WASHINGTON -- The Atlantic hurricane season has proved to be much tamer than the "above average," "very active" range government and private forecasts predicted last spring.
All the models used by the National Oceanic and Atmospheric Administration, university researchers and private weather services pointed the same way last May.
A steady supply of warm water in the tropical Atlantic (80 degrees or higher), a steady jet stream coming off Africa to deliver tropical waves that breed storms, and the lack of a Pacific El Nino that often generates winds that break up tropical storms pointed to as many as 20 systems developing in the next six months. As many as six or seven of those storms were forecast to become "major" hurricanes with top winds greater than 111 mph.
"Historically, this combination of climate factors produces an active hurricane season about 70 percent of the time,'' Gerry Bell, NOAA's lead seasonal forecaster said.
Instead, there have been 10 tropical storms, some that lasted only a few hours, and only two that reached hurricane strength. The first of those, Humberto, reached hurricane strength early on Sept. 11, just shy of the record for the latest date a storm has achieved winds greater than 74 mph. And no major storms have formed.
The first storm, Andrea, raked Florida and soaked much of the East Coast in early June, but caused no major damage. Several Gulf of Mexico storms -- Barry, Fernand and Ingrid-- caused severe flooding and hundreds of deaths in Mexico, but experts say the overall amount of energy released by all tropical systems so far this season is only about 30 percent of average because they've been so weak and short-lived.
"The season does look to be a huge bust,'' said Phil Klotzbach, lead author of Colorado State University's hurricane forecast, which initially called for 18 storms. "We're working hard to figure out why the season has been as dead as it is."
At NOAA, spokesman Chris Vaccaro said forecasters believe conditions conducive to storm formation have been offset by increased vertical wind shear, drier air sometimes mixed with dust from the Sahara, and sinking air patterns that prevent thunderstorms and tropical storms from forming.
And Vaccaro pointed out that the eastern Atlantic formation of Tropical Storm Jerry as the 10th named storm of the season is actually ahead of the average date a 10th storm forms -- Oct. 19.
"October is part of the peak period of the hurricane season and the season doesn't officially end until Nov. 30. And as Sandy showed us last year, devastating storms can still strike land during the second half of the season."
Historically, 53 hurricanes known to have formed since 1851 have come in October, with 16 of those considered major. And 21 hurricanes have crossed South Florida in October during that timespan, including Wilma in 2005 and a devastating though unnamed storm that hit Tampa Bay in 1921.
Still, Klotzbach says "unless we have the mother of all Octobers, this is going to be a below-average season. And I just don't see anything significant on the horizon. The later you get in the season, the factors are not that great for sustaining a major storm."
Klotzbach and other experts say there are several things that may have kept the Atlantic tropical basin dry and with winds inhospitable to hurricane development: an abnormal pool of cold water off Portugal; a drought in northern Brazil; strong westerly winds coming off Africa; inversions that kept air dry both near the surface and at higher altitudes; even a more northward track for waves coming from Africa.
At Florida State University, an associate research scientist with the Tim LaRow Center for Ocean-Atmospheric Prediction Studies, said his group's model joined everyone else in over-predicting (12 to 17 storms, 5 to 10 hurricanes). "It will be an interesting post-mortem to try and track down what made the models wrong. For instance, we don't account for Saharan dust, so if that turns out to be a big contributor, we might have to address that.
"But being off one year doesn't make or break a model or a forecast. We have to see if this is just noise in the climate system, or if there's something new we have to consider in seasonal forecast,'' LaRow said.