Friday, September 21, 2012

"Active" In Numbers, Not Active In Intensity

There's been a fair bit of hay made about the "active" nature of the 2012 hurricane season in terms of quantity because were a large number of storms developing early in the season and because it was the "X" earliest season in which a certain letter storm would develop, blah blah blah.  While we were off to an active start in terms of tropical development from a quantity standpoint, the season's activity has slowed markedly in the last few weeks.

With September down to a bit more than a week to go before the calendar page gets ripped off, we're reaching the 60%, even 65% mark in the season.  In terms of storm numbers, we're one tropical storm behind the 1995-2011 average for tropical development of 15 named systems and eight hurricanes.  That benchmark of 15 and 8 is a more reasonable benchmark to compare to than the long-term averages of 10 and 6 for a couple of reasons -- lack of satellite data prior to the 60's and the current warm cycle in the Atlantic ocean.   Within the 'apples to apples' comparison of the last seventeen years, this season from a quantity standpoint is pretty close to normal.

However, in terms of Accumulated Cyclone Energy (ACE), which is a metric that gives points to storms based on intensity, we're running below the current seventeen year cycle.  This year's ACE is "just" 89 -- about forty points below the cycle average of 127.  One of the big reasons for that is the lack of major storms this year -- just one storm has made major hurricane status (Michael) this year and it just barely made it there.  We've also lacked a significant Cape Verde season this year thanks to dry air in the tropical mid atmosphere and some wind shear in the tropics.  The average season in the 95-11 cycle has had just under four majors a year.  Looking back, 1995, 2004, and 2005 were all hyperactive years in our cycle with high cyclone numbers in 1995 and 2005 and very high ACE tallies in all three years.

While the season still has a couple of months left to it, the odds for a "hyper" boost to the season's totals will dwindle markedly over the next few weeks as the influence of Nino in the Pacific continues to impact the deep tropics with unfavorable wind shear.  Storm development this year has been above average in the subtropical Atlantic and it wouldn't surprise me if any additional development this Fall takes place in the middle of the Atlantic from nontropical systems or upper level systems that acquire tropical characteristics.