As the peak of hurricane season arrives Tuesday without a major storm having formed, forecasters are starting to rethink predictions of a busy year for high-powered tropical systems.

As of the National Hurricane Center's 5 a.m. update, Tropical Storm Humberto was located roughly 150 miles west of the Cape Verde Islands, at latitude 14.4 north and longitude 26.6 west. The storm was moving west-northwest at 9 mph, expected to turn more to the northwest and slow later today.

Forecasts call for Humberto to reach hurricane strength later today. The storm's current maximum sustained winds are near 65 mph, with higher gusts, and tropical storm-force winds extend outward as much as 80 miles.

Meanwhile, Tropical Storm Gabrielle has reformed in the Atlantic Ocean, some 165 miles south of Bermuda. Gabrielle is moving to the north at roughly 14 mph, a track expected to continue through the night before the storm turns to the north-northwest and slows before reaching Bermuda tomorrow. Maximum sustained winds are 40 mph, and extend outward up to 70 miles.

Depending on what happens with Tropical Storm Humberto this week, 2013 could mark the slowest start to a hurricane season by one measure in more than 100 years.

Humberto formed off the coast of Africa Monday and is expected to curve out to sea, staying far away from Florida. The system is only notable because forecasters believe it has a good chance of becoming the first hurricane of the season. If it does, it would be one of the latest first hurricanes on record.

On average, two hurricanes should have formed by now. Past seasons have started slow and finished with a flurry of tropical cyclones, but the window is starting to close.

Phil Klotzbach, a research scientist with Colorado State University's Tropical Meteorology Project, has been perplexed by the hurricane season's progress so far. Klotzbach predicted there would be 18 named storms this year, including eight hurricanes. The average is 12 named storms and six hurricanes.

Other forecasters, including the National Oceanic and Atmospheric Administration, also looked at the conditions that contribute to storm formation earlier this year and similarly predicted a busy season.

"As with any statistical model, there are going to be busts in some years," Klotzbach wrote in an email, adding: "I'm certainly going to be doing a lot of study over the next couple of months to try and figure out exactly what happened with this year's hurricane season. It's a very strange one so far!"

Hurricanes feed off moisture, but there has been an unusual amount of dry air at mid-levels in the atmosphere this year.

Additionally, strong upper level westerly winds have hampered hurricane development. Such winds usually are less prevalent during years when the El Nino weather pattern is absent, as it is this year.

"The combination of wind shear and dry air have certainly been the surprise factors," said Accuweather meteorologist Dan Kottlowski.

Kottlowski's team predicted 16 named storms this year and eight hurricanes. He no longer believes that many hurricanes will form.

"We do not think we will have an above-normal year for hurricanes" anymore, he said.

Yet in some ways this has actually been a busy year for tropical cyclones. Humberto is the eighth named storm of the season. That mark usually is not hit until Sept. 24. But none of the storms have generated sustained winds of 74 mph, the minimum required to qualify as a hurricane.

There are still more than 2 1/2 months left in the hurricane season, but the tropics must heat up fast for an above-average season to materialize.

The probability of major storms forming starts dropping after today. Hurricanes are rare after late October.

Still, it's not unusual for a series of storms to form quickly in September and October, even after a slow start to the hurricane season.

The first hurricane of 2001 did not form until Sept. 9, but the season ended with nine hurricanes and 15 named storms.

"We're only at the midway point," said Hurricane Center spokesman Dennis Feltgen. "We still have a lot of season to go."

The record for the latest first hurricane of the season is Oct. 8, 1905, although before satellites were invented some storms likely went undetected if they stayed out at sea. The six-month hurricane season ends Nov. 30.

The latest first hurricane of the modern satellite era is Hurricane Gustav, which formed on Sept. 11, 2002.

Forecasters are predicting that Humberto will become a hurricane late Tuesday or Wednesday, possibly eclipsing Gustav but leaving 1905 in the record books.

"It's the old baseball saying: Records are made to be broken," Feltgen said. "This is a record I don't mind breaking."