Thunderstorms, some severe, with very heavy rain have moved into the Mid South and Tennessee Valley today. Flash flooding and severe thunderstorms will be possible from the central Gulf Coast to the Upper Great Lakes. The greatest threat of excessive rains will be in portions of the Lower and Middle Mississippi Valleys. This storm system has a history of producing life threatening flash floods. Read More >
The 19 January 2002 Ice Storm Failure over the
Northern Foothills and Mountains of North Carolina
Bryan P. McAvoy
Author's Note: The following report has not been subjected to the scientific peer review process.
An in situ wedge ridge, with a temperature contrast that would eventually exceed 40 degrees F across the thermal-moisture boundary, developed over the western Carolinas during the morning hours of 19 January 2002. Appying a "barrier jet" conceptual model, and based on ambiguous temperature data, a Winter Storm Warning was issued in the mid-morning hours on 19 January for a part of the northern Mountains and northern Foothills of North Carolina. The Eta model did a good job forecasting the surface pressures and temperatures, though it exhibited a cold bias the day before, something seen in other events in the winter of 2001-2002. The primary problem in forecasting the event was dealing with conflicting temperatures reported in the North Carolina Foothills and Piedmont by automated sensors.
2. Synoptic Evolution and Model Forecast
The operational version of the 12 km Eta had been rather warm up until the run initialized at 1200 UTC on 18 January. This run was dramatically colder than previous runs, around 4 degrees C in the lowest 2 km based on a comparison of model soundings (This was observed at the time). The next two runs of the Eta had more consistant temperatures, though with a slight warming trend.
However, the 1200 UTC run of the AVN model on 18 January came in warmer, clearly indicative of a rain event. Later runs were similarly warm. The 0900 UTC Short Range Ensemble Forecast (SREF) run from 18 January, valid at 1500 UTC on 19 January, showed a consensus 0 degree C 2-meter temperature isotherm over northern North Carolina. This turned out to be a good verification for the 0 degree C line for this event. The 2100 UTC SREF run from 18 January, valid at 1500 UTC on 19 January (Fig. 1), was a little farther north with the 0 degree C 2 m temperature. However, as you can see from the 1400 UTC and 1600 UTC maps below (Fig. 2), this was still not too far off. The blue lines on the map above are the Eta members while the black lines are the AVN members. (NOTE: There is a SREF forecast run that also includes the operational runs. Images from that version were not saved. These images are simply from the 10 "perturbed" members of the SREF.)
Figure 1. SREF 2 meter surface forecast 0 degree C isotherm from the 2100 UTC 18 January 2002 model cycle valid at 1500 UTC on 19 January.
Figure 2. Surface observation plot and analysis of sea level pressure and isotherms on 19 January 2002 at 1200 UTC (upper left), 1400 UTC (upper right), 1600 UTC (lower left), and 1800 UTC (lower right). Click on images to enlarge.
Another concern was the translation of a synoptic scale frontal boundary across the region. This was seen by the rapid eastward push of the reservoir of cold air in the western Carolinas to the east during the afternoon hours (after the period of the 1800 UTC map in Figure 2 below). This process is the classic decaying stage of a wedge ridge. In this case, the wedge ridge eroded only a few hours after it developed. This can also be seen as the wind turned to the west at Flat Top Mountain, a reporting station at 4000 feet, east of Asheville, North Carolina. Typically, this station stays at the top of the barrier jet cold dome until the end of a damming event. The disruption of the wedge, and some downslope warming at the time of heaviest precipitation certainly did not help in what was already a very close forecast. This warming can be seen when the 1600 UTC and 1800 UTC maps are compared in Figure 2. Note the warming, as much as 3 degrees F, in the the immediate lee of the Appalachians. However, farther to the east, 2 degrees of cooling occurred. The eastward translation of the barrier jet could also be seen in a time section from the Charlotte, North Carolina, wind profiler. This was observed in real time by the forecasters. It was not recognized until after the event, however; and we do not have the archive data from the Charlotte profiler.
3. Forecast Concerns - Automated Sensor Headaches
A Winter Storm Warning was issued by the day shift on the morning of 19 January. Prior to this, the midnight shift had issued winter weather advisories for the northwest Piedmont and northern Foothills of North Carolina, and the northern Mountains of North Carolina north of Asheville. There were no significant damage reports associated with icing that day, which is the criteria used by the National Weather Service at the Greenville - Spartanburg office to verify ice storm warnings. The warning was issued ahead of a large band of precipitation that was moving out of Tennessee, the result of strong frontal forcing. There was also a strong contribution due to low level warm advection. In the end, while there was some icing, there was no significant damage, nor great inconvenience. The exception was in Avery County, North Carolina, where there was a rather significant glaze of ice, but no known damage.
The decision to warn was based upon the existence of an in situ wedge and real-time observations. The forecaster believed that a barrier jet would develop in response to the in situ wedge. It was believed this would maintain temperatures just below freezing in a narrow part of the North Carolina Foothills, just east of the higher elevations of the Appalachians. A report of 0.5 inch of ice accumulation in the South Mountains, 14 miles southwest of Morganton, North Carolina, lends a great deal of credence to the idea that a barrier jet did form, as there was certainly a decrease in temperature aloft which had to be maintained by some process in the face of strong warm advection. However, surface temperatures were simply a few degrees too warm for there to be much icing.
One problem was determining exactly what the surface temperature and dewpoint was upstream, and hence the resultant wet bulb temperatures. There were differences in the ASOS and AWOS readings. Whether these differences were the result of actual topographic effects, as well as their location relative to the heart of the barrier jet, is difficult to discern. In particular, note the difference between the Hickory ASOS and the Morganton AWOS on the 1200 UTC map in the upper left panel of Figure 2 (the Morganton observations had to be penned-in as they were too close to Hickory display legibly on the printout). It appears that the Hickory dewpoint was too low, and that the Morganton temperature was too high. We placed more faith in the ASOS than the AWOS. Perhaps if the Hickory wet bulb had been correct, the decision to warn for the three foothill counties would not have been made.
It is interesting to note that a few icy spots were reported as late as early afternoon in Lincoln County, North Carolina. This was south of where the advisories were issued, but does make sense if the cold nose at Hickory is extrapolated southward.