Tropical depression 14 forms as Leslie and Michael fade out of the picture

Today starts the second half of the Atlantic hurricane season following yesterday’s calendar peak. It’s been an extremely busy season thus far, almost 50 percent more active than normal.

In recent weeks, storm after storm has developed with hardly a break. Presently we have three active systems although two are decaying. Leslie has transformed into a post-tropical (extratropical) storm as it pounds Newfoundland and Michael is a shell of its former self. Tropical depression 14 is the new kid on the block, which will become tropical storm Nadine should it intensify as expected.

Leslie


Visible satellite image of Leslie racing across Newfoundland. (NRL-MRY)

At 11 a.m., Leslie’s official intensity is 70 mph, and its tropical storm force winds extend out an impressive 345 miles from the storm’s center. However, the final advisory has been written on it since it’s now fully extratropical.

Newfoundland is no stranger to these fast-moving storms transitioning to intense extratropical cyclones. Just in the past ten years, they’ve gotten hit by six storms: Maria and Ophelia (2011), Igor (2010), Bill (2009), Isaac (2006), and Gustav (2002). Of course, the storms don’t know about land being there, but like Florida and North Carolina, Newfoundland “sticks out”, making it a more common target.

Michael

Michael, a Category 3 major hurricane five days ago, is now a ghost of a storm. It has moved over very cold ocean waters and into strong vertical shear, leaving behind a tiny low-level swirl in the clouds. It’s still hanging on as a 45 mph tropical storm, but will dissipate later today. It’s located 1090 miles west of the Azores.

Tropical depression 14


Model guidance is in good agreement on the future of this storm. It should begin recurving toward the north, and turn back toward the northeast by the time it reaches around 55W in three days. Conditions appear favorable for it to become the next tropical storm (Nadine) and then a hurricane.

Seasonal update

September is climatologically the most active month in the Atlantic. Conditions are generally favorable over the bulk of the basin, from the deep tropics in the eastern Atlantic, to the Caribbean Sea, Gulf of Mexico, and off the U.S. East coast.


Typical tracks for tropical storms and hurricanes during September (NOAA)

Prior to September, conditions can be favorable over a large area at times, but the extent and depth of warm ocean waters is less, large plumes of African dry air get ejected westward, mid-latitude troughs or upper-level lows introduce strong shear, etc. There are always exceptions and the interannual variability of these factors is large, but these are the long-term climatological patterns.

The ACE (Accumulated Cyclone Energy) as of 8 a.m. this morning stands at 77.2, which is 148% of an average season’s ACE on this date. As a refresher, ACE is a standard metric for tropical cyclone activity used around the world. In a nutshell, it’s a scaled sum of the squares of every storm’s wind speed from each 6-hourly advisory. Only tropical storms and hurricanes count, and the stronger they are and the longer they last, the more rapidly ACE accumulates. If two storms are occurring simultaneously, it racks up that much quicker. The average season finishes up with an ACE of 104. The graph below shows where 2012 stands in comparison to seasons since 2004 -- it’s lagging behind 2004, 2005, and 2008, but ahead of the rest.


Time series of ACE for the past nine years. (Ryan Maue)

* Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.

Brian McNoldy works in cyclone research at the University of Miami’s world-renowned Rosenstiel School of Marine and Atmospheric Science (RSMAS). His website hosted at RSMAS is also quite popular during hurricane season.

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