As we inch closer and closer towards the winter months, a specific large scale setup becomes slowly more evident and relevant as temperature contrasts build between the north and south and we trend toward the colder months. It’s a setup that arises as high pressure develops to the north and warm, moist air to the south becomes aligned in just the right way that signals a typical setup of what’s known as cold air damming.
Cold air damming (CAD) occurs most often on the east side of large mountain ranges such as the Appalachians and Colorado Rockies as cold air at the surface is funneled alongside these topographic features. This layer of cold air becomes trapped beneath a layer of much warmer and more saturated air to form various precipitation types and localized temperature patterns.
This process can occur during both summer and winter, with the winter events being much stronger due to larger temperature contrasts between the cold northern states and the warmer southern states. There are multiple types of cold air damming set ups, but for now we’ll focus on the most “classic” one.
Classic set up:
An area of strong high pressure sets up to the north of the region, usually originating from Canada, creating clockwise flow around it. This wind around the high pressure provides an east/northeasterly flow as it circles around the high. Those winds will then push up against the Appalachian mountains that run from northeast to southwest as it flows down the mountain range. They ultimately become trapped alongside thee topographic feature while warm, moist air is funneled in from southerly flow. Check it out in the image below:
What’s happening in the atmosphere in the upper levels is important as well in determining the strength and type of precipitation the event will produce, which provides many challenges for the forecaster when determining both temperatures and precipitation type.
In this process, the warm air coming from the south overrides the cooler air. This allows large amounts of precipitation to fall through a shallow layer of cold air to create potentially hazardous weather such as freezing rain. As the image explains, the type of precipitation that falls depends on the depth of the cold air. The larger the layer of cold air, the more likely the precipitation will fall as snow, or sleet. If the layer of cold air is really shallow at the surface with a large area of warm air aloft (known as an inversion), you can get freezing rain to occur.
In a classic set up of CAD, the surface winds may maintain a northeasterly component rather than easterly as the high pressure continues to exit eastward. As the air rises up the mountain, it cools and an area of low pressure can develop on the western side of the mountain range. This can enhance the pressure gradient and strengthens the cold air damming process.
- Locally cold temperatures
- Different precipitation types such as snow, sleet, and freezing rain
- Enhancement of winds
- Fog and cloud development
These topographically driven weather events are not limited to just winter weather, but it can affect other important weather parameters during the warmer seasons such as visibility and cloud heights, precipitation type, and both wind speed and direction.
We currently have an area of high pressure building over Quebec that will maintain an east/ northeast flow not only across the Great Lakes, but into parts of the northeast into tomorrow. This situation is slightly unique as we’re still dealing with mainly warm season precipitation, and we have a post-tropical system passing by just to our south. A big reason western NY will miss out on most of the precipitation associated with this system is because of the building high to our north that will keep the post-tropical system just to our south and east as it exists on Monday. As the system passes it will turn our northeasterly flow southwesterly as it feeds slightly more mild air into the region come Tuesday, before a cold front settles in after.
Check out the surface temperatures along the northeast in the latest euro model run below:
You can start to see where the colder temperatures in blue are being concentrated along the Appalachians from parts of West Virginia to the western edge of North Carolina.
A classic set up of CAD at the 500 millibar level will also often indicate a ridge in the east with an approaching trough out west, similar to the one shown below:
It is not the most impressive set up by any means and would look a lot different if we were in the heart of winter, but you tend to get these types of set ups to occur more often and with more wintry outcomes the closer we get into the colder season. And we’re definitely starting to head in that direction as this setup only becomes more relevant and important in our forecasts going forward, especially when dealing with multiple types of precipitation over an area.
~Meteorologist Christine Gregory