The 6-7 January 2002 Snow and Ice Storm
over the Western Carolinas and Northeast Georgia
Bryan P. McAvoy
NOAA/National Weather Service
Author's Note: The following report has not been subjected to the scientific peer review process.
While half the warned counties in the National Weather Service
(NWS) Weather Forecast Office (WFO) Greenville-Spartanburg (GSP)
county warning and forecast area (CWFA) verified in this event,
a sizable part of the warned area did not receive the amount
of winter precipitation, or even the precipitation type, that
was expected. This included much of the South Carolina
Mountains and the southern and central North Carolina Mountains.
It was also a missed forecast as significant icing did not
develop in the Piedmont and parts of the Foothills of North
Carolina. Areas which verified were the higher elevations of
the North Carolina Mountains, including the Tennessee border
counties. These counties primarily verified with a northwest
flow snow event, as was expected. The northern North Carolina
Foothills verified, mainly with sleet, and parts of the Georgia
Mountains verified, receiving significant snow early in the
event, before warm advection changed the precipitation over
to rain (Fig. 1). The Upstate of South Carolina and the
southern North Carolina Piedmont were not included in watches
or warnings for this event.
Figure 1. Map of snow, sleet and ice accumulations from the
6-7 January 2002 winter storm.
2. Synoptic Overview
This was not an easy forecast by any stretch, with offices all
along the East Coast verifying poorly on their watches and
warnings. Principally this was caused by a significant
Quantitative Precipitation Forecast (QPF) error by the Eta
model for points farther north. The track of the heaviest
QPF was expected much farther east than what verified,
illustrative of the more westward track of the storm and hence
warmer verification. Some offices farther north saw snowfall
totals of up to one foot where only light snow had been
forecast just 12 to 24 hours before the event
The path of the low verified a little farther inland than the
models had projected, though the errors were fairly small over
the southern states. Precipitation rapidly translated across
the region, with moderate to heavy precipitation only lasting
3 to 4 hours at most locations. The 12 km Eta model exhibited
a cold bias on select runs and exhibited a considerable amount
of run-to-run variability. The AVN model was more consistent
than the Eta and if anything was a little warm. The Canadian
GEM model (both the global and regional versions), while perhaps
the most consistent model in forecasting the general evolution
of the event (it nailed the pattern days in advance), was also
the model with the most significant cold bias.
In the end, however, the primary contribution to the inaccurate
forecast may have been forecaster misjudgment. It was believed
that the Eta soundings were a little too warm as dynamic cooling
would subtract a degree or two Celsius. In reality, what
forecasters often call "dynamic cooling" is handled well by the
Eta and should not be compensated for without a good reason.
Also, the brief period of precipitation should have been
considered, though this may not have been as big an issue as
precipitation amounts were generally equal to or greater than
what was projected.
The following sections will discuss the various aspects of this
event touched on in the paragraphs above. Some will be little
more than a statement of purpose or intent at this time, while
others will have a more significant amount of data.
Before proceeding any further, it will be beneficial to first
view this reflectivity loop from the event.
a. Run to Run Variations in the Eta Model and Erroneous Forecaster
Across most of the central and southern North Carolina
Mountains, where 4 to 8 inches of snow was forecast, a light
glaze of freezing rain fell and temperatures warmed into the
upper 30s. Higher elevations had up to 2 inches of wet snow
before the changeover. The period of heaviest precipitation
verified around 1300 UTC on 6 January in the mountains. Bufkit
soundings for the Asheville, North Carolina, airport (KAVL)
from the Eta model valid at 1300 UTC 6 January for model cycles
starting at 0000 UTC 5 January and ending with the 1200 UTC
run on 6 January are shown in Figure 2. There were timing
differences in each run of the Eta. The slightly different
timing of the warm nose at KAVL in each model run was captured
by the Bufkit soundings at 1500 UTC (Fig. 3) and at 1700 UTC
(Fig. 4). The 0000 UTC run on 5 January was coldest Eta run
of the group. It was this run that the mid shift on 5 January
used to issue the first Winter Storm Watch for the event.
