptic conditions on January 2, 1999

Figure 3 shows the surface analysis at 1700 UTC on January 2, 1999, revealing a cold air damming situation associated with a stagnate high pressure system over the Carolinas and northern Georgia. Temperatures remained in the 20s and 30s most of the afternoon on the eastern side of the Appalachians. On the western side of the Appalachians, warmer air in the 40s and 50s was being advected into East Tennessee by strong southerly winds. A strong pressure gradient producing gusty south-southeast winds was created by this contrast in airmasses, as well as the approach of a strong low pressure system located over the mid-Mississippi Valley.

The Regional Atmospheric Measurement and Analytical Network (RAMAN) meteorological tower network, located primarily over central East Tennessee (one site is in western North Carolina), showed the contrast in temperatures between the western slopes and eastern slopes of the Smokies (Fig. 4). By 3 pm EST the Bat Creek Knob tower near Sweetwater, Tennessee and the Sharp's Ridge tower near Knoxville both reported temperatures in the 50s, reflecting the narrow band of warm temperatures generated by the downsloping southeast winds.

Morning soundings from around the southern Appalachian region at 1200 UTC (Fig. 5) revealed a stable airmass with a temperature inversion in the lower levels. A layer of cold, dry air between 900 and 700 mb was located east of the Appalachians, with a relatively warmer and more moist airmass to the west of the Appalachians. The 1200 UTC 850 mb analysis (Fig. 6), which is the best pressure surface to analyze in this case since it is usually found around 5000 feet MSL (roughly the elevation of the Smoky Mountains), revealed a strong southerly inflow around the eastern side of a strong low, while the 500 mb analysis revealed a strong southwest flow aloft.

The 6-hr forecast from the 1200 UTC Eta model run on January 2 (Fig. 7) indicated a strong low-level 850 mb jet from the south-southeast in excess of 60 kt over the southern Appalachian region. In addition, relatively warm 850 mb temperatures of 80 C (460 F) were being advected into the region. Satellite images from January 2, 1999 (Fig. 8) revealed breaks in the clouds due to the warm, dry air sinking into the valley. In general, these images did not show any lee wave activity (lenticular clouds) which sometimes occur downwind of the Smoky mountains when the wind is perpendicular to the mountains. Lee waves are usually formed when a sufficiently strong inversion forms a reflective surface for wave maintenance and propagation.


Conclusions

An unusually warm and narrow band of temperatures observed on January 2, 1999 - some 10 to 150 F above the surrounding area temperatures - were the result of adiabatic warming of stable air as it was dynamically forced downward into the central East Tennessee valley. A strong low pressure system over the mid-Mississippi valley combined with a strong high pressure system over the Carolinas to create a strong pressure gradient which induced south-southeasterly winds, perpendicular to the Smoky Mountains. Also, a strong low-level jet advecting relatively warm air at 850 mb contributed to the magnitude of the warming observed in the valley downwind of the Smokies.

The magnitude of this event was unexpected and had a significant impact on the temperature forecast for the central Tennessee Valley. It also likely had an impact on the rain forecast, as the descending air stabilized the local atmosphere. Forecasters will have a formidable task in predicting the magnitude of these atypical events, but with knowledge of the synoptic conditions necessary for the occurrence of downslope winds, forecasters can reasonably expect to predict the onset of such events.


References

Houghton, D.D., 1985: Handbook of Applied Meteorology. John Wiley and Sons, Inc., 50-53.

Shumway, S.M., 1993: A mountain/lee wave situation over the Smoky Mountains and implications to future WFO Knoxville/Tri-Cities. Unpublished local study, 13 pp.