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.