Weather Topics

North America and the Jet Stream in Winter

Weather systems are steered by the several Jet Streams that circle the globe from west to east. A Jet Stream is a river of air that flows at speeds that sometimes exceed 150 miles per hour, located tens of thousands of feet aloft. It can increase or decrease air travel times significantly, depending on whether it is a headwind or a tailwind. The particular Jet Stream that has the greatest influence on winter weather in North America is known as the North Polar Jet Stream, and that is the one we concentrate on in this column.

There are essentially two different extreme patterns for the Jet Stream: a flat, more or less straight west to east flow, called a zonal flow, and a "hills and valleys" up and down flow that is in the north in some parts and the south in others, which we can call an sinusoidal flow, or a flow with amplitude. The greater the amplitude, the more extreme are the differences in air masses between the "hills and valleys."

We will treat the zonal flow first. This pattern lends itself to a mostly calm movement of weather systems straight across the continent. Cold air is limited to those parts north of the Jet Stream; areas to its south are mild to warm. This rarely results in any particularly cold weather anywhere in the Continental United States since in this case our air masses have their origin over the Pacific Ocean. Storm systems tend to be weaker than normal; they tend to follow a track straight from west to east.

The more complicated type of flow is the non-zonal one. There are several variations of this possible over the continent. By a "hill" we mean that part of the Jet Stream that is located to the north of the rest of the flow; conversely, a "valley" refers to the more southerly part of the flow. One possible alignment has a hill over the western part of the continent and a valley over the eastern part. Another possibility is just the opposite: valley in the West and hill in the East. Still other possibilities are a hill in the center with a valley at each coast, and a valley in the center with a hill at each coast.

Whichever pattern sets up, it tends to be stable for some period of time, until some influence - typically a wave disturbance west of the continent - disrupts it. This period of stability is often two to four weeks, although it can be anywhere from about a week to six weeks long.

The Jet Stream up to Mid-Winter 2008

The Jet Stream has been (literally) all over the map so far this winter. Half of the time it has had a valley in the West and a hill in the East, bringing record amounts of snow and some rather cold temperatures to parts of the West. For a few brief periods, there was a zonal flow that set up between shifts in the pattern. The valley was in the East during the following periods: for a week starting 6 November and again starting 16 November; for about four weeks starting 30 November; and for two weeks starting 13 January.

Here in Emmitsburg, MD we have had a total of 12 inches of snow; 6 inches fell on 5 December and 6 inches fell on 17 January. Twice in January, lakes and streams froze over, during the first several days of the month and again for several days starting on the 21st, when we recorded a low of 1 degree above zero, the coldest temperature seen here in some time. Several times we "dodged the bullet" from Nor'easters which formed either too far north or too far east to affect us. (And I predicted many Nor'easters for this winter.)

On the other side of the coin, we observed record warmth here on 7 January with a high of 66 degrees. On 6 February we had 65 degrees here. The very warm four-day period starting on 6 January was caused by a huge Pacific storm that came onshore along the West Coast. The storm was so large and powerful that its (counterclockwise) circulation pulled up warm air from the South for over 1000 miles ahead of its path. This was enough to create a huge ridge across the eastern part of North America extending (briefly) into Canada.

You may have seen stories in the news about snow falling in Baghdad and Jerusalem on different days in January. While snow in Jerusalem is not unknown, this was the heaviest seen for a while. Snow in Baghdad is so rare that even several elderly residents said they had never seen any in the city all their lives. Heavy snow was also observed early in February in southern China, causing massive (thousands of vehicles) traffic tie-ups.

This winter has seen the growth of large pools of arctic air from Siberia across Alaska into central Canada. In fact this supply of arctic air is colder and more extensive than it has been over the last several winters. Twice this air broke through the Jet Stream "dam" and flowed south and east: at the start of December (causing a winter storm from Colorado to New England) and in the middle of January. Both outbreaks were very short-lived.

Late in January, reports out of Siberia told of a week of temperatures between 70 and 75 degrees below zero! While these were not record-breaking, they were colder than has been observed there in several years. Often such pools of arctic air migrate eastward over Alaska and Canada. When such air travels over land that is warmer than itself, it expands and moderates. The expansion nearly always involves a southward movement; this is a common source of cold waves in the Continental United States.

So if you are a skier or a fan of cold winter weather, and are lamenting the scarcity of (natural) snow here in the Mid-Atlantic this winter, don't give up just yet. Just keep in mind that Punxsutawney Phil saw his shadow on Groundhog Day.

Bill O'Toole
8 February 2008