|Cold Air Damming Along the East Coast
Cold Air Damming is synoptic phenomena which can be present throughout the world; anywhere a mountain
range is present. This weather pattern occurs when low level cold air is blocked by topography. Cold
air damming occurs within the United States in two places, East of the Rockies, and East of the Appalachian
Mountains. In this paper I will focus on the cold air damming which takes place east of the Appalachians, and
how it affects the nature of precipitation through a real-life forecasting example.
Cold air damming can occur anytime throughout the year, but the majority of the time is occurs is during
the winter. According to Bell and Bosart, the average cold air damming scenario will last approximately
four days during the winter. In the graphic below you can see there is a sharp peak of cold air damming
events which occur in December and March.
There are three types of cold air damming that occur. The first of the types is classic cold air damming; this
type will be discussed later. The second type is in-situ cold air damming, this is where little or no cold air
advection takes place, but diabatic cooling generates the cold air which is dammed east of the
Appalachians. The third type is hybrid cold air damming. Hybrid cold air damming is a combination
of both classic and in-situ cold air damming. In this paper I will focus on the classic cold air
There are three main contributing factors which lead to cold air damming. The first of which is cold air
advection. This is generally induced by a high pressure system which is located to the north of the affected
region. As the surface winds flow from the north and northeast the winds become trapped east of the
Appalachians. The second factor is adiabatic cooling. As the surface wind flow comes in contact
with the mountains is the air is forced up causing the air to cool further due to adiabatic cooling,
30% of cooling occurs due to adiabatic cooling. The third influence on the cold air is diabatic
cooling. This is caused by latent heat loss due to melting or evaporation of any precipitation
which is falling into the underlying cold dry air.
The classic cold air damming scenario is the easiest form of cold air damming to spot and forecast, but be aware
it does have its challenges. Classic cold air damming is setup when a surface high is situated in the Northeast
United States; this usually follows the passing of a strong east coast cyclone. As the cold air is forced
south and southwest from the high the air is trapped in the low levels by the mountain, this is the
beginning of a classic cold air damming situation. This situation is easily seen on the surface
charts by the dipping of the isobars between the Appalachians and the East Coast. As the air
hits the mountains it is force to rise and cool, therefore reinforcing the cold air in place. If
a low pressure system is to the south, overrunning may occur which could lead to a strong stratiform
deck and possible precipitation. This is where the challenge of forecasting
cold air damming comes in.
The graphic below is an example of a classic forecasting problem which faces forecasters
in the Northeast every winter.
In this classic example of cold air damming, the underlying cold air is well entrenched east off the Appalachian
Mountains and there is a weak-moderate low situated over the Southeast United States. As this situation sets
up overrunning from the low forms a stratiform cloud deck north, into the mid Atlantic region. As the
precipitation begins to fall the freezing level is at 2000 feet. Many forecasters in the Northeast
have seen this situation before. The models predict the onset of the cold air damming event but the
precipitation forecasts from around the area tend to favor rain with a change to snow throughout
the area. Here is where your experience comes in to play, do you forecast by what the models say,
or do you use your experience and understanding of the area with the model data as a guide to
tailor your forecast?. The models predict that the cold flow from the north would continue for
the next few days, but you are seeing the models lag behind in the forecasted strength of the
high. Also the low to the south intensified faster than the models have predicted. This sets
up a situation where if everything would have stayed true to the initial model rain would fall
into a dry atmosphere, and through diabatic cooling, the freezing level would have descended
to the surface and snow would have occurred, but this is not the case. As the rain would
start to fall, the evaporational cooling near the surface would drop temperatures below
the freezing point, but at 3000 feet warm air would intruded from the south. This would
be a dangerous setting for a large scale freezing rain event. Instead of the snow blanketing
the area overnight the Mid-Atlantic region would wake up to a layer of ice. Power and
telephone lines would be down, there would be numerous vehicle accidents, and flights
at the airport would be cancelled and delayed.
This situation is similar to what occurred on 13-14 February 2007 in the Baltimore area. For this storm most
forecasters were playing catch up as the storm progressed instead of being aware of the situation and seen
the signs that the pattern was changing. Had the forecasters been on top of the pattern change they would
have been able to inform the public in a timely manner and possibly lessen the blow
this storm had on the region.
This was a very hard storm to forecast, but we can learn from this situation. Stay ahead of your forecast. Donít
be afraid to amend your forecast when the situation change, learn and live by model biases. Models are a
wonderful invention but they do have their problems. To make a good forecaster you need to be aware
of past weather events and how the synoptic situation played out in your area. At least once every
year cold air damming in the Northeast tends to blow a forecast. Hopefully by learning from past
mistakes and keeping ahead of the situation, you wonít be in this
situation with a busted forecast.
Bell, G.D., and L. Bosart, 1988: Appalachian cold-air damming. Mon. Wea. Rev., 116, 137-161.
Hartfield, G., 1999: Cold air damming/coastal fronts.
CSTAR Project, 2000: Improving Forecasts of Topographically-Forced Weather Systems in the Carolinas
and Virginia. Cooperative effort involving North Carolina State Univ., and WFOs at Raleigh and
Morehead City, NC, Wakefield, VA, and Greer, SC.
COMET/ University Corporation for Atmospheric, 2001: Cold Air Damming.