08 October 2015

State of the Season

Through October 8, the Atlantic has had 10 named storms, 3 hurricanes, and 2 of those became major hurricanes (Category 3+).  The strongest by far was Joaquin, the only depression to not become a tropical storm was TD9, and the shortest-lived storm was Bill.

The final advisory was just written on Joaquin last night as it transitioned to an extratropical cyclone, so I'll go over its fairly unique evolution.  Joaquin's origins could be traced all the way back to around September 17-18 as an upper-level low pressure system. While tropical development from upper-level "cold core" lows is not unheard of, it is rare for storms of that pedigree to become nearly Category 5 hurricanes.  Here is a quick timeline as a recap:

Sept 18: upper-level low identifiable
Sept 25: becomes disturbance of interest, or "Invest"
Sept 28: upgraded to tropical depression
Sept 29: upgraded to tropical storm
Sept 30: upgraded to hurricane
Oct 1: upgraded to major hurricane
Oct 3: strengthens to 135kts, just short of Category 5 status
Oct 7: weakens to tropical storm
Oct 8: final advisory written as extratropical cyclone

Water vapor images at select milestones in Joaquin's lifecycle leading up to its peak intensity.
Joaquin was the strongest hurricane in the Atlantic since Igor in 2010.  It was the first major hurricane to impact the Bahamas since Irene 2011.  It maintained hurricane intensity up to 40.5°N (same latitude as Long Island, NY).  In terms of Accumulated Cyclone Energy, or ACE, Joaquin racked up the same amount as all nine previous named storms combined!  That brings 2015 up to approximately 65% of an average season by this date... so still decidedly "quiet" overall.

Average seasonal cumulative ACE (purple) and 2015's accumulation so far (yellow).  
Even in distant 'model-land', the Atlantic looks quiet, and with a very strong El Nino in high gear in the central and eastern Pacific Ocean, tropical Atlantic activity should remain suppressed for the rest of the season.  However, suppressed does not necessarily mean dead.  October is historically still a month when extremely powerful hurricanes have formed and some of those made landfall.  As Joaquin proved, we can still get very intense storms even though the overall seasonal activity is below average.

Recent October hurricanes that reached Category 4-5 intensity are:
Gonzalo 2014
Omar 2008
Wilma 2005
Michelle 2001
Iris 2001
Mitch 1998

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