Coastal storms

Hurricanes are the most familiar of the storms that ravage coastlines, but they aren't necessarily the sturdiest. For all its immense power, a hurricane can be quite finicky. Should the sea surface be a touch too cool, or should a stiff wind tilt the central chimney of rising air a bit too much, the balance of forces is destroyed and the hurricane becomes history.

There's another kind of oceanic storm, one that doesn't wilt in the presence of cold air and isn't easily torn to shreds. You've seen such a storm if you've ever tried to walk the shores of Cape Cod during one of the vicious storms known as nor'easters, or stood on the Normandy coast in the teeth of a wet January gale. These systems – great swirls of low pressure that pull cold air south and warm air north – don't rate a name as they approach. Some of them are named afterwards, however, once they've blasted miles of beachfront, smashed windows, unroofed homes, and robbed people of their lives. Years later, they're remembered by location or date: the Ash Wednesday storm, the Columbus Day storm, the Great Storm of 1703, the Storm of the Century. Less renowned than hurricanes or tornadoes, coastal storms can still pack hurricane-force winds and cause tremendous damage to buildings and ecosystems along the shore.

A product of contrasts

Coastal storms are part of the huge family of low-pressure centres that wend their way from west to east around the surface of the globe at mid-latitudes. Most places, whether inland or on the coast, see dozens of these lows come and go every year. Each low might bring rain, thunderstorms, snow, ice or perhaps just a shift in the wind. Most of these lows form ahead of equatorward dips in the upper-level jet streams that encircle the Northern and Southern hemispheres. Such dips, or troughs, help nourish the circulation of a surface low and tend to keep it moving eastward, especially in the presence of jet streaks. These bundles of wind, blowing at speeds that can top 320kph/200mph, descend from the upper atmosphere and help enhance upward and downward motion at lower levels.

Once a low is generated, it needs moist air in order to make rain or snow. The ocean needn't be the only source, since humid winds can travel hundreds of miles inland, or a large lake can provide moisture. However, lows that form or move over the sea have little trouble staying moist. Like a hurricane, a coastal low feeds off the warmth of the ocean and the water that evaporates from it Unlike a hurricane, a coastal low can ingest warm, moist air from the ocean even as it pulls in cold, continental air on its other side. Coastal lows are structured so that this asymmetry in temperature doesn't hinder them. In fact, the contrast between warm and cold can actually energize their circulations. Some of the greatest coastal storms have occurred when a powerful low moves close to a mass of cold air located near the coast As warm air rises on top of the blob of cold air, heavy rain and snow can be wrung out, and the tightening temperature and pressure contrasts often produce gale-force winds. Waves can build for days across thousands of miles of ocean before they batter the shore.

Along the US east coast, cold air often lodges against the Appalachians as far south as the Carolinas for days at a time. This “cold air damming” shifts the effects of the mountain range closer to the coast and heightens the likelihood of a coastal storm producing snow or ice rather than rain.

In general, east coasts around the world are usually favoured for the worst coastal storms, particularly snowstorms. That's because of the huge anti-cyclonic oceanic gyres that tend to bring warm water toward the poles along the east coasts of continents, just at the same place where cold air may be moving off the continent In Australia, the intense cyclones that spin up in the Tasman Sea are dubbed East Coast lows. The action is a bit further offshore in Asia, where cold air is typically entrenched across the continent in mid-winter, so lows in this region usually pull east of Japan toward the Gulf of Alaska.

Western Europe has also experienced its fair share of coastal storms, thanks to the Gulf Stream. This mild ocean current flows up the US east coast; an extension continues all the way northeast toward Norway. The warm waters encourage some coastal storms to spin up near the British Isles or the continent, while others manage to migrate across the North Atlantic. Similarly, coastal lows can surf the Pacific's Kuroshia current and its extensions and intensify all along the way to the Gulf of Alaska. Many end up slamming into the mountains of the southeast Alaskan coast, where the town of Ketchikan is drenched with an average of close to 4000mm/157in of rain each year.

Europe's big ones

Between December 26 and 28, 1999, two intense coastal lows swung inland across France, killing over 100 Europeans. The first low – the more northerly and stronger of the two – brought wind gusts of up to 219kph/136mph and knocked out much of Frances electric power for days. The strongest winds in half a century littered Paris with debris.

