The First Snow of the 2000-2001 Winter Season
is One of the Earliest and Heaviest on Record
Bryan P. McAvoy
Author's Note: The following report has not been subjected to the scientific peer review process.
Snow fell across the mountains of the western Carolinas most of the night Saturday, 18 November 2000, and into the day on Sunday 19 November. An accumulating late November snow is not that unusual in the mountains. What happened in the foothills and piedmont, however, was quite unusual for the region. During the late morning hours on 19 November, the rain/snow line began to advance east across the foothills and into the piedmont. By late morning, most of the Greenville-Spartanburg County Warning Area was experiencing snow. Snow fell moderately over this area until mid afternoon, when the precipitation ended abruptly from west to east, the result of subsidence around the upper jet. Snowfall totals averaged from 2 to 5 inches over the mountains (a bit more in spots), and from a trace to 3 inches east of the mountains (Fig. 1).
Figure 1. Total snow accumulation for the 18 November 2000 Winter Storm. Click on image to enlarge.
The 2.5 inches of snow recorded at the Greenville-Spartanburg Airport for the day was the heaviest snowfall ever recorded in November, beating the old record of 1.9 inches set back in 1968. For additional November snow data visit this link: Greenville-Spartanburg November Precipitation Records. The record for the day was tied at Charlotte Douglas International Airport where 2.5 inches of snow also fell. The previous record was also set in 1968. The Asheville Regional Airport recorded 2 inches of snow for the day, quite a bit less than other mountain locations.
Why the surprise snow? Without getting too technical, the lower levels of the atmosphere were expected to be too warm for snow east of the mountains. And for much of the morning, this was the case, with only some occasional mix-over to sleet. Two cooling processes, however, conspired to lower the temperatures in the lower atmosphere enough to support snow. One process was evaporational cooling. This occurs when rain evaporates as it falls through a dry air mass. This alone was probably not enough to cause the transition to snow. The other process was the melting of snow. As snow falls and melts, it also robs the atmosphere of heat, just as rain does when it evaporates. This process, however, can occur even in a saturated atmosphere, unlike evaporative cooling. Based on the precipitation rates observed late Sunday morning, it is estimated that melting of snow resulted in about 2 degrees Celsius of cooling. This cooling process was likely what lead to the Interstate-85 corridor snows.
An examination of critical thickness for freezing and frozen precipitation revealed that every line except for one was northwest of the foothills and piedmont (Fig. 2). Generally, locations northwest of these lines (particularly the yellow and turquoise lines) will experience snow and locations to the south will experience rain. This data was taken from the morning run of one of our best numerical models, and is valid for 2 pm Sunday afternoon, the height of the snow event east of the mountains. As computational power increases, the models will be better able to resolve cooling due the melting of snow.
Figure 2. Critical thickness lines from the numerical models on 18 November 2000. Image courtesy of The College of DuPage.
This Public Information Statement provides detailed information of snowfall across the Greenville-Spartanburg County Warning Area. Note though that some of these totals reflected the amount on the ground by late evening, and some reflected estimated storm total snowfall.