Author's Note: The following report has not been subjected to the scientific peer review process.

1.  Introduction

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.

2.  Discussion

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.