As Isaac slowly decays over the U.S. continental interior, two freshly named systems are spinning up during the second most active Atlantic hurricane season to date on record (in terms of number of named storms).
The figure to the right shows the latest track model guidance from a large number of models (and ensembles of models) for both storms. There is very good agreement on Kirk’s track, and fairly good agreement on Leslie’s track at least for the next 3-4 days until the models diverge.
At the very least, the U.S. East Coast, southeastern Canada, and Bermuda should keep one eye on this storm for a possible encounter in 7-10 days.
Yesterday, it was a beautiful symmetric hurricane with an open eye, but today it’s a bit less organized and is forecast to gradually weaken as it passes over colder and colder water.
Though previously plagued by moderate amounts of vertical wind shear, large-scale environmental conditions now appear to be in Leslie’s favor. It should intensify over the next several days, very likely becoming a hurricane shortly. We have the luxury of time to monitor this storm before it’s a threat to any land.
Climatologically, an Atlantic hurricane season only produces 11 named storms, so with Leslie as the 12th named storm, we have crossed into an above-average season by that very basic metric. Not only did we reach the 12th named storm, we did so before the peak of the season, not at the end!
Since 1995, the Atlantic has been in a very active phase, so in 15 of the past 18 seasons, we’ve hit the “L” storm. But over a longer averaging period, it’s not so normal. The activity comes and goes in multi-decadal peaks and valleys of favorable conditions.
ACE, the accumulated cyclone energy, is a global standard metric for measuring tropical cyclone activity. It’s basically a scaled sum of the square of the wind speeds for all tropical storms and above. If we define a base climatology of 1981-2010, the annual average ACE is 106 (the units are 104 kt2, but that isn’t relevant). By this date, the average ACE is 32.3. In 2012, as of 8am on August 31, the ACE is 46.3, or 143% of average. And again, we haven’t even reached the peak of the season yet. The mega-seasons of 2005, 1950, 1995, and 2004 finished up with ACE values of 248, 243, 228, and 225, respectively.
* Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.