Figure 2. Eta model Bufkit soundings for KAVL, valid at
1300 UTC 6 January 2002. The soundings are taken from the
0000 UTC 5 January model cycle (upper left), 1200 UTC 5 January
cycle (upper right), 0000 UTC 6 January cycle (lower left), and
the 1200 UTC 6 January cycle (lower right). Click on each
image to enlarge.
Figure 3. As in Figure 2, except for 1500 UTC 6 January.
Click on each image to enlarge.
Figure 4. As in Figure 2, except for 1700 UTC 6 January.
Click on each image to enlarge.
The Air Resources Lab (ARL) has an online archive of both the
EDAS and GDAS data. We are still looking into whether or not
we can compare the ARL archive of EDAS and GDAS data since the
GDAS analysis sounding from Asheville at 1200 UTC was much
warmer than the EDAS analysis sounding from 1200 UTC. At
1200 UTC 6 January the Asheville airport was reporting freezing
rain and a temperature of 32 degrees F. A location a few miles
northeast of the airport, at an elevation of 4,320 feet
reported a temperature of 36 deg F at 1330 UTC. If we can
rely on the GDAS data, this would be a very interesting case
of the Eta model verifying too cold for the onset of the event.
However, until we learn how these soundings are mapped, it is
best to rely on the Bufkit soundings for point specific model
The Bufkit precipitation algorithm appears to run too warm, as
suggested by the Partial Thickness Universal Nomogram plots at
1700 UTC 6 January (Fig. 5). The larger red dots indicate the
most recent hourly thickness plot, thus it should be fairly
easy to extrapolate back through the four or five hours of
significant precipitation using the thickness images. Even
though the 0000 UTC run on 6 January was cooler than the
1200 UTC run on 5 January, the thicknesses were still leaning
a little toward a mix rather than all snow. Based on the cooler
GEM model, and a belief that "dynamic cooling" should help hold
the warm nose at bay during the critical 1200-1500 UTC time
frame, the cool side of the Eta solution was favored when the
mid shift made the decision to warn on 6 January. In fact,
the low tracked a little farther west, bringing the warm nose
across all of the mountains by midmorning, and verifying a
warmer solution during the most intense precipitation than
suggested by the 0000 UTC Eta run on 6 January.
Figure 5. Partial Thickness Universal Nomogram plots from Eta
model Bufkit soundings at KAVL, valid at 1700 UTC 6 January 2002.
The nomograms are taken from the 0000 UTC 5 January model cycle
(upper left), 1200 UTC 5 January cycle (upper right), 0000 UTC
6 January cycle (lower left), and the 1200 UTC 6 January cycle
(lower right). Click on each image to enlarge.
The 850 mb height and temperature fields from the last four
runs of the Eta and AVN are quite interesting (Figs. 6 and 7).
Between the 1200 UTC run on 5 January and the 0000 UTC run on
6 January, 850 mb temperatures cooled nearly 4 degrees C over
the North Carolina Mountains, and there were significant
temperature fluctuations in all four runs shown.
Figure 6. Eta model 850 mb geopotential height and temperature
(deg C) valid 1200 UTC 6 January 2002. Model run time is below
However, the temperature fluctuations in the AVN solution were
considerably smaller. Notice that the 0 deg C line stays nearly
over the Tennessee - North Carolina border for each of the last
three runs herein (those being 0000 UTC 5 January, 1200 UTC
5 January, and 0000 UTC 6 January, respectively). While the
last three runs, valid at 1200 UTC on 6 January, were very
consistent, they also appeared to verify reasonably well in the
mountains, though perhaps a little too warm. Unfortunately,
a more detailed analysis of the AVN is not possible at this
time due to lack of data. Still, these gross fields are still
very interesting. Is it possible that the 12 km Eta is subject
to more run-to-run variability than pervious versions of the
model? An event which happened less than a week before also
exhibited run-to-run differences, though those differences
were not as important since that event was all snow.
Figure 7. As in Figure 6, except for the AVN model.
