By H. Michael Mogil
Special to The Washington Post
Wednesday, February 12 1997; Page H01
The last week of December brought record-breaking winter storms to the Pacific Northwest -- floods, hurricane-force winds and deep snowfalls. Yet, unlike actual hurricanes, nobody gave these storms names, individually or as a type of storm. Most people just called them storms.
Here, in the eastern part of the country, however, meteorologists do have a name for a very similar type of storm. It's nor'easter, a contraction of "northeaster."
The "Blizzard of '96" that dumped two to three feet of snow from Boston to Washington Jan. 6-8 was a nor'easter. So was the snowstorm that paralyzed this area in February 1958 and another February 1983 that dumped 18 inches.
And so were many of the other memorable storms that have pummelled the East Coast over the years, a high proportion of them about this time of year.
Though many meteorologists find these great storms to be major events in their weather-tracking careers, they do not agree on a formal definition of the phenomenon.
Do you know what a nor'easter is? Here's a one-question test.
A nor'easter is a low-pressure system or storm that:
(a) affects only the northeastern part of the United States
(b) brings northeast winds
(c) moves toward the northeast
(d) brings heavy snow.
If you chose (a), you would be almost correct. Nor'easters occur northward along the East Coast from about Cape Hatteras, N.C. However, they have several counterparts. These are storms called black northeasters near Australia and New Zealand, strong southeast-winds called the suestado along the east coast of South America
and storms with strong southerly or southwesterly winds called Kona or leeward storms in Hawaii.
If you opted for (b), you'd be more correct. Nor'easters are known for their strong winds that blow from the northeast. As these winds blow ashore from the North Atlantic, they often bring high tides and violent waves, causing significant coastal flooding and beach erosion. Just ask the folks in Ocean City, Md. Almost every year there, nor'easters keep the Army Corps of Engineers and local officials busy dredging sand from offshore to replenish the beach.
Winds usually are weaker in the Washington area than along the coast because its distance from the shoreline allows lots of trees and buildings to blunt their force.
Coastal northeast winds mean that the center of the storm lies offshore because a nor'easter is a counterclockwise rotating mass of air something like a hurricane. The diagram shows how the wind circles the low-pressure center.
If you answered (c), you'd be correct, too. Almost without exception, nor'easters move from southwest toward the northeast.
Benjamin Franklin discovered this. In fall 1743, rain-filled skies prevented Franklin from watching a lunar eclipse in Philadelphia. However, his brother saw the eclipse in Boston. When Franklin learned that storm clouds rolled into Boston several hours after the eclipse, he began a research effort that included gathering reports from newspapers along the entire Atlantic seaboard.
What he discovered, according to David Laskin in his book Braving The Elements, was that " ... such storms originate in Florida or Georgia and move [toward the] northeast at about a hundred miles an hour." We now know that
they move at about 30 mph to 50 mph, but 250 years ago, Franklin's was "cutting edge" knowledge.
Answer (d) often is correct, and many area residents seem to feel that heavy snow is a requisite for a nor'easter. Although many winter nor'easters do bring snow, especially inland, they often leave coastal locales and the eastern and southern parts of the metropolitan area dripping in a chilly rain.
So, what is a nor'easter?
According to Paul Kocin, a meteorologist with the National Weather Service in Camp Springs, the beasts are hard to define. One approach is to compare them with hurricanes.
Often referred to as "the greatest storm on Earth," hurricanes are well-defined, Kocin notes. Their winds move in a circle around a low-pressure center with sustained winds of at least 74 mph. In the northern hemisphere, the winds move counterclockwise.
"Unfortunately," Kocin says, "it is impossible to be so exact when it comes to nor'easters."
That's because nor'easters are born from clashes of air masses and strong jet streams, while hurricanes form more slowly within a warm, humid tropical air mass. Because air masses differ and can clash in different ways, the resulting storms also can be highly variable. Hence, Kocin says, there's no such thing as a typical nor'easter.
The chief factors that give birth to these storms, he says, are:
* Cold Air Mass -- Typically, this is Canadian or Arctic air circling clockwise around a high-pressure center and moving into the Mid-Atlantic region. There the cold air meets the second factor.
* Low-pressure system -- Air moves counterclockwise around a low-pressure center. The combination of a high to the north and a low to the south causes the circling air masses to mesh like gears, each reinforcing the other. The result: strong northeast winds.
* Warmth and humidity -- Air moving on the eastern side of the low is warm and humid after being influenced by heat from the Gulf Stream flowing just off shore. Where warm air meets cold, it rides up and over the denser cold air that hugs the ground. The rising air cools, and its water vapor condenses into rain or snow, depending on the temperature.
