The 6-7 January 2002 Snow and Ice Storm
over the Western Carolinas and Northeast Georgia
Bryan P. McAvoy
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.
a. Run to Run Variations in the Eta Model and Erroneous Forecaster Presumptions.
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 sounding data.
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 each image.
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 Table 1.
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.
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 Foothills.
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 and Neil Dixon converted the web page to the standard NWS template.