09 September 2014

Peak of hurricane season comes quietly this year

Although the Atlantic is fairly calm today, if you average activity over the whole 163 years of records, September 9 is actually the climatological peak!

There are numerous ways to define "activity" of course, and there are numerous time periods one could use to create the average.  But for this claim, I'm using a standard metric called Accumulated Cyclone Energy (ACE), and the full 1851-2013 period.

 It is not a sharp peak, but rather a broad hump that spans about two weeks (roughly the first half of September).  On average, Sep 2-16 generates 1/4 of the entire season's ACE!

[ACE is calculated for each named storm every six hours.  The higher the peak wind and the longer it lasts, the more ACE a given storm will accrue.  It's the sum of the squares of the six-hourly peak winds,... for example, if a storm's intensity is estimated to be 65 kt in an advisory, the ACE is 4,225 kt^2, but it's commonly shown in units of 10,000 kt^2, so that value would become 0.4225.]

Now let's look at just the average counts of tropical cyclones through the season (tropical storms, hurricanes, and major hurricanes).  The pattern is similar, but additional interesting details come through.  Major hurricanes (Category 3+) are almost exclusively found in Aug-Sep-Oct.

Rather than breaking it down into daily counts, the next figure shows the average cumulative counts.  This is handy if you wish to see that the third hurricane typically occurs by September 12, for example.

These figures and dates do change if the period used for the climatology changes.  One could look at more recent times, like 1950-2010, 1981-2010, or whatever is desired!  However, the fewer years that go into the averages, the more noisy the plots will become and the less robust the results will be.

Now to wrap up with current activity.  There are no named storms, but there are a couple areas of interest that could form in the coming 2-5 days.  The 5-day formation outlook from NHC highlights where those areas are now (X) and where formation might occur within the next five days (shaded blobs).  Note that these are NOT track forecasts!  The yellow area is given a 20% chance of development, and the red area a 70% chance.  I will probably send out a more complete update on these disturbances tomorrow if they continue to develop.

Visible satellite images of the two disturbances are shown here for reference:

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