* High altitude low-pressure system -- Often referred to as an upper level "trough" or "disturbance" associated with the jet stream, this pattern can enhance the rising air flow, creating even more precipitation. Also, it can remove air from above the low-pressure system, causing the low to deepen, further increasing wind speed near ground level. Lows at the surface and at higher altitude often are two ends of a tube-shaped, low-pressure zone similar to a tornado but larger and much less powerful.
Some storms share only some features of nor'easters. The "superstorm" of March 1993, for example, started over Texas, crossed the Gulf of Mexico, picking up moisture, and moved up the Appalachian Mountain range, using warm humid air from the gulf.
Coastal sections of Maryland and Delaware had southerly winds and warm temperatures because they were on the eastern side of the low, while the District and points west received heavy, wet snow. New England's infamous Halloween storm of 1991 arrived from the east and brought mainly strong winds and flooding to the coast.
Although meteorologists have known the basic nor'easter recipe for years, several major government and university research projects in the North Atlantic during the last decade have helped meteorologists to learn more details about the structure and evolution of these storms.
Several studies also have shown that intense ocean storms may even have an "eye," much like a hurricane. This knowledge, plus continually improving numerical models, ever faster computers and new satellite and radar technology have led to improved forecasts and warnings about these storms.
Although nothing in the nor'easter recipe guarantees snow, everything guarantees high winds. If the colder high-pressure system to the north remains strong and moves slowly while the low deepens and is steered rapidly northeastward by the jet stream, the difference in pressure between the high and the low will increase [see diagram].
On a weather map, this often appears in the way lines of equal barometric pressure look. The more closely spaced these lines, called isobars, surrounding the low, the greater the difference in air pressure over a given distance. Hence, the winds will blow more strongly.
Think of pressure gradients as slopes down which air falls, moving from the surrounding high pressure toward the center of the low pressure. The steeper the pressure gradient, the faster the air moves. The air flow also curves, its path bent by the Coriolis force, a consequence of Earth's rotation.
In the Washington region, with its many buildings, only exposed, suburban areas usually experience higher winds and blowing snow. In coastal sections, winds can be two to three times stronger than in or near the city.
"These strong winds, coupled with wind-driven waves, are what drives much of the evolution of the barrier islands and beaches along the mid-Atlantic shore," notes Carl Zimmerman, a natural resource manager at Assateague Island National Seashore.
"Powerful storms like nor'easters create most of the movement of sand along the shoreline," he says. "Storms can wash sand out to sea or back to shore, move it into the bays behind the barrier islands and even create inlets in a barrier island."
Scientific studies show that storms, which occur less than 5 percent of the time, account for about half of all sand transported along the Delmarva coast.
Zimmerman also notes that jetties and groins, structures built into the water perpendicular to the shoreline, can reduce sand movement and keep inlets from being clogged. But there are serious tradeoffs.
For example, jetties at the south end of Ocean City, Md., are one reason that Assateague, just to the south, is suffering from a lack of sand. What the jetties capture for Ocean City, Assateague never receives. A major project is underway to develop better sand management.
A nor'easter is not necessarily noteworthy to the public because of wind strength, the amount of erosion it causes or its rain or snow. Rather, its notoriety is enhanced because about 37 million people, 15 percent of Americans, live in the Boston-Washington corridor.
Even a small storm can wreak havoc to vehicle, train and plane travel and choke the region. In Minnesota and the Dakotas, residents generally live much farther apart and and less affected by severe storms. This winter has been an exception.
How the nor'easter gained its contracted name is uncertain. Scott Lindstrom, a meteorologist at the University of Wisconsin in Madison, says he suspects that Rhode Island fishermen called them no'theasters.
Paul Cousins, a television weather reporter, says "lexicology police" in his southern Maine broadcast area have advised him against using the term nor'easter because it is "incorrect." Still, he notes, some of these same people use the word "so'wester" for strong southwest winds associated with St. Lawrence River storms.
Whatever their name and however variable the conditions that cause them, these storms occur most often at this time of year.
Four of of what are generally considered the 10 most severe nor'easters happened in February. Six of the Washington area's worst snowstorms happened between Feb. 10 and Feb. 20. You might want to look out the window.
H. Michael Mogil is an educational meteorologist who lives in Rockville. Visit his home page at www.weatherworks.com for answers to other weather questions and links to weather sites.
© Copyright 1998 The Washington Post Company
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