FLHurricane.com: Season Over?!?

I’ve observed some comments about the lack of tropical cyclone activity so far this season when compared against the high pre-season storm total forecasts. While I was one of the few that felt that this season would not be hyperactive (and I haven’t changed my thoughts on that), the season is far from over – in fact it has barely begun.

In the past (1880-2009), by July 25th only 13.3% of the total number of storms (1 May – 30 Nov) had been observed. That means the season still has about 87% to go, thus it is premature to suggest that the season is over. However, it is not premature to suggest that a hyperactive season of 18 or more named storms may not occur. Conditions in the Atlantic Basin are still quite hostile with significant areas of subtropical windshear, evidence of an active Saharan Air Layer (although it looks like it could finally be starting to wane), and a equatorial easterly low-level tradewind that continues to dive to the west southwest off the west African continent. Until these negative factors modify, the likelihood of having a new storm every week is pretty slim.

If you look at all of the storms since 1880 (all 1,223 of them) you don’t reach the halfway point in the season until September 16th. defines that midpoint date as September 10th, but they only examine the totals during the satellite era (starting in 1966, although the era actually started in mid 1964). Twelve of the most active seasons (75% of the total) and one of the least active seasons (5% of the total) have occurred during the satellite era. Of the 130 seasons from 1880 – 2009, 12% had a seasonal total of 14 or more storms and 14% had a seasonal storm total of 5 or less. Since highly active seasons are just as uncommon as highly inactive seasons, the forecast skill for predicting either extreme is rather poor.

The average number of storms for the entire period (1880-2009) is 9. Using that long-term period, the average date for the first storm was July 3rd, the average date for the second storm was August 1st and the average date for the third storm was August 18th, so currently we are still ahead of the average based on the activity thus far this season. From 1965 – 2009 (that satellite era again) there have been exactly 500 storms in 45 years – and that’s where the current average of 11 named storms per year comes from.

The most active seasons were: 2005 (28), 1933 (21), 1887 and 1995 (19), 1969 (18), 1936, 2003 and 2008 (16), 1916, 2000, 2001, 2004 and 2007 (15), and 1953, 1990 and 1998 (14).

The least active seasons were: 1914 (1), 1930 (2), 1929 (3), 1883, 1884, 1890, 1917, 1925 and 1983 (4), and 1902, 1904, 1905, 1907, 1910, 1919, 1920, 1922, 1939 and 1962 (5).

Its interesting to note that low activity seasons can cluster into periods of multi-year inactivity just like active seasons can cluster into periods of multi-year increased activity – which does lend some support to the notion of a 25 year cycle (or a 20 year cycle, or whatever it turns out to be). In the long-range forecasting weather business, patience prevails.
ED