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 "peaks and valleys" up and down flow that is in the north in some parts and the south in others, which we can call a sinusoidal flow, or a flow with waves. The greater the amplitude (the north-south distance between peaks and valleys), the more extreme are the differences in air masses between the peaks 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 "peak" 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 peak 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 peak in the East. Still other possibilities are a peak in the center with a valley at each coast, and a valley in the center with a peak 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.

Summer and Autumn 2009 in the Mid-Atlantic

The early outlook for the summer was for above normal temperatures in a wide swath of the western states (except the coastal areas) and the Rockies, the Gulf Coast region, and up the Eastern Seaboard to New York State and New England. Cooler than normal temperatures were forecast for the Dakotas and Minnesota. Close to normal temperatures were predicted for the rest of the continental U.S. These predictions were released at the end of May by the Climate Prediction Center in Camp Springs, MD.

Indeed these forecasts were not surprising, considering that many areas experienced unusually high readings in the 90s as early as April, breaking many long-standing records. May saw several mini heat waves in the East. Your Conjecturer recorded 90 and 91 on April 26 and 27 respectively – less than three weeks after the last snowfall, on the 8th. Our area escaped the worst of the May heat; our highest reading in May was 88 on the 23rd, not all that unusual.

From the beginning of June until the time of this writing (mid September), this area has experienced one of the coolest summers on record. Over 3,000 low temperature records were broken in the Northeast in the months of June and July. The highest your Conjecturer recorded in June was 87 (on the 2nd, 25th, and 26th). The maximum in July was 92, on the 16th. August’s highest was also 92, recorded on the 10th. There were only nine days in the 90s here all summer; it has been many years since that last happened. Air conditioning was needed very few times, saving wear and tear on the power grid, and lowering the utility bills.

Of course there were many remarkable low temperatures recorded. Here are some: 35 on May 13; 32 on May 19; 48 on June 1; 52 on July 6 and 9; 47 on July 14; 49 on July 15; 57 on August 7; 50 on August 31; 45 on September 1; 44 on September 2; and 47 on September 3. Many weather aficionados have asked your Conjecturer for an explanation of this aberrant pattern. How could the Climate Prediction Center have gotten it so wrong?

There are two reasons. First, a developing El Niño caused a strong west to east flow across the Gulf of Mexico, preventing the Bermuda High from setting up long-term residence off the East Coast. The few brief hot spells we had were caused by attempts on the part of the Bermuda High to establish itself. But within a few days, the upper-level winds broke it up. (These same winds suppressed the development of hurricanes in the Atlantic basin, giving us a relatively worry-free season – so far! Hurricane season ends on November 30.)

Secondly, there has been a persistent northwest flow across the northeastern third of the country, meaning that cool Canadian high pressure systems made their influence known much more frequently than in most summers. These were kept moving along by upper-level steering currents, meaning that there were few times when stagnant air could sit and overheat for days at a time, which often happens during the average summer in the East.

What about precipitation? The Climate Prediction Center was quite accurate in this aspect of their forecast. They predicted continuing drought in California, Washington, Nevada, and Hawaii. California had a plague of disastrous, large-scale wildfires, exacerbated by the drought there. The Center predicted an easing of the drought in Florida and the Southeast; this also was correct. Heavy rains have fallen in these areas this summer.

Your Conjecturer will do a detailed analysis of the Almanack’s forecasts in the 2011 edition. (The 2010 edition treats the accuracy of our forecasts for the 12-month period from July 2008 to June 2009, due to printing and production schedules.) A preliminary comment: There have not been as many hurricanes and tropical storms as your Conjecturer expected.

Outlook for Winter 2009-2010 in the Mid-Atlantic

The winter will start early because the cooler-than-normal trend experienced much of the time since the start of 2009 will continue into 2010. Even so, there will be the occasional warm spell, but only a few days at a time. The winter should not spill over into April like it does some years. All months from November to March will be colder than normal, except for February, which will be close to average. Overall, the winter will be about 2 degrees below normal.

November and January will have close to average precipitation, but December, February, and March will have below average precipitation. Overall, there will be a deficit of about 5 inches of precipitation for the five-month period. About half of the moisture that will fall should be in frozen form because of the depressed temperatures. There will be about 41 inches, or about 104 cm, of snow, which is about 8% above average. January will be the snowiest month, but February and March will be close behind. Expect several Nor’easters, including one in late February and another a week later in early March.

All of this is based on the assumption that the El Niño will not be strong near the end of the year. On the other hand, if it becomes a strong El Niño, our winter will be warmer and less snowy. In that case, there will be a series of strong storms entering California from the Pacific, with strong winds and heavy rains, causing disastrous mudslides because of the loss of vegetation caused by this summer’s wildfires.

Bill O'Toole
18 September 2009