Visible satellite image of Tropical Storm Henri from 8:15am EDT today. The center of the surface circulation is marked with a blue “L”.

It’s usually the most active time of year for tropical storms in the Atlantic and, right on schedule, Henri has been born.

Henri began as Tropical Depression Eight early Wednesday morning about 220 miles southeast of Bermuda.  It was upgraded to Tropical Storm Henri (pronounced ahn-REE) late Wednesday night (Eastern time) but is not expected to become a hurricane.

Models and the National Hurricane Center forecast it to slowly head north over the next couple of days then transition to an extratropical cyclone as it gets whisked away to the northeast by a cold front this weekend.

At 5 a.m. EDT today, Henri’s peak winds were estimated at 40 mph and it was nearly stationary.  It is very disorganized and sheared from the south, as is evident in the satellite image above.  But the shear is forecast to weaken tomorrow which should allow Henri to strengthen a bit more in the short-term.


Probabilities of tropical storm force winds over the next five days. (NOAA)

September 10 is the so-called climatological peak of for tropical storms in the Atlantic, when we’re most likely to see at least one active named storm. So this year is no exception.  The statistical peak for storms attaining hurricane intensity was Wednesday, and for major hurricanes (Category 3+) Tuesday.

The climatological peak of Accumulated Cyclone Energy (ACE) – a measure which accounts for the strength and duration of all storms, was also Wednesday (see graph).

Note that tropical cyclone activity typically ramps down slowly through the end of October, and is quite low during November, the final month of the official hurricane season.


Although the number of named storms is above average for this date (we’re at 8, which would normally be seen on October 16), the storms that have formed have generally been on the weak side, so the ACE is only around 50 percent of average for this date.

In the chart below, the actual storm counts as of September 10 are shown by the colored dots, and the cumulative activity for each intensity bin are the shaded curves.  The exact dates and counts vary depending on which period of record one uses, but here I’m using the 1851-2013 record (adding 2014 wouldn’t change anything).


Typically, El Niño acts to reduce — not eliminate — Atlantic tropical cyclone activity, and this year fits that description very well.  Although storms are forming, strong vertical wind shear across the basin has limited their development and duration.

The only other feature we’re watching is an easterly wave that has just entered the far eastern Atlantic today.  It was generated in the Ethiopian Highlands last Friday and remained a vigorous feature during its long trek across Africa.  Should it develop, the next name on the list is Ida.


Visible satellite image over the west coast of Africa from 8:30am EDT. (RAMMB)