Rains from the second storm forced the Seine to overflow its banks. Some 10,000 trees in the grounds of Versailles palace were destroyed, including 80 percent of the palace's historic trees, such as the pine from Corsica planted during Napoleon's reign.

The French nightmare reminded many British people of their own hurricane-like experience, the Great Storm of 1987, whose intensity caught many by surprise. Winds and rain had been forecast for the night of 15-16 October, but the emphasis had been on heavy rain. As the coastal low approached, BBC weatherman Michael Fish announced there would be no hurricane, in response to a viewer's phoned-in query about a separate system far out in the western Atlantic. Although technically accurate, his reassurance may have been lost on Gorleston, Norfolk, where winds gusted to 196kph/122mph. Six of the namesake trees at Sevenoaks were felled, along with 15 million others across southeast England. London's power was knocked out for the first time since the Blitz, and the total damages came close to one billion UK pounds.

The 1987 storm was dubbed by some as the nation's worst in three hundred years, calling to mind the Great Storm of 1703. Two weeks of bad weather that November had driven hundreds of ships to the southeast coast. On the night of November 26-27, winds topped hurricane strength as a low crossed England from southwest to northeast. Coupled with the work of a seasonally high tide, the winds piled some 700 ships into a bend of the Thames. The storm knocked over some 800 houses and killed about 125 people on land and as many as 8000 at sea. The most famous account came from Daniel Defoe, who lived in London at the time and surveyed the damage in town and at sea. Defoe edited a set of journalistic accounts of “the particular dreadful effects of this tempest”, and he penned a poetical essay full of political and spiritual portent: “And every time the raging element/Shook London's lofty towers, at every rent/The falling timbers gave, they cry'd REPENT.”

Although it didn't do its worst in London, the deadliest modern-day coastal storm to strike Europe brought massive destruction to England's east coast and the low countries on February 1, 1953. A rapidly deepening low pulled hellacious gales onshore from Norfolk to Kent. Seawalls were breached en masse, and coastal flooding was so severe that some residents reportedly escaped only by punching holes in the ceilings of their flats. Just over three hundred died in England, but the picture was far worse in The Netherlands, where the storm's main surge coincided with high tide. A massive dike failure flooded over 2000 square km/770 sq miles and killed some 1800 people. The dual catastrophe led to decades-long flood control projects in both countries, including the Thames Barrier that now protects London.

High points of US coastal lows

Record-setting storms happened to strike both sides of North America during 1962. The Ash Wednesday coastal storm takes the prize as the nation's most relentlessly damaging of the twentieth century, due largely to its timing and longevity. The low moved slowly up the coast over a four-day period (March 6-10) that coincided with the year's highest tides. An unusually strong mass of high pressure centred over maritime Canada helped feed cold easterly winds that battered the US coast from Massachusetts to North Carolina. The easterlies went on to help dump heavy snow as they ascended the Appalachians. Later in the year, on Columbus Day (October 12), the Pacific Northwest was mauled by a fast-moving system that cruised up the Oregon and Washington coasts – much closer to land than usual – in a matter of hours. With the remnants of a typhoon tucked in its core, the low brought wind gusts to 201kph/131mph at Oregon's Mount Hebo and damaged over 50,000 homes across the state.

One of the most powerful and most accurately-predicted lows in American history was dubbed the Storm of the Century even before it struck on March 12-14,1993. A jet stream plunging well into the Gulf of Mexico gave this low a kick-start unusually far south. Feeding off warmer than normal Gulf waters, the low intensified and barrelled into the northwest coast of Florida with a 3.7m/12ft storm surge. A ferocious line of thunderstorms extended south all the way to Cuba, causing widespread damage with wind gusts estimated to be 220kph/137mph. Staying just inside the East Coast as it moved north, this low lived up to the dire forecasts spun by computer models. It produced windblown snows from north Florida to Maine, as deep as 127cm/50in in places. The storm dropped the barometric pressure to unprecedented lows as it became the first weather event ever to close all the major East Coast airports from Washington, DC, northward Some 270 people died – many of them the victims of heart attacks due to shovelling heavy, wet snow.