News•August 7, 2014

Forecasters Lower Hurricane Season Expectations

Hurricane activity in the Atlantic Ocean basin is likely to be even more lackluster for the remainder of the season than forecasters originally thought when it began on June 1. Forces like cool ocean waters and a stable atmospheric environment are keeping storms from forming and growing, and the El Nino that is faltering but still expected to develop could further quash storms, forecasters say.

Click image to enlarge. Credit: NOAA

But even relatively subdued seasons can produce storms that cause considerable damage to coastal communities, as Hurricane Andrew did when it devastated South Florida in 1992, an otherwise fairly slow season. There has already been one hurricane strike this year: Hurricane Arthur made landfall in North Carolina as a Category 2 hurricane on July 3, causing power outages, toppling trees and storm surge flooding along the coast.

“We know hurricanes strike in below-normal seasons,” said Gerry Bell, the lead seasonal hurricane forecaster with the National Oceanic and Atmospheric Administration’s Climate Prediction Center in College Park, Md.RELATEDTropical Trends - Hurricanes and Climate Change
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The 2014 hurricane season, which officially lasts until Nov. 30, has a 70 percent chance of below-normal activity according to the latest updated forecast from NOAA, released on Thursday, as the season enters its historic peak period. When the initial forecast was issued on May 22, there was a 50 percent chance for below-normal activity.

Forecasters expect to see 7 to 12 named storms (which includes tropical storms and hurricanes), of which 3 to 6 are likely to become hurricanes, with a maximum of only 2 of those likely to become major hurricanes (Category 3 or higher on the Saffir-Simpson scale). The 30-year seasonal averages for storm numbers are 12 named storms, 6 hurricanes and 3 major hurricanes.

Only two named storms, both of which became hurricanes, have formed this year: Arthur and Bertha. The remnants of Bertha are still crossing the North Atlantic and could bring unsettled weather to the British Isles this weekend, according to the UK Met Office.

Of course, storm numbers aren’t the only indicator of the level of activity in a hurricane season. Another is called ACE, for Accumulated Cyclone Energy, which is a measure of the destructive potential of all of the storms across a season that incorporates wind speeds and a storm’s longevity.

The forecast from the Tropical Storm Risk consortium, affiliated with the reinsurance group Aon Benfield, predicts an ACE of 70 for the 2014 season, compared with the climatological average of 102. Their storm numbers are similar to those predicted by NOAA (as are those from another major forecast group at Colorado State University).

“A low ACE-to-storm ratio implies that storms forming will either tend to be weak for the majority of their life, or be short-lived,” Adam Lea, a tropical storm researcher at University College London who helps write the TSR forecasts, said in an email.

These numbers are so low because of the prevailing conditions across the basin, which are suppressing storm development. And there is “every indication is that those suppressive conditions will continue,” Bell said.

One such set of conditions is the ocean temperatures across the tropical Atlantic, the prime nursery of tropical cyclones (the generic term for hurricanes and tropical storms). Warm ocean waters (of at least 80°F) are necessary to fuel the convection engine at the heart of a tropical cyclone.

The ocean surface has been cooler than normal this summer and “exceptionally cool relative to the remainder of the global tropics,” according to a NOAA statement. The temperatures are also cooler than models predicted at the beginning of the hurricane season, contributing to the increased chances of a below-normal season in the updated forecast.

The atmosphere is also more unfriendly to storm formation than was anticipated at the beginning of the season, with strong vertical wind shear, or the change in direction and speed of winds at different heights in the atmosphere. When the shear is strong it tends to rip budding storms apart.

Another storm “killer,” as Bell called it, is the stable conditions that have also reigned over the Atlantic, with a general sinking motion — hurricanes need instability and uplift to fuel them.

The GOES East satellite caught this picture of Hurricane Arthur on the morning of July 3, 2014, a day before it was expected to make a direct hit on North Carolina.
Click image to enlarge. Credit: NOAA

The West African monsoon has also been subdued, which means fewer tropical systems sprouting off the coast of the continent and developing into cyclones.

“So we’re seeing a whole list of things,” Bell told Climate Central.

The environment could become even less favorable to would-be tropical cyclones if the El Nino that has been flirting with forming since March does finally come to fruition. El Nino tends to intensify the atmospheric conditions already holding back storm development, increasing wind shear and stability over the Atlantic.

“It would contribute to making even more unfavorable the conditions that are already unfavorable,” Bell said.

Right now, there is a 65 percent chance that an El Nino will form by late fall or winter, which is down from last month’s 80 percent chance, but still twice the normal probability of a winter El Nino, according to CPC forecasters.

If the El Nino does develop, the storm numbers are likely to be at the lower end of the forecast range, Bell said. If it doesn’t, they’re more likely to be at the upper end.

But, Bell warned, a below-normal season “doesn’t mean the season’s shut down.”

“If you’re a coastal resident you need to be prepared every hurricane season,” he said.