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).
Kirk was first declared as a tropical depression this past Tuesday afternoon, and is now nearly a major hurricane. Leslie, born from another easterly wave Thursday morning, is rapidly on its way to becoming the season’s 6th hurricane. Both are located in the central Atlantic, and only Leslie poses a threat to land, and it’s remote.
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.
Hurricane Kirk became the season’s fifth hurricane yesterday morning, and as of 11 a.m. EDT this morning, is a Category 2 storm packing 105 mph sustained winds. It’s a very compact system centered about 1430 miles west-southwest of the Azores islands and moving toward the north-northwest at 13 mph.
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.
Tropical storm Leslie, formerly and briefly known as tropical depression 12, is a 65 mph tropical storm centered about 845 miles east of the Lesser Antilles and moving west-northwest at 17 mph.
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.
This season, the 12th named storm formed on August 30... one day AHEAD of the 2005 season, 1 day behind the 1995 season, and 22 days ahead of the 2004 season. Finally, we haven’t had a major hurricane (yet), but climatologically, the first one comes on September 4. As an update to where we stand on the climatological activity chart (above), today’s date is marked by the green line in the above figure... so the bulk of the season should still be ahead of us.
* Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.