Nineteen named storms, 12 hurricanes, and five major hurricanes ranked Category 3 and higher on the Saffir-Simpson scale.

That was the 2010 Atlantic hurricane season.

It was the second most active season on record in number of hurricanes, and the third most active with respect to the number of named storms (tropical cyclones with winds of at least 39 mph). Yet, not a single hurricane made landfall in the United States.

This exception is unprecedented.

In over a century–since at least 1900–there has been no other Atlantic hurricane season that has had ten or more hurricanes in which not one has struck the United States.

As a result, many insurers may be asking how this reflects on the catastrophe models they rely on to manage hurricane risk.

AN ACTIVE SEASON

In June, Hurricane Alex became the first Category 2 hurricane in more than four decades to form so early. In September, eight named storms developed, tying 2002 and 2007 for the most tropical cyclones on record ever to have developed in that month. In fact, from August 21 through September 26–36 days–not a single day passed in which there was not at least one tropical cyclone active in the Atlantic basin. And for only the second time in more than a century, two Category 4 hurricanes, Igor and Julia, were active in the Atlantic at the same time (also in September).

Just how active was it?

The 2010 season was about twice as active as the “average” season in terms of storm production.

o In a (statistically) average year, 10.7 named storms develop; 2010 produced 19.

o In the average season, six storms develop into hurricanes; 2010 saw 12.

o On average, three (2.8 to be precise) major hurricanes develop in a season. In 2010, five major hurricanes developed.

WHY SO MANY STORMS?

A number of environmental factors, both long-term and short, may have contributed to the conditions that together produced the 2010 hurricane season's unusual activity. One was a warm Atlantic.

A hurricane's “engine” is fueled by the heat and moisture of the ocean surface. The warmer the ocean, the more readily available is fuel; the warmer the air, the more moisture it can hold for clouds–and eventually precipitation–to form and fall. Understandably, warm sea surface temperatures (SSTs) are positively correlated with hurricane activity.

During the 2010 hurricane season North Atlantic sea surface temperatures were significantly warmer than average. While the jury is still out on the cause–be it the result of a natural multidecadal cycle as some have argued, or global warming–SSTs in the Atlantic have been consistently above the long-term average every year since 1995. At the same time, while each individual season has been unique, over this 15-year period of warm SSTs, counts of U.S. landfalling hurricanes, major hurricanes and aggregate annual losses fall in line with landfall frequency and average annual loss (AAL) encapsulated by the AIR U.S. Hurricane Model.

A second large-scale climatological factor is the “El Ni?o/La Ni?a-Southern Oscillation,” or ENSO, a climate cycle that occurs across the tropical Pacific Ocean and typically lasts for three to seven years.

ENSO is characterized by temperature fluctuations of the tropical eastern Pacific–a warming of the sea surface known as El Ni?o and a cooling known as La Ni?a. (Many years are neither warm or cool enough to classify as an ENSO event and are deemed “neutral.”)

Just before the onset of the 2010 Atlantic hurricane season, the Pacific basin entered a new La Ni?a ENSO phase. La Ni?a is associated with a seasonal reduction in destructive (to hurricanes) wind shear over the Atlantic, thereby providing a more favorable environment for tropical cyclone formation and development.

WHY SO FEW LANDFALLS?

While large-scale, long-term climate conditions had a strong effect on the formation and development of tropical cyclones in 2010, shorter-term weather patterns strongly influenced where the storms tracked. For the most part, the most threatening storms were steered away from the North American coastline.

In 2010, the “Bermuda High”–a large, semi-permanent center of high atmospheric pressure in the eastern Atlantic near the Azores (and roughly at the latitude of Bermuda)–was at times situated further east than usual and/or was weaker than usual. Additionally, for much of the core of the 2010 hurricane season, a low pressure system was in place over the U.S. East Coast. Because of this pattern, storms tended to track away from the United States.

Additionally, the jet stream's position over North America acted as a barrier that kept many storms over open water.

