A Review of the 17 April 2001
Northwest Flow Snow Event
Bryan P. McAvoy
NOAA/National Weather Service
Author's Note: The following report has not been subjected to the scientific peer review process.
This was an interesting event in that while it brought snow to the usual locations of the mountains, it also resulted in brief snow showers across parts of the North Carolina Foothills, Piedmont, and the Upstate of South Carolina. Many of these locations saw snow while the temperature was in the upper 40s. A few areas in the mountains reported between 2 and 4 inches of snow. This text statement details the snow reports. Satellite data taken the next day revealed that amounts in eastern Yancey and Mitchell counties may have been higher than reported. However, no follow-up calls were made to these locations.
Using a locally created color curve, locations where snow was falling were well correlated with areas with cloud top temperatures at or below 14 degrees Centigrade. Click here for a java-script loop of cloud top temperatures during the morning and afternoon of 17 April 2001. This was the time during which snow fell outside the mountains, and most of the accumulation in the mountains occurred. Surface observations are also plotted on the map. Notice how much the areas in the lee of the mountains warmed up before the snow showers developed.
To orient yourself a little better, we also have a loop of visible satellite imagery valid for the same time. Waves of clouds developed during the day and areas of convection developed downstream of the mountains, possibly the result of mesoscale areas of convergence in the lee of the mountains acting on a rather unstable low level airmass.
As stated above, there was some potential instability ahead of an approaching shortwave trough. Eta model forecast soundings valit at 1200 UTC on 17 April (Fig. 1) show some of the steep low level lapse rates and small areas of Convective Available Potential Energy found that day over the mountains.
Figure 1. Eta model forecast soundings for Asheville, North Carolina,
valid at 1200 UTC on 17 April 2001: 12-hour forecast from the 0000 UTC
17 April run (left) and initial hour profile from the 1200 UTC 17 April
run (right). Click on images to enlarge.
Eta model forecast soundings at Asheville, North Carolina, for 1500 UTC (0000 UTC run, 1200 UTC run), 1800 UTC (0000 UTC run, 1200 UTC run) and 2100 UTC (0000 UTC run, 1200 UTC run) on the 17 April continued to have an unstable layer, though by 2100 UTC mid level winds were forecast to veer sharply to the north northwest, and a strengthening subsidence inversion was projected to develop behind the departing short wave trough. This verified quite well, though other weaker troughs, and persistent northwest flow, would keep light precipitation going for several more hours.