An active hurricane season is now likely, with a record warm Atlantic, forecasters say

With record-warm sea surface temperatures in the Atlantic’s hurricane-breeding regions, an already active season is likely to continue to produce above-normal numbers of tropical storms, government meteorologists warned Thursday.

NOAA bumped up its projected totals from its earlier forecast, even though well-above normal sea-surface temperatures in the tropical Pacific — an El Niño event that typically generates winds that keep Atlantic storms from developing — are expected to persist through the season.

Those competing forces, said Matthew Rosencrans, lead hurricane season forecaster with NOAA’s Climate Prediction Center at a media briefing, are “at the root of the uncertainty” in the revised outlook.

NOAA now is calling for a total of 14 to 21 named storms, those with winds of 39 mph or more. It also says six to 11 of those would become hurricanes, with winds of 74 mph or higher, and two to five of those, “major” hurricanes, with winds at least 111 mph. Five tropical storms and one hurricane already have formed.

In May, NOAA had called for a near-normal season. The long-term averages are 14 named storms and seven hurricanes, three of which become major.

» READ MORE: Cleaner air may be contributing to hurricane activity, researchers say

NOAA’s revised forecast is in line with the updated outlook posted last week by Philip Klotzbach, hurricane specialist at Colorado State University, a longtime tropical meteorology research center.

But, he cautioned, “The uncertainty this year is off the charts.”

Sea-surface temperatures in the Atlantic’s hurricane-development region are nearly 2 degrees Fahrenheit above normal, the highest they have been in observations dating to 1979, said Klotzbach.

That’s related to worldwide warming, and — perhaps surprisingly — cleaner air that has reduced aerosols and allowed for more solar energy to reach the ocean surface, according to NOAA scientists.

» READ MORE: The hedging began early with this year's hurricane outlooks

Research continues regarding “which one has the more prominent impact in the Atlantic,” said Rosencrans.

What is clear is that the warm waters would be a ready supply of storm fuel.

It is likely that a moderate to strong El Niño, with sea-surface temperatures 1.5 to 2 degrees Fahrenheit above normal over a vast expanse of the tropical Pacific, will take hold, Klotzbach said, and perhaps continue into the winter.

Interacting with the overlying air, those warm waters generate strong west-to-east winds in the upper atmosphere that can rip apart Atlantic disturbances before they can grow into hurricanes.

Moderate to strong El Niños have coincided with 10 Atlantic hurricane seasons since 1950, said Klotzbach. In only one case, 1951, was tropical-storm activity above normal, and then only slightly.

However, it’s the first instance since at least 1950 that such an El Niño has occurred when the Atlantic temperatures were above normal.

Rising seas and water levels are adding to storm surges and flooding, and worldwide warming also is making at least subtle contributions to hurricane strengths, Rosencrans said.

» READ MORE: Sept. 10 is usually the peak period for Atlantic hurricanes.

He estimated that the warming has added a 2% to 3% increase in tropical-storm rainfalls, and that is expected to grow to 7% by 2100. It also has increased storm intensities by 2%, he added.

The climatological peak of the season is about Sept. 10, but Rosencrans said that if the warmer waters persisted, the hurricane season could continue well into late fall.

The season ends officially on Nov. 30.