In retrospect, the situation was handled the best by the
1200 UTC Eta model run from 5 January. The previous runs of
the Eta had featured a stronger, slower, and more cutoff 500 mb
short wave than the AVN or GEM models. This resulted in a
low farther to the south and a colder airmass. The 1200 UTC
run on 5 January came more into line with the other two models
in showing a more rapidly translating wave. Compare the Eta
model's surface low position at 1200 UTC 6 January to the
analyzed position to see that the Eta was pretty much right
on the money with this run. However, the day shift on
5 January strongly considered the colder Canadian model and
the previous, colder run of the Eta, when the decision was made
to upgrade the warnings in the mountains.
As we were unsure of how the Eta model handled cooling due
to strong upward vertical velocities, there was a tendency
throughout the forecast to assess a small amount of cooling
which it was assumed the model would not generate. The model
should take into account cooling by expansion as a stable
layer is lifted (Dr. G. Lackmann, NC State University,
personal communication). The only process which could result
in unmodeled cooling would be creation of a stable layer by
melting of snow. This layer would be more subject to cooling
than the Eta would project due to shallower lapse rates.
Considering that strong low level warm advection should
completely overwhelm cooling from melting, it is best not to
assess any kind of "dynamic cooling" fudging to the Eta data
unless it could be proven that vertical velocities were in
fact stronger than predicted.
Additionally, it would have been helpful if a rudimentary set
of data from other events which affected the CWFA in the past,
exhibiting similar characteristics to this one, was available.
In particular, a look at the "surprise" snow event in
Asheville in January 1998 would have been of benefit. It
appears that the Eta actually did a good job with this event,
though we have only anecdotal evidence of this.
b. Freezing rain verification in the Piedmont and Foothills
There has been talk about the Eta 2-meter temperatures being
too cold, and warming of the boundary layer by "warm" clouds.
Both of these effects were slightly in evidence that morning.
However, the differences were not great as can be seen in
A more basic mistake was assuming that a weak in-situ wedge
would provide enough support to overcome latent heat released
by freezing rain. It was not. In fact, later in-situ wedge
events which occurred on 19 January 2002 and 6 February 2002,
which had higher surface pressures, also failed to result in
damaging ice accumulations. While ice did form on trees at
least as far south as Greer, South Carolina, there was only one
pocket of ice accumulation that even might have been close to
winter storm criteria, in Catawba County, North Carolina. It
is simply very difficult for in-situ events to generate the
necessary cold advection to overcome the latent heat released
by freezing rain. Coupled with the Eta's tendency to run 1 to
1.5 degrees too cool with surface temperatures this year, this
is something that forecasters need to take into account when
forecasting damaging ice accumulations.
Table 1. Temperatures at Hickory, NC vs Eta 2 meter
temperatures for 6 January 2002
||Observed surface temperature
||Eta 2 meter temperature
c. Other models - the Canadian GEM and SEF
It is with some trepidation that I supply these maps from the
0000 UTC run on 6 January from the GEM (Fig. 8). Since we do
not have any archived data from the 0000 UTC or 1200 UTC runs
on 5 January we can only speculate based upon what we remember
the GEM doing for those runs. However, while we have a great
deal of faith in the skill of the GEM at forecasting the overall
evolution of synoptic scale patterns (it proved to be right
when the Eta was wrong in a couple of the more significant
winter weather busts in the GSP CWFA over the past two years),
it does exhibit a cold bias most of the time. Below are the
1200 UTC and 1800 UTC surface fields from the 0000 UTC 6 January
run of the Canadian GEM. Compare how much farther south the
surface low is to that of the verified features. This southward
bias is what resulted in a colder airmass. We need to
investigate whether or not the cold bias might simply be the
GEM's developing systems a little too far south at our latitude.
Figure 8. GEM model 12-hour forecast of sea level pressure isobars,
1000-500 mb thickness, and 12-hour precipitation valid at 1200 UTC
(upper left) and 1800 UTC (upper right) 6 January 2002. Surface
observations plot with sea level pressure and fronts analysis at
1200 UTC (lower left) and 1800 UTC (lower right) 6 January 2002.
Click on each image to enlarge.
d. The Short Range Ensemble Forecast (SREF)
Could the Eta ensemble have been a useful tool? We looked at
the ensembles, but what they provided was inconclusive. For
borderline events where we need to understand in great detail
how the model is handling the vertical structure of the
atmosphere, the SREF ensembles are essentially useless as the
data provided is of too coarse a resolution to be of any good.
