By Tom Brown
MIAMI (Reuters) - The 2013 Atlantic hurricane season looks set to go down as a big washout, marking the first time in 45 years that the strongest storm to form was just a minor Category 1 hurricane.
There could still be a late surprise in the June 1-November 30 season, since the cyclone that mushroomed into Superstorm Sandy was just revving up at this time last year.
But so far, at least, it has been one of the weakest seasons since modern record-keeping began about half a century ago, U.S. weather experts say. Apart from Tropical Storm Andrea, which soaked Florida after moving ashore in the Panhandle in June, none of this year's cyclones has made a U.S. landfall.
That meant relief for tens of millions of people in U.S. hurricane danger zones. But 2013 has been a bust for long-range forecasters who had predicted a stronger-than-usual burst of activity in the tropical Atlantic.
It has been "a very strange sort of year" in the unpredictable world of cyclones, said Jeff Masters, a hurricane expert and director of meteorology at Weather Underground (www.wunderground.com).
"We've been in this multi-decadal pattern of activity but it just didn't happen this year," Masters said, referring to the prolonged period of increased hurricane activity that began in 1995.
That period is still playing out, fed primarily by warm ocean temperatures in the tropical Atlantic that fuel hurricanes. But instead of increased activity, 2013 almost seems like a year when an enormous tranquilizer dart was fired into the heart of the main breeding ground for hurricanes.
A confluence of factors, including abundant sinking air and dry air, and possibly dust flowing out of North Africa's Sahara desert, kept a lid on hurricane formation in 2013, according to many cyclone experts.
That wreaked havoc with most leading seasonal forecasts like the one issued by Colorado State University on August 2. The errant forecast said 2013 would see above-average activity, with eight hurricanes and three that would develop into major hurricanes of Category 3 or higher on the five-step Saffir-Simpson intensity scale.
An average season has six hurricanes and three major hurricanes. But an August 8 outlook from the National Oceanic and Atmospheric Administration called for six to nine hurricanes, three to five of which would become major hurricanes.
There were two short-lived Category 1 hurricanes this year, making it the first Atlantic season since 1968 when no storm made it beyond the first level of intensity, according to the National Hurricane Center.
It has also been a year marked by the fewest number of hurricanes since 1982 and the first since 1994 without the formation of a major hurricane.
In terms of so-called "Accumulated Cyclone Energy" (ACE), a common measure of the total destructive power of a season's storms, 2013 ranks among the 10 weakest since the dawn of the satellite era in the mid-1960s, said Dennis Feltgen, a spokesman for the Miami-based National Hurricane Center.
"The ACE so far in 2013 is 33 percent of normal," he said.
Long-range hurricane forecasts are eagerly awaited in U.S. financial and energy markets, which quiver every time a storm bears down on the U.S. oil and gas-producing region of the Gulf of Mexico.
Phil Klotzbach, a Colorado State University climatologist, readily admits that the forecasts are based on statistical models that will "occasionally fail," since the atmosphere is chaotic and subject to fluctuations that cannot be predicted more than a week or two in advance.
But Klotzbach and other experts say the models, and seasonal forecasts, still provide useful insight into something as unpredictable as extreme weather even if they do not always pan out.
"Obviously, individuals should not plan differently for a specific season based upon a seasonal forecast," Klotzbach told Reuters by email. "They are purely there to satisfy the public's curiosity based on our best knowledge of how large-scale climate features impact tropical cyclone seasons."
Despite the season ending with a whimper, Masters said long-range forecasts are still worth betting on.
"They have a point, as long as you understand their limitations," Masters told Reuters.
"You expect that they will have bust years, like this year. That's part of the game," he said. "But if you consistently bet the way they're going, then eventually over the long run they'll pay off."
(Reporting by Tom Brown; Editing by David Adams and James Dalgleish)