|Cold Air Damming:|
Setup, Forecast Methods/Challenges for the Eastern US
As one moves south along the Appalachian spine, through the southeastern US, the general thought is that
the climate is getting warmer and during the winter time, warmer temperatures would make it much harder
to see frozen precipitation. While this may be true with an ordinary synoptic scale setup, on occasion
a mesoscale phenomena occurs which totally alters one’s general thinking. This phenomena is known
as cold air damming (CAD), aka “the wedge” and most often occurs on the east side of the Appalachian
Mountain chain and the east side of the Colorado Rockies. When CAD establishes itself in these
areas, a forecaster can be faced with quite a challenge, one that can wreak havoc on forecasting
high temperatures and precipitation types. The following paper discusses the various setups of
cold air damming, the challenges these setups present to forecasters, and what can be done to
better predict the conditions experienced during cold air damming situations, specifically during
winter-time setups. CAD setups east of the Rockies and east of the Appalachians are very
similar with some differences; however, in this paper emphasis will be
placed on Appalachian setups.
Cold Air Damming occurs when a low-level cold air mass is topographically trapped on the east side of a
mountain range. An area of high pressure at the surface will be positioned to the north of the region
and the clockwise flow around the high means the low-level winds coming into the region will have an
easterly component. These easterly winds will push up against the east side of the mountain range. Air
moving up the mountain range ends up cooling at a greater rate than the dry-adiabatic lapse rate
and this causes the upslope flow to slow down allowing the cold air to build up on the eastern
slopes. When looking at a surface chart, a CAD setup will often be noted by a distinct “wedging” pattern
of cool temperatures trying to push their way down the mountain range spine which can clearly be
seen on the East Coast in Fig. 1 below.
Surface charts best depict the wedging pattern because warm air will overrun the cold air and warm the upper
levels to temperatures above what they are at the surface. This can often lead to an upper level
temperature chart looking more like a normal surface temperature chart. If an area of low pressure
develops to the south of the region, a blocking situation can occur which causes winds at
the low levels to run parallel to the mountains. This process is known as a barrier jet
and it can affect the weather by displacing precipitation maxima, enhancing low-level cloudiness, reducing
surface temperatures, and increasing precipitation coverage/intensity. If upper level forcing
is leading to upward motion, precipitation can form and the characteristics of the cold layer
at the surface will ultimately determine what form of precipitation will fall to the ground. For
the east facing slopes of the Appalachian chain, CAD events most often occur during the
winter months, particularly in December and July while a lesser amount of events will occur
during the summer months with a minimum in July. Winter CAD events typically last longer
than summer CAD events and are usually much stronger than summer CAD events.
There are 3 CAD types which most often affect the east facing slopes of the Appalachians. These
are: Classic, In-Situ, and Hybrid. It is important to look at surface charts, 850mb charts, and 500mb
charts when trying to determine the specific CAD type for the area. Below, each type of CAD is
explained using sample images from the surface, 850mb, and 500mb.
A Classic CAD event will occur when surface winds east of the mountain range maintain a northeasterly component
rather than easterly as the surface high to the north moves east. The terrain blocking which is occurring
is preventing an upslope flow from taking place and while this flow is trying to establish itself, the
air starts cooling adiabatically and stratification in the low levels occurs which ultimately hinders
the wind shift. An area of low pressure can develop to the west of the mountains and this can enhance
the pressure gradient which helps increase cold air advection and strengthen the damming. An
onshore flow will also begin to take shape which will allow warm air to rise up over cold air. This
overrunning process can lead to significant amounts of precipitation falling through the cold
air at the surface. The surface chart below depicts this setup (see Fig. 2).
For a classic setup at the 850mb level (see Fig. 3), we would not find a high over New England, while extensive
ridging would take place across the eastern US. This proves there is a shallow layer of cold air at the
surface while the southerly flow aloft is creating the overrunning process.
For a classic setup at the 500mb level (see Fig. 4), often times a deep trough will develop across the
central/southern plains which teleconnects to a ridge over the Great Lakes and Mid-Atlantic region. This
pattern progresses at a slow rate allowing the cold air at the surface to become better established along
with a northeasterly flow east of the Appalachians and a southerly flow aloft providing
the overrunning of warm air.
An In-Situ CAD setup (see Fig. 5) is one in which little or no cold air advection is occurring because
the high pressure center which we relied on in the classic setup is too far to the south. Cold air is
established through evaporational cooling caused by a pool of dry air east of the Appalachians. Surface
winds still tend to be from the northeast along with some mountain-parallel ridging but no cold air
advection is occurring from a high to the north. 850mb and 500mb patterns will be very similar to
those of a classic setup.
A Hybrid CAD setup (see Fig. 6) is one in which cold air advection and melting/evaporation work together
to establish the cold layer at the surface. With this setup, we can find an area of high pressure at
the surface to the north, say over New York State, but it is weak and moving fairly quickly with a
north to south orientation. This does not bode well for an easterly flow and so very limited cold
air advection takes place. Through a synoptic scale forcing mechanism, precipitation can form over
the area experiencing the small amount of cold air advection, and this helps add more cooling to the
layer of cold air at the surface.
When attempting to forecast conditions during a CAD event, often times the grid spacing from models will
be too coarse to show certain features. Don’t rely on the grid population. Models will typically
handle the start of CAD better than its erosion. They handle erosion via a cold frontal passage
and a coastal low better than they handle erosion via a surface low passing to the northwest and
a surface low moving southwest to northeast through the area. If the model has a surface low
passing to the northwest, often times it will bring the warm front too far northwest. If the
model is moving the surface low southwest to northeast, it will often try to drive the low
straight through the wedge which is a very rare occurrence. Other errors can include: MOS
data being premature in warming daytime CAD temperatures back to normal, model QPF being too
high in the cold dome during summer CAD, and the ETA increasing solar radiation too much so
that the CAD erosion occurs much quicker. The forecast is at stake so picking up on these
traits can help the forecaster make a more accurate forecast. Perhaps the easiest way to
do this is the monitoring of real time data. Perform hand drawn analysis on surface maps
and upper air maps. Check the evolving wind and temperature profiles through soundings. Look
at satellite and radar imagery to determine where moisture is headed and how surface and upper
level features are moving. Finally, check stability and determine where convergence or
divergence boundaries are taking place. Following these simple steps can go a
long way in improving the forecast for temperature and precip. type, especially
in winter weather situations.
Billet, J. (2006, November 23). Appalachian Cold-Air Damming. Powerpoint lecture presented
at Wakefield NWS Forecast Office, Wakefield, VA.
COMET/University Corporation for Atmospheric Research. “Cold Air Damming: Causes, Examples,and Forecast Parameters.” 2001.
(April 7, 2007).
Iowa Environmental Mesonet/Iowa State University Department of Agronomy.
“Week 2 Cold-air damming and Lake-Effect Snow.” February 18, 2002.
(April 7, 2007).