Weather Break -- The Siberian Express

This is a transcript of the Weather Break radio show for Monday, January 21, 2008. The episode was written by Dr. Jon Schrage.

Late last week, very cold air began to penetrate in the continental United States from the north. TV meteorologists were calling this weather pattern “The Siberian Express”. This term isn’t particularly scientific or formal; rather, it’s just an expression for an outbreak of extremely cold air over North America. This cold air mass didn’t necessarily come from Siberia, of course. Siberia, which is in extreme northeastern Russia, certainly is a very good source region for extremely cold air masses, but it is fairly uncommon for these airmasses to travel all the way from Russia to the United States without significantly modifying. “Modification” is the technical term meteorologists use to describe the processes that adjust the temperature and moisture in an air mass once it has left its original source region. In the case of a Siberian air mass, there are lots of ways in which that extremely cold, extremely dry air would modify between Russia and the United States. As the air mass passes over bodies of open water, evaporation and the conduction of heat would change the moisture and the temperature properties of the air, making it less dry and less cold, for example.

On the other hand, the Siberian Express weather pattern can be the result of very cold air masses pushing their way south from northern Canada or even the Arctic. There is relatively little opportunity for the air mass to modify between northern Canada and the United States, since the surface over which it will be passing is probably very cold and covered with snow at this time of year.

The Siberian Express is an example of a weather pattern in which air masses are in motion. Air masses don’t just move themselves; they are moved by the weather patterns around them. Meteorologists talk about air masses as being moved by the jet streams. The jet streams are like rivers in the atmosphere. They are fast-moving channels of wind found about 5-6 miles above the surface of the earth. All of the jet streams that are important to weather in the United States are generally from the west, although they typically meander north and south quite a bit. Picture the way a river meanders from side to side on its way to the sea. In the same way, jet streams meander north and south as they head off to the east. Meteorologists call these meanders troughs and ridges, and the position and movement of troughs and ridges in the jet stream cause air masses to move.

The weather pattern known as the Siberian Express is often set up by a particular pattern in the jet stream known as PNA. PNA, or the Pacific-North America pattern is actually one of the most common patterns in the atmosphere. In the PNA pattern, the jet stream swings far to the south of Alaska before turning sharply to the north. The jet reaches almost all of the way to the North Pole over western Canada before making a sharp right turn and heading south across the central United States. At the coast of the Gulf of Mexico, the jet stream makes a left turn and heads east across Mississippi, Alabama and Georgia before turning even farther to the left, going northeast up along the east coast of the US. A pattern like this in the jet stream creates conditions at the surface that are favorable for the coldest air up in northwestern Canada to slide down across the plains of Canada and the central US, producing extremely low temperatures across much of the country.The Siberian Express and the PNA pattern illustrate one of the most important principles in meteorology—namely, that the weather and climate at one location are strongly influenced by weather patterns at locations that might be thousands of miles away. In the case of the Siberian Express, the conditions that favor this cold weather pattern are set up by the PNA pattern in the jet stream, which itself is set up by patterns of thunderstorm activity in parts of the tropical Pacific ocean. The best forecast models incorporate global weather features into their depiction of the atmospheric state so that they can produce the most accurate representation of how the atmosphere will evolve over the next few days.