In addition the SREF is a set of models run with a far coarser
resolution than the operational Eta and AVN, and with a
different physics package. This seems more like comparing
apples to oranges rather than an apple to lots of apples. The
ensembles are a useful, if not necessary tool, which could
greatly help reduce forecast busts, but only if we are provided
with real-time, high resolution data through AWIPS.
e. Rapid translation/coupled jet
Most of the CWFA was free of significant precipitation by
1700 UTC on 6 January. Contrast this with a time section of
Eta precipitation at Asheville from the 0000 UTC run on
6 January. A weak deformation zone turned out to be
nonexistent as the upper forcing associated with the storm
raced to the north (as seen by the 9 mb/3 hr pressure falls
on the surface maps below).
Finally, this was a rapidly moving system. Precipitation took
a while to reach the surface. When all was said and done, the
duration of precipitation was only 3 to 4 hours, all of it
associated with lower and mid-tropospheric isentropic upglide.
There was no backward wrapping deformation zone precipitation
to speak of. Again, reference the loop of radar data for the
event to see how short a period of time it precipitated over
the area. In our defense, the color curve used here is a
little "cooler" that the one we employ in the office.
The fact that the system tracked farther inland than expected
may also have had to do with the impressive coupled upper jet
associated with it. There was tremendous upper divergence and
attendant upward vertical velocities, associated with this
feature. Perhaps the low level low developed farther into the
cold air as a result of this forcing. Obviously the track
errors, which were fairly small over the southeast, became
much larger as the low deepened and moved north.
3. Other work to be done
One thing that would be of some benefit would be to take the
0000 UTC soundings from 6 January and compare the observed
thickness to the Eta initialization. Because the 0000 UTC
6 January run came in much colder than the previous run, it
would be interesting to see if other upper air soundings from
around the region (GSO, RNK, FFC, BNA and CHS) were appreciably
different from the Eta.
4. Summary and conclusion
The 6-7 January 2002 winter storm was a good learning experience
for the staff at WFO GSP. Taken at face value, the Eta model
would have supported a forecast of mixed precipitation over the
mountains of the western Carolinas and northeast Georgia, and
perhaps some warning criteria icing over the North Carolina
However, forecasters erroneously applied a small amount of
"dynamic cooling" to Eta model soundings, allowing them to
issue watches and warnings for a considerably larger area. In
reality, the Eta already takes such cooling into effect, at least
in situations dominated by strong low level warm advection.
Forecasters relied on the Canadian SEF and GEM models during
the storm as well. While the GEM does a good job in forecasting
the evolution of an event, both the GEM and SEF frequently have
a large cold bias over the region, making it tricky to determine
the proper precipitation type.
The Eta exhibited rather significant temperature fluctuations
in the runs leading up to the event, more so than the AVN or
Canadian models. A short range ensemble forecast may be
helpful in these cases, though the data currently available
on the Internet is difficult to use and takes time away from
the rest of the already busy forecast process. Having this
data in AWIPS may prove useful.
Finally, damaging accumulations of ice were forecast over much
of the northwest Piedmont and the Foothills of North Carolina.
While a mix of sleet, snow and freezing rain verified the
Foothills, the Piedmont saw only light accumulations of ice.
It is difficult for in-situ damming events to generate damaging
ice accumulations. And, as the Eta runs a little cold, it may
be in the best interest of the forecasters at GSP to hold off
in issuing ice storm warning on events that appear "borderline"
per the Eta low level temperature fields.
The images in Figures 6 and 7 were obtained from the NWS State
College (PA) Lagged Average Forecast Page. NWS GSP Senior
Forecaster Harry Gerapetritis provided the temperature data
for Table 1. Patrick Moore converted the web page to the
standard NWS template.