Forecasters predict fewer but stronger northeasters Weather

The jet stream is the key to North American storm patterns

climatologists know the pattern of this river of air has changed, but they don't know why.


For people who have suffered through the gales, heavy rains, floods, deep snows, property destruction and lost power of the nasty northeasters of the 1990s, there is little comfort in the fact that recent decades have seen fewer of these classic storms. That is because northeasters generally are more severe than they used to be.

The main reason, experts say, is a long-term shift in the predominant winter path of the North American jet stream - the high-altitude, west-to-east river of air in which counterclockwise spinning northeasters are born, from which they draw their energy and by which they are transported. The jet stream shift tends to throw large air masses together in a pattern that strengthens the storms. The northeaster that have pounded the Northeast this fall, as well as last January's blizzard, were variations on the main theme.

It is unclear whether the shift in large-scale atmospheric circulation that has brought more severe northeasters to the Middle Atlantic and New England states is a natural variation in the climate system or is related to changing global temperature patterns that accompany global warming.

"The outright honest answer is that we don't know" what is

behind the change, said Dr. Robert E. Davis, a climatologist at the University of Virginia.

His opinion, he said, is that the shift in circulation is from natural causes.

Other experts say that while climatic change caused by heat-trapping industrial gases like carbon dioxide might be contributing, there is virtually no way to distinguish the human influence from natural factors.

Origin of storms

Like the hurricane, the northeaster is a cyclone, meaning that its winds circulate counterclockwise around a center. But while hurricanes are born in warm tropical waters in the summer and early fall, northeasters originate as ordinary low-pressure systems in temperate latitudes in fall, winter and early spring.

The storms typically form in one of seven places: near the Bahamas or north of Cuba; over or near Florida; in the Gulf of Mexico; along the Appalachians; off Cape Hatteras, N.C.; off the Middle Atlantic coast, and far inland, around Colorado. The jet stream transports them to the heavily populated Northeast.

Northeasters also wreak their havoc differently from hurricanes. While hurricanes are more tightly wound and have more powerful sustained winds, they affect a smaller area when they hit land; a northeaster, on the other hand, can ravage almost the entire coast. And while a hurricane can punch far inland and spin off tornadoes there, a northeaster's direct physical damage is usually limited to the coast. It causes that damage mainly by sending high waves ashore over a long period, often several days.

Davis and a colleague at Virginia, Dr. Robert Dolan, have devised a scale for rating the severity of northeasters. Like the Saffir-Simpson scale for hurricanes, it classifies northeasters in five categories of intensity. The categories for northeasters are based on the combination of two factors: wave heigh and duration of the storm.

Using this classification system, Davis and Dolan have analyzed northeasters over the last half century and found that the strongest storms tend to occur in October, January and March. They have also found that from the mid-1940s through the mid-1960s, there were about 33 northeasters a year. This dropped to about 22 from the mid-1960s through the mid-1970s. Since then the frequency has varied but has not consistently reached pre-1965 levels.

But since the mid-1960s, the storms have generally been stronger. Research to date, say Davis and other researchers, implicates the shift in the jet stream as the cause. Instead of

following a more-or-less straight path across the continent, the sinuous, constantly shifting jet has tended in recent decades to follow a curvier course, often dipping far to the south in the Eastern United States and offshore Atlantic, then turning northward along the coast.

Dangerous permutation

In this pattern's most dangerous permutation, cold air to the west of the jet and warm ocean air to the east clash, and the sharp pressure difference between the two air masses creates strong winds around the rotating storm as the jet carries it northward. Further, a high-pressure system over New England or eastern Canada blocks the storm, allowing it to pound the coast for days at a time. The rotating storm typically hurls its winds from the northeast; hence, the northeaster.

There are a number of variations on this theme. In the Oct. 18-21 northeaster, for instance, the storm detached from the jet stream.

Deprived of the jet's transport mechanism, it stalled and was blocked even more effectively by the typical high-pressure system over eastern Canada. At the same time, it drew extra moisture from nearby Hurricane Lili. This was largely responsible for the heavy rains that set records from New Jersey northward and that amounted to nearly 20 inches in some parts of Maine.

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