The 2013 Atlantic Hurricane Season officially ended on Saturday, and unlike some recent years, it's reasonable to assume that there will be no "after season" storms developing in the basin.
Over the past several weeks there has been much discussion about the failed pre-season expert outlooks for 2013. Early-season forecasts - and even the mid-season updates - called for a fairly-active hurricane season, following a trend that has been the general rule since the mid-1990s.
In the end, however, the 2013 season proved to be one of the least active hurricane seasons in quite some time.
Although there were 13 ‘named' storms - above-average for ‘named' storms in a season - only two achieved hurricane strength, the fewest in a season since 1982. What's more, these two hurricanes - September's Humberto and Ingrid - topped out as Category 1 storms. That means no ‘major' hurricanes during 2013, the first season that has occurred since 1994.
In addition to storm counts, another index that tropical researchers review is the Accumulated Cyclone Energy (ACE) index. This index estimates the wind energy produced by each ‘named' system over the course of its lifetime. By adding up the total energy for all ‘named' storms, the ACE provides a guide for comparing total storm energy across all seasons. Based on the ACE, 2013 is the ‘weakest' tropical season since 1983 (a season with only 4 ‘named' storms, but 3 were hurricanes). In hindsight, the 2013 season was far less productive in terms of tropical energy than the 13 ‘named' might suggest.
It's not that the season didn't get off to a good start. 2013's first ‘named' storm - Andrea - formed over the Gulf in early June and made landfall as a tropical storm along Florida's Big Bend. So there we were - a June storm in the Gulf and making a U.S. landfall - three traits that seemed to support the concern for another busy season, especially along the Gulf Coast.
Furthermore, you may remember that 2010, 2011 and 2012 all had 19 ‘named' storms. That persistence over the past three seasons was enough to get everyone's attention. Then add in several climate signals at the start of the 2013 Hurricane season that supported another active season: there was no El Niño (a hurricane ‘inhibitor' for the Atlantic Basin), early-season Atlantic sea-surface temperatures were normal to above-normal across most of the basin, surface pressures in the tropical Atlantic were normal to below-normal, wind shear was near-normal for the season's start, and tropical waves were developing regularly over the basin during the early weeks of the season.
So what happened?
Given the "supportive" factors listed above, it made sense to expect another active hurricane season. The climate signals listed above provide long-range guidance as to what to expect over the course of the season. But this year, some "unpredictable" factors trumped that list of supportive elements, namely "dry" air and subsidence.
The combination of "dry" air in the mid-levels of the atmosphere over the tropics coupled with subsidence (sinking air from above) serve as inhibitors for thunderstorm development, and without developing thunderstorms you simply can't have tropical systems.
So why didn't the experts anticipate the "dry" and sinking air? Because we simply don't have the meteorological skill to forecast these two features by more than a week or so ahead of time. While the "supportive" elements listed above tend to remain in place for weeks or months at a time once they become established, the atmospheric set-up for subsidence tends to come and go.
What made 2013 unusual was not the occurrence of "dry" mid-level air and subsidence in the tropics - these features come and go throughout the hurricane season. What made 2013 unusual was the persistence of these two features.
Since the science is not yet good enough to anticipate these features over the long-term, the general opinion was that these inhibiting factors in the tropics would eventually disappear, at least for some periods of time. Once the "dry" air and subsidence abated, the tropics would kick-in with more frequent and stronger hurricanes. So the tropical community waited … and waited … and waited …
In fairness to those pre-season forecasting groups, they've been doing an ‘okay' job with their projections in recent years: not great, but ‘okay.' Well, ‘okay' until the 2013 season.
What the 2013 Hurricane Season does tell us all is that we are still a very long way - maybe even decades - from being able to provide the long-range vision that many in the public sector seem to expect. That's why you prepare yourself, your family and your business interests for tropical weather each and every season.