Why Hurricane Forecasts Went Bust This Season

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Looking just at hurricanes alone, it`s been the quietest Atlantic season in 75 years.

What`s remarkable about this is that forecasters had predicted a busy season. The National Oceanic and Atmospheric Administration went so far as to call it a possibly "hyperactive" season.

Thirteen storms formed, but most were weak systems that stayed far out at sea.

Rather than simply counting storms, probably the best way to measure a season`s overall activity is to sum up the number, strength and duration of its tropical storms and hurricanes, a value known as Accumulated Cyclone Energy. For the 2013 Atlantic season, this value was 31.1, or about 30 percent of normal levels.

Put another way, it was among the five lowest ACE values recorded since 1950.

So what happened?

"A combination of conditions acted to offset several climate patterns that historically have produced active hurricane seasons," explained Gerry Bell, NOAA`s lead seasonal hurricane forecaster. "As a result, we did not see the large numbers of hurricanes that typically accompany these climate patterns."

Temperature, pressure

Perhaps it`s best to step back and look at why a busy season was predicted. It wasn`t to scare people, or to drive up insurance rates. Seasonal forecasters often under-predict hurricane activity as well. As a science, seasonal hurricane prediction is in its infancy.

Prior to the beginning of this season, which started June 1, forecasters were expecting to see higher-than-normal water temperatures and lower-than-normal pressure in the deep tropics, where most tropical systems form. Forecasters also expected water temperatures in the tropical Pacific Ocean would remain in the cool or neutral range through the season.

All of these factors tend to boost hurricane activity in the Atlantic.

And during this season all of these things happened. And yet, there were no big storms.

Atmospheric moisture

"It turns out that there is an additional parameter that was not generally considered when making seasonal predictions," said Chris Hebert, a hurricane forecaster with ImpactWeather, a company based in Houston.

Hebert said earlier this year, after it became obvious that the Atlantic activity would be well below normal, he searched for other factors at play and discovered that moisture levels in the midlevel of the atmosphere, about 18,000 to 25,000 feet above the surface, were well below normal.

Storms often weaken or die after ingesting dry air.

In their post-season analysis, Colorado State University hurricane scientists William Gray and Phil Klotzbach wrote: "It seems like this dry air was one of the critical reasons why the season was very quiet."

It seems like a good bet that next year`s seasonal forecasts will attempt to incorporate the likelihood of such dry air into their calculations.

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Copyright Houston Chronicle 2013