Similarly, since many storms formed in the extreme eastern Atlantic (because of the favorable ocean/wind shear conditions there), they often began their turning to the north and east well before ever reaching land. Storms that form off the coast of Africa have a relatively high probability of intensifying into hurricanes owing to their long trek over warm tropical waters, but a relatively low probability of making landfall along the U.S. coastline because of the Coriolis force (an artifact of the Earth's rotation), which inevitably deflects storms northward.

A LOOK BACK AT THE FORECASTS

The several government agencies, academics and private organizations that forecast the Atlantic hurricane season every year predicted that 2010 would be “an active year”–and they proved to be correct.

With respect to forecasting specific numbers of named storms or hurricanes that would form in 2010, most of the early predictions were actually conservative. Nonetheless, in May, just before the formal start of the season on June 1, the National Oceanic and Atmospheric Administration forecast a very successful range of counts: 14-23 named storms (there were 19), 8-14 hurricanes (there were 12), and 3-7 major hurricanes (there were 5).

NOAA does not forecast landfalls. Indeed, many forecasters do not, for the very reason that the factors that determine how storms track–atmospheric steering currents–are more closely associated with short-term weather variability that cannot be anticipated more than a few weeks in advance.

Most forecasters that did venture a guess on the number of hurricane landfalls expected a higher than average count. The average is about two U.S. landfalls per year. Yet in 2010 only Bonnie, which was barely a tropical storm at the time, came ashore, and with little adverse effect.

ONE NEAR MISS

The passage of one of the 2010 season's hurricanes elicited a loud and collective sigh of relief. Hurricane Earl–the season's third hurricane and second major hurricane–approached Puerto Rico and the U.S. Virgin Islands at the end of August as a Category 4 hurricane with winds near 150 mph. As the Labor Day weekend approached, Earl's projected path was becoming clear: north along the U.S. East Coast with a possible landfall near downtown Boston!

Hurricane landfalls in the Northeast are relatively rare, occurring on average once every ten years. The last was Hurricane Bob in 1991; the last major hurricane landfall in the Northeast–an even more rare event–was the Great New England Hurricane of 1938.

On September 2, the National Hurricane Center distributed its forecast track and accompanying “cone of uncertainty” for Hurricane Earl. AIR estimates that had the left-most side of the cone of uncertainty been realized and the storm had indeed cut up through Rhode Island and eastern Massachusetts, total insured losses from Earl would have been about $4 billion.

A few days later Earl made a more northerly turn than projected and eventually passed well east of Cape Cod. All the same, Earl–like Hurricane Bill last year–serves as a not so gentle reminder of the very real hurricane risk faced by the densely populated Northeast.

AFTERTHOUGHTS

The 2010 Atlantic hurricane season was unusually active and broke or matched several records of hurricane formation, number and behavior. All the same, not one hurricane made a U.S. landfall.

Climate conditions such as warm sea surface temperatures and the El Ni?o/La Ni?a-Southern Oscillation can significantly influence tropical cyclone formation and development. However, even under favorable conditions driven by a warm Atlantic, the number of hurricane landfalls can be average or even well below average.

This was in fact the case in 2010.

It serves as another reminder that neither warm sea surface temperatures nor elevated activity in the Atlantic basin will necessarily produce elevated losses. Loss projections depend on a host of factors, many of which are not easily forecast.

What matters–in costs and in lives–are the storms that strike where people live and work. When, where and how often that will happen is uncertain. Given the low accuracy of forecasting the number and location of hurricane landfalls and therefore the consequent volatility in hurricane losses, there is currently insufficient skill to predict losses in advance of a season.

The extreme variability of hurricane losses has led to the use of catastrophe models, which include thousands of scenarios that represent the full range of plausible outcomes. Catastrophe models should be expected to capture seasons like 2010–however unusual they may be. Through the use of catastrophe models, estimated probabilities can be determined for different levels of loss, which are essential input to robust catastrophe risk management.

Peter Dailey is assistant vice president, director of atmospheric science at AIR Worldwide.