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Local forecast by
"City, St"

The 19 January 2002 Ice Storm Failure over the

Northern Foothills and Mountains of North Carolina

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
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.)
SREF 0 deg C 2 m temperature isotherm from 2100 UTC 18 January model cycle valid at 1500 UTC on 19 January 2002
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.

Surface analysis at 1200 UTC on 19 January 2002Surface analysis at 1400 UTC on 19 January 2002
Surface analysis at 1600 UTC on 19 January 2002Surface analysis at 1800 UTC on 19 January 2002

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.

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