BOSTON — `IT'LL be a real nor'easter.'' That note of warning has been heard more often this winter than most New Englanders - indeed most people from Virginia to Nova Scotia - would like. This breed of storms, traditionally the bane of fishermen and sea captains, now typically brings burdens ranging from severe beach erosion to sky-high snowplowing bills for municipalities, businesses, and homeowners.
There have been 13 nor'easters - or ``northeasters,'' as the dictionary stodgily puts it - this year, with room for more before March blows out. The last one, which struck the Boston area March 3, dumped just enough snow to make this winter the whitest ever here, with 89.5 inches of snow so far.
Where do the big storms come from, and why so many this year? Weather watchers as far back as Ben Franklin remarked on the strong cold winds out of the northeast and assumed the storms came from that direction, says Richard Tinker, a meteorologist at the National Weather Service's Climate Analysis Center in Camp Springs, Md. In fact, just the opposite is true. The storms generate far to the south, often off Cape Hatteras, N.C., then migrate north.
The counterclockwise cyclonic swirl of the storm can produce the cold northeasterly winds along the coastal edges of the storms, however.
Nor'easters are a complex weather phenomenon, says Kerry Emanuel, a meteorologist at the Massachusetts Institute of Technology in Cambridge, Mass.
Some have suggested that nor'easters, like hurricanes, might be rated for severity to give the public a better idea of what to expect. ``The nor'easter offers such a diverse set of phenomena that the public is already baffled,'' Professor Emanuel says. ``A high rating might scare someone who lives inland, when the real destruction, from wind and rain, will be on the coast.''
A lively ``mix'' of precipitation is standard for nor'easters. Who gets snow and who gets rain, and when, has a lot to do with subsidiary ``coastal fronts'' that form along with the main storm, Emanuel says. These secondary fronts cause wind shifts, with cooler air on one side and warmer on the other.
There are also ``complex temperature setups'' within the storm, Emanuel continues, with warmer layers of air sandwiched between colder ones. Precipitation can start as snow, turn to water as it goes through a warm belt, and then refreeze down lower, hitting the surface as ice pellets or sleet.
Mr. Tinker points out that the swirl of air in a nor'easter can result in temperature anomalies at ground level, too. During the early March storm it was raining in Baltimore while snow fell in Norfolk, Va., much further south. ``There's almost no way to forecast that,'' he says.
Actually, forecasting the direction and approximate severity of nor'easters once they get underway down south is not a great problem, says Tinker. ``Our computer models tend to do better with nor'easters than with hurricanes,'' he says, explaining that hurricanes are influenced by more-subtle conditions.
But hurricanes also have a much longer and more-easily tracked life cycle. Nor'easters churn up and disappear out to sea in a matter of a few days. The most recent one, says Tinker, began as a ``very weak reflection'' in the Gulf of Mexico. And predictions of types and amounts of precipitation as a storm nears can be educated guesses at best.
The storms originate with a confluence of opposing air masses on the East Coast of the United States. They develop as a ``wave'' on the boundary between cold air to the north and ocean-warmed air to the south, says Hal Nagle, a technical editor with the American Meteorological Society in Boston.
The strength of a given storm, he explains, is determined by the ``gradient,'' or difference over distance, between the low pressure to the south and the high to the north. The low pressures registered in a robust nor'easter can rival those of a moderate hurricane, Tinker notes.
A tough winter
And why the series of big storms this year? Mr. Nagle emphasizes the influence of the jet stream. Irregular patterns in those high-altitude winds interact with surface developments, generating the cyclonic action of storms, he says. Tinker mentions the influence of a ``ridge'' of polar air fixed over Canada for much of this winter.
But basically, Emanuel says, the complexities of atmospheric interactions, typified by nor'easters, defy hard-and-fast explanation.
There's no quasi-cyclical phenomenon, like the warm ``El Nino'' Pacific Ocean currents to fall back on here in New England, Emanuel says. There's only the change in the seasons. And to the relief of many, spring should be just a few weeks away.