The piedmont trough is a relatively new discovery in the eastern part of North Carolina. The key effects
of a piedmont trough are soil type and diurnal heating. The North Carolina Sandhills region of the Southeast
is an elongated area of sandy soil that is located adjacent to regions of varying soil type. The Piedmont
area in the eastern part of the state has soils that are mainly loam and clay-loam, while a variety of
soil types exists in the Coastal Plain to the southeast. It is the differences in these soil types
that create differential heating of the surface. Areas with sandy soil will more easily release
their moisture to the area, allowing for a higher sensible heat flux than loam or clay-loam soils. This
allows more energy to be used for heating the surface rather than evaporating moisture in
the soil. Also, the heat capacity for sand is much less than that of clay or loam, so that
given the same amount of energy, sandy soil would increase in temperature more than the
loam or clay soils.
This hot air rises, while the cooler air over nearby regions doesn't, causing a small area of circulation
to develop. If there is a southerly wind carrying moisture from the Gulf of Mexico, as is often the
case in the Southeast in summer, rain clouds can form or intensify in this circulation. Having moist,
densely vegetated regions on both sides of the Sandhills, together with the presence of the sea
breeze which reaches as far inland as the Sandhills in the Carolinas, makes the process all the
more potent by creating circulation on both sides.
Urbanization also plays a role. As the Sandhills' native vegetation is cleared to make way for new
development, its soil can dry out even more quickly, increasing the heat differential between it and
the moister soils in adjacent regions.
These differential surface heating patterns is what creates the "Piedmont Trough". This trough acts as
a focusing mechanism for thunderstorms in the summer. Much like a sea breeze influences the location
of summer thunderstorms in Florida, the piedmont trough impacts the weather in North Carolina.
The trough can act as a lifting mechanism and can help develop convective thunderstorms. The piedmont trough
is often a cause for strengthening storms in the area east of the piedmont. The trough is on a
smaller, mesoscale level and is not always detected by weather models that look for large scale
features. This being the case, this trough is often overlooked and developing thunderstorms in
this area are generally missed. The trough is also responsible for strengthening weaker systems
and can also revitalize convective systems that pass through the area.
The piedmont trough is a common occurrence in the eastern part of North Carolina during the summer
months, mainly from June through August, with the most occurrences happening in July. It is
responsible for enhancing severe weather and also generating pop-up thunderstorms. Being that
this trough is on a smaller scale it is not easily detected. This creates a problem when forecasting
the weather for this region. If this feature is missed the forecast will be significantly affected. This
can result in underestimation of severity and also an underestimation in precipitation amounts. With
its mild winters and fast-draining soil, the region is home to many golf courses, horse
farms, retirement communities and resorts whose activities hinge upon the weather forecast.
Until recently this particular forecast problem occurred on a daily basis during the summer months. With the
discovery of this occurrence there are methods being developed to try better forecast for this weather
event. The solution to this problem is to utilize many forecasting tools together to identify the
location and intensity of this trough. When combining the WSR-88D radar, the GOES satellite, and
surface data, the trough was located 95% of the time. The role that
each played is described below:
1) The WSR-88D radar is capable of detecting boundaries even in clear air to a limited range; however, displays
from several regional radar sites are necessary to define convergence boundaries in the PBL beyond
approximately 50 n mi from the local radar.
2) The GOES satellite is useful for the detection of boundaries well beyond the range of the radar, but
an unobscured cloud line must be present in order for it to be of use; in addition, linear cloud features
are not always associated with low-level boundaries (over detection problem).
3) Conventional surface data is a relatively weak boundary detection component (under detection problem), yet
it usually allowed the nature of the features observed by radar and satellite to be clearly identified; surface
data can be used in many instances to detect subtle temperature, moisture, and/or stability
discontinuities that define weak, shallow fronts.
In conclusion, with the discovery of the Piedmont trough this forecasting problem can now be assessed. With
the combination of several forecasting tools this feature can be identified, located, and properly assessed
to give a forecaster a more in depth understanding of what processes in the piedmont region that is driving
the summertime severe weather.
Location and orientation of Piedmont troughs
July Averaged Monthly Precipitation around the Sandhills Region over the 40 years (1960-1999). The Sandhills
are outlined in red.
Soil surface texture in the Carolinas and Georgia
State Climate Office of North Carolina
Businger, S., W. H. Bauman III, and G. F. Watson, 1991: The development of the Piedmont front and
associated severe weather on 13 March 1986. Mon. Wea. Rev., 119, 2224-2251
Steven E. Koch and Charles A. Ray: Mesoanalysis of Summertime Convergence Zones in Central and Eastern North Carolina
Weather and Forecasting Volume 12, Issue 1 (March 1997) pp. 56-77