BioEd Online

The United States and Mexico should brace themselves for another set of powerful hurricanes this year, forecasters have warned.

Conditions are right for a bumper crop of hurricanes to form in the Atlantic Ocean, they say. Sea-surface temperatures are warmer than usual in the region where hurricanes develop, providing more fuel for potential storms. And wind patterns, which if strong can rip apart a nascent hurricane, are relatively weak.

Hurricane season begins in the Atlantic on 1 June, and scientists are trotting out their predictions of what to expect. On Monday, the National Oceanic and Atmospheric Administration (NOAA) released its forecast: that 13 to 16 tropical storms would form in the Atlantic, with winds of at least 62 kilometres per hour. Eight to ten of those storms could reach speeds of 118 kilometres per hour, making them a hurricane. And four to six might get all the way to 178 kilometres per hour, making them a major hurricane.

That's higher than the average, but lower than last year's massive hit of 28 named storms. "We don't anticipate reaching or exceeding last year's extraordinary tally of storms," says NOAA chief Conrad Lautenbacher.

That said, hurricane forecasting can be a tricky business, especially this early in the season. Last year, NOAA forecast just half of the true number of Atlantic storms in its May announcement ( see box).

Stormy weather

The predictions rely on computer models that look back at past hurricane seasons and calculate how likely storms are to form if certain conditions, from ocean temperature to wind strengths, are present. The models serve as a general guide for what to expect, but rarely get the exact number of hurricanes right.

This April, sea-surface temperatures were 0.6 °C warmer than usual in the swath of tropical ocean between Africa and the Caribbean, where hurricanes usually get started. This warming is unlikely to dissipate in the coming months.

At the same time, easterly winds in the middle atmosphere have been blowing more weakly than normal, says NOAA forecaster Gerald Bell. So any tropical depression that develops would have little to stop it from building into a storm. Although winds might be expected to be more variable over shorter time scales than water temperatures, researchers have found there are patterns that mean spring measurements can forecast something useful about the coming months.

There's also no sign of an El Niño this year. That phenomenon, marked by a warming in the eastern tropical Pacific, tends to suppress hurricanes in the Atlantic.

All together now

Other forecasting groups have come up with similar numbers to NOAA's, using similar logic but different models. A team led by William Gray at Colorado State University predicts 17 named storms, with 9 of them hurricanes. A group at the Benfield Hazard Research Centre, at University College London, UK, predicts 15 tropical storms and 8 hurricanes.

Still, Lautenbacher points out, "it only takes one hurricane in your neighbourhood to make it a bad season."

NOAA will update its forecast in early August. Adding in the data from June and July should dramatically improve predictions for the rest of the year, experts say. July winds have been previously shown¹ to be a good indicator of whether hurricanes will make landfall or not, an important factor in the damage that they can incur.

Hurricane season officially ends on 30 November, although nature does not always comply with the calendar. Last year, the final storm of the season did not die until early January, forcing the National Hurricane Center to yank forecasters back from their new year's holiday to keep working.

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