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WFO Greenville-Spartanburg, SC

Local forecast by
"City, St"

The 2-3 January, 2002 Snow Event

over the Western Carolinas and Northeast Georgia

Bryan P. McAvoy
NOAA/National Weather Service
Greer, SC

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

1.  Introduction
This was an unusual event for the Greenville-Spartanburg (GSP) 
County Warning and Forecast Area (CWFA) in that the precipitation 
fell as all snow.  Local research shows that only about 10 percent 
of our winter precipitation events are of this type (Moyer 2001).  
Every county that was included in the initial winter storm watch 
and warning verified with significant lead time.  However, snow 
began earlier than expected and farther to the northwest.  While 
the southeast part of the CWFA was almost perfectly forecast (see 
Fig. 1 for snowfall totals) the surprise snow in the southeast 
slopes of the mountains the afternoon of 2 January will be a large 
part of this paper's focus.  As it became evident that the area 
of warning criteria snow would extend farther to the north and 
west, additional warnings were issued shortly after midnight on 
3 January.  Most counties in this second area of warnings received 
a very short lead time but they did verify.
Snowfall amounts for 2-3 January 2002
Figure 1.  Snowfall map, in inches, from the 2-3 January 2002 
winter storm. 
2.  Synoptic Overview
Snowfall amounts ranged from an inch or less over most of the 
North Carolina Mountains, to as much as 10 inches in the eastern 
part of the CWFA (Fig. 1).  Typical amounts for the storm were 
from 4 to 6 inches.  Amounts exceeded 1 foot over a large part 
of central and eastern North Carolina.  Much of the precipitation 
in eastern North Carolina fell late on 3 January and early on 
4 January, after a nearly twelve hour break in significant 
precipitation between the warm advection snow and the 
intensification of the upper level deformation zone, as the 
upper level energy with the system crossed the area.  Figure 3 
shows how far upstream the 500 mb upper low was when heavy snow 
was falling over parts of the GSP CWFA during the late afternoon 
hours of 2 January.  Accumulations associated with the passage 
of the upper low, and attendant intensification of the deformation 
zone, were only around one inch in the GSP CWFA.  The significant 
secondary snowfall was mainly along and east of Raleigh, North 
Carolina.  As a result, this review will not focus much on the 
contribution of the upper low and intensification of the 
deformation zone, but the interested reader may view this 
summary from WFO Raleigh (RAH).
Eta 500 mb height/vorticity at 2100 UTC 02 Jan 2002
Figure 2.  Eta model 500 mb geopotential heights and vorticity 
valid at 2100 UTC 2 January from the 0000 UTC 2 January 2002 
cycle.  Moderate to heavy snow was occurring over the GSP CWFA 
at this time.  Click on the image for a loop of the 500 MB 
heights from this run.
While the primary contribution to the heavy snow over the eastern 
part of the GSP CWFA was from low level warm advection, the 
precipitation which fell over the mountains of northeast Georgia,
South Carolina, and southern North Carolina was not in a region 
of strong low level isentropic upglide seen on the 292K surface
at 2100 UTC from the 0000 UTC 2 January run of the Eta model. 
While this run of the Eta model did not handle upper forcing 
well for the afternoon hours of 2 January, all indications are 
that its lower to middle tropospheric upglide was fairly accurate 
if a little too far east.  However, even though there was little 
adiabatic omega, the atmosphere over the entire CWFA was saturated, 
and not particularly stable, so it should not be surprising that 
the forcing which developed resulted in a snow band. 
Up to three inches of snow accumulation had occurred by late 
afternoon with the narrow snow band, quite a problem as the 
warnings did not go into effect until the second period (Wednesday 
night) for areas farther to the east.  While higher terrain was 
certainly a factor in these snow totals (it was anticipated that 
a dry low level airmass would delay precipitation onset in all 
but the southernmost zones), this base reflectivity loop (valid 
from 1200 UTC 2 January through 0700 UTC 3 January) clearly 
indicates a northeast to southwest oriented band of precipitation 
over this area in the afternoon from 1700-2100 UTC.  Thus, the 
precipitation in the mountains was likely the result of upper 
level forcing, as well as the higher terrain.  Even as significant 
snow was falling in the mountains, the central part of the Upstate 
had nothing but flurries and few heavier snow showers until the 
early evening hours.  This can be seen in these hourly decoded 
METARS from KGSP for 2 January 2002.  This part of the forecast 
was exactly as expected.  Snowfall totals did reach 2 to 3 inches 
in the southeast-most zones by later afternoon as well.  It was 
anticipated that this might happen, and is not considered a 
significant bust.
Eta model RH cross section over GSP from 0000 UTC 02 January cycle
Figure 3.  Eta model cross section of relative humidity over 
Greenville-Spartanburg, SC (KGSP) from the 0000 UTC 02 January 
2002 cycle. 
The easiest way to compare the differences between the 2 January 
0000 UTC run of the Eta and the 1200 UTC run, is through the use 
of BUFKIT time sections, centered over KGSP.  Figure 3 is from 
the 0000 UTC run on 2 January.  This was the run available to 
the mid shift, when the warnings were issued.  Notice how dry 
the low levels were.  Precipitation was not expected to make 
significant "landfall" until the evening hours over most of 
the CWFA.  Not too far to the south, however, the Eta model 
projected a much earlier onset time as can be seen on the 292K 
surface at 1500 UTC.  Figure 4 is from the 1200 UTC run of the 
Eta on 2 January.  Notice that the low levels are now completely 
saturated over KGSP through the afternoon hours.  Time sections 
of omega over KGSP illustrate why the moistening occurred so 
much more quickly.  The 0000 UTC Eta omega time section showed 
weak upward vertical velocity (UVV) over the region from 1800 UTC
to 0000 UTC.  Strong UVV was not expected until overnight on 
2 January and early on 3 January.  The 1200 UTC Eta omega time 
section was completely different, showing the strongest UVV 
during the afternoon hours of 2 January. 
Eta model RH cross section over GSP from 1200 UTC 02 January cycle
Figure 4.  As in Figure 3, except for the 1200 UTC 
2 January 2002 model cycle. 
A couple interesting features highlight why the upper-level 
forcing developed.  The EDAS 400 mb winds valid at 1800 UTC on 
2 January show a distinct double jet structure, right above the 
level of the strongest UVV on the 1200 UTC time sections.  At 
300 mb the jet was more consolidated, though the region was in 
the entrance region of the jet.  Considering the slight 
anticyclonic curvature of the jet, the deep forcing is fairly 
easy to intuit.
3.  Forecast Performance
The first watches were issued on the afternoon forecast shift 
of 1 January.  Figure 5 shows the total model QPF from Eta model 
runs from 0000 UTC and 1200 UTC on 1 and 2 January. It was the 
1200 UTC run on 1 January which moved the precipitation track 
farther to the west.  Per the Meet-Me conference call held that 
afternoon, a decision was made to issue a watch for parts of the 
South Carolina Upstate and the southern North Carolina Piedmont.

