|Forecasting Precipitation Type|
During Cold Air Damming Episodes
In The Piedmont of North Carolina
Residents in the Piedmont of North Carolina don’t experience a great deal of wintry weather. Typically, the moisture
associated with winter systems that move in from west to east is rung out over the Appalachians, leading
to smaller precipitation totals in the Piedmont. There is one scenario that can usher in snow, sleet, and
freezing rain into the greater Charlotte area, a Cold Air Damning event. When these events hit the
region, forecasting them is difficult as the precipitation type can vary considerably and ice
storms are possible. You’ll often hear your local television or radio meteorologist refer
to this as “a wintry mix.”
Cold Air Damming occurs when a low-level cold air mass becomes trapped because of topography, typically on
the east side of a mountain range. The trapped cold air affects the overall weather pattern of the area
producing widespread cloud cover, colder temperatures, and various types of freezing precipitation. In
the Piedmont of North Carolina this happens when a ridge of strong surface high pressure in the Northeast
sends cold air southward (cold air advection) down the east coast. As the air is forced upward by the
higher terrain, adiabatic cooling takes place. Bell and Bosart found that up to 30% of cooling
occurs through this process. If precipitation is underway, the latent heat loss caused by evaporation
of rain or snow or the melting of frozen precipitation leads to diabatic cooling. This process is
believed to cause nearly the same percentage as adiabatic cooling according to Bell and Bosart. Each
cold air damming event is different, some are cold air advection dominant while others are controlled
more by diabatic cooling.
In order for cold air damming to occur, a low level anticyclone is required to force upslope flow. Also needed
is terrain blocking for adiabatic cooling and an inversion along the top of a cold air pool. Terrain blocking
enhances cold air advection because the flow becomes more vertical as winds turn across isobars toward low
pressure situated to the west. It may also lead to a greater inversion and the formation of a barrier jet, which
is a jet on the windward side of the mountain. The barrier jet may also lead to an increase
in surface wind speeds.
Cold air damming is a year round occurrence in the Appalachian Mountains. A maximum number of these events
take place in the winter months, while few events take place in the summer. Over the last fifty years, the
peak of these events has been in December and March. The winter events also last longer than the summer
events, often leading to two-day ice storms.
Numerical models are often of little help to meteorologists and forecasters in cold air damming situations
because most of the important features are happening in the lowest half-mile of the atmosphere. Model
ability to handle the depth and strength of cold air at these levels and at the surface is difficult. Terrain
effects are also hard for models to handle. Use of some mesoscale models like COAMPS may be better at
forecasting the low-level features. A small change in lapse rate whether at the surface or in the middle
levels of the atmosphere make huge differences in precipitation that a forecast area will experience
when temperatures are hovering near freezing. Understanding the temperature profiles at various
levels will help in determining whether snow, sleet, freezing rain or rain is going to occur.
For snow to occur, temperatures below freezing need to occur throughout the atmosphere. Since the snow does
not pass through any warm layers on its way down, it reaches the ground as snow. Sleet on the other
hand, starts as snow at higher altitudes but falls into a shallow warm layer typically about one
mile above the surface where melting takes place. It then falls into another cold layer towards
the surface and the water droplets begin to refreeze.
Freezing rain is considered the most dangerous form of precipitation in these storms as it can lead to
significant icing. Residents of Charlotte and the central Piedmont found this out in December of 2002
when a cold air damming event led to the largest ice storm to ever hit North Carolina. More than three
quarters of an inch of ice accumulated on trees and power lines. Over 1.2 million Duke Energy customers
were left without power, many for more than two weeks.
Freezing rain begins as falling snow. The snow falls into a warm layer deep enough that total melting occurs
and the snow becomes rain. Now falling as rain the precipitation passes through a thin cold layer, above the
surface and cools below freezing. Supercooling takes place and the drops themselves don’t freeze until
they hit the frozen ground, power lines, or tree limbs and branches.
WCCB-TV Charlotte Chief Meteorologist Reg Taylor has forecasted cold air damming events in the Carolinas
for more than ten years. “It’s a forecaster’s worst nightmare. You really have to fly by the seat of
your pants because the models are very little help.” Taylor says experience is the key to accurately
predicting these types of winter events. “Years of experience with this kind of weather, seeing it
and knowing what to expect is the best teacher. It’s really impossible to say when forecasting, in
Charlotte it will be freezing rain, in Gastonia you’ll have sleet, and in Rock Hill you’ll have
cold rain. There are two many variables even twelve hours before the precipitation begins.” Taylor
says when the event is taking place in real-time it’s important for the forecaster to explain why
this is such a difficult forecast and the small differences that lead to the
varying precipitation types.
Forecasting and end to cold air damming events is often just as difficult as forecasting the event when it
began. There are several things to look for as a damming event weakens. Warm air advection at the
surface, including a shift to a southerly wind flow will weaken a damming situation. Surface pressures
falling in the cold air or a decrease in cold air depth are also signs that
the end of the event is near.
In order to give the best forecast possible in a cold air damming situation, model biases should be learned
an incorporated during the forecast process. Shallow cold air is not handled well by numerical models, often
leading to a forecast that does not support a large draw of cold air when in fact there is sufficient cold
air advection taking place. Most importantly, learn from past events and journal information you learn
or patterns you see in previous cold air damming events.
“Cold Air Damming: Uses, Example, and Forecast Parameters.” University Center
for Atmospheric Research.
“Appalachian Cold-air Damming.” Bell, G.D., and Bosart, L.F. 1988
Mon. Wea. Rev., 116, 137-161.
“Cold Air Damming Forecast Tips”
NWS Raleigh, NC & North Carolina State University
“Worst Ice Storm In Years Devastates The Carolinas.”
Associated Press, 12/16/2002
Haby, Jeff. The Weather Prediction