Figure 5.  Eta model total QPF from the model cycle at 
0000 UTC 1 January (upper left), 1200 UTC 1 January (upper 
right), 0000 UTC 2 January (lower left), and 1200 UTC 
2 January (lower right).
A second period warning was issued for the southernmost 
counties in the CWFA by the midnight shift early on the 
morning of 2 January (Greenwood, Abbeville and Elbert).  The 
reason for issuing the second period warning was that only 
around an inch of snow was anticipated by late in the first 
period, but there was expected to be 4 inches of snow fairly 
early in the second period.  The reasons for this are detailed 
in the previous section.  The previous watch was continued and 
also expanded to include the rest of the Upstate (the so-
called I-85 corridor), but not the mountains, and a significant
portion of the Piedmont of North Carolina.  Even though the 
2 January 0000 UTC Eta run had backed off a little in its QPF, 
the Canadian GEM continued to advertise heavier precipitation 
for the eastern half of the CWFA.  The forecasters on the mid 
shift had a great deal of confidence in the Canadian, a model 
which had outperformed (in the estimate of the forecasters) 
the Eta in significant winter events over the past two years. 
The day shift on 2 January issued warnings for an area that 
included the original watch area (the one from the previous 
day shift), and issued advisories for the counties included 
in the watch by the midnight shift. 
During the early morning hours on 3 January, the mid shift 
upgraded the advisory to a warning for the I-85 corridor of 
the Upstate as well as the South Carolina and northeast Georgia 
mountains.  As near-winter storm criteria snow had fallen in 
the mountains, these zones were upgraded to account for light 
to moderate snow which continued to fall much of the night.  
In the North Carolina Foothills and western Piedmont, warnings 
were issued as the deformation zone developed farther west 
than expected, spreading moderate to heavy snow into this 
area during the late evening hours on 2 January and early on 
3 January.  This rather weak deformation zone diminished 
considerably during the early morning hours.  However, the 
warnings were kept in effect for the entire day of 3 January 
as some precipitation was expected along and north of the 
track of the 500 mb low.  Around an inch of light snow fell 
over the southern part of the CWFA during the day associated 
with this feature. 
4.  Discussion and Conclusion 
A winter storm affected the GSP CWFA starting the afternoon 
hours of 2 January, and lasting until the late afternoon hours 
of 3 January.  Precipitation fell in association with several 
different forcing mechanisms during the event.  A band of snow 
associated with an upper level jet streak affected the southern 
mountains during the afternoon hours of 2 January.  This feature 
was poorly handled by both the Eta model and the forecasters at 
GSP.  Lower to middle tropospheric isentropic upglide developed 
over the eastern part of the CWFA during the late afternoon hours 
of 2 January.  This was fairly well handled by the Eta model and 
forecasters.  Snow did develop farther west then forecast during 
the late evening of 2 January.  This was the result of a weak 
deformation zone in the nose of another 300 mb jet streak 
ejecting out of the trough, coupled with the western most edge 
of the strong low level upglide.  Finally, light some persisted 
most of the day on 3 January as a strong 500 mb upper low crossed 
the southern CWFA.  This low eventually lead to another bout of 
heavy snow over eastern North Carolina late on 3 January.
While the event was handled well on the whole, snow consistently 
developed farther west then expected.  Mechanisms responsible for 
this were upper level forcing caused by a series of jet streaks 
that crossed the region in southwest flow ahead of a strong 
500 mb trough.  The Eta model had a poor handle on the ascent 
caused by these features almost up until the time they developed. 
In the future, it may benefit the office to issue watches and 
warnings father west than what low level upglide would suggest, 
perhaps as far west as the core of the 300 mb jet. 
Here are a series of Javascript loops from the 0000 UTC and 
1200 UTC runs of the Eta model from the 2 January 2002 model cycle.
Moyer, Benjamin W., 2001: A climatological analysis of winter 
     precipitation events at Greenville-Spartanburg, SC.  Eastern 
     Region Technical Attachment 2001-01. 19 pp.
Patrick Moore reconfigured the page with the GSP web page template.

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