Tropical Storm Hermine rapidly developed and has moved into South Texas soaking rather moist ground from previous Hurricane Alex and TD2. Scattered areas of interest in the Atlantic include the remnants of Gaston and a couple African Easterly Waves about 3,000 miles from the United States mainland. Each has a low chance of tropical cyclone development in the next 48-hours according to the NHC. Long-range forecast models are bullish on wave number 3 eventually developing as it moves westward.
Accumulated Cyclone Energy to date (thru September 8) in the North Atlantic is 63. In a previous post (Earl Sputters…), the climatological trace of ACE showed the Atlantic being about 8-10 days ahead of normal (1950-2009 historical records). Many other images also float around the internet and show that the half-way point of the Atlantic season is roughly September 11.
From 1950-2009, there are many seasons in which Atlantic hurricane activity is very weak, with only a few storms of marginal strength which produce low amounts of ACE. Out of the last 60 seasons, there are 17 that have an ACE of less than 63 (including last year’s 53), which is coincidentally the tally through September 8, 2010. Thus, 2010 is number 18 from the bottom in terms of ACE so far, with still half the season to go. As each storm develops, more ACE will move 2010 up the rankings, but how far?
Figure: the percentage of the total season’s ACE left in the second half of the season (y-axis) as defined as September 11, with the bubbles size scaled by the overall season’s ACE. The mean is 48% but the standard deviation is 20% indicating a huge spread. Data source.
This figure exemplifies part of the reason why seasonal forecasting is so difficult for ACE in April or even in August. Tropical storms and hurricane activity in the North Atlantic varies intraseasonally as well as interseasonally in terms of genesis locations and track configurations, which are the critical components of the ACE metric (frequency x duration x intensity). The North Atlantic is sometimes called the “marginal basin”. The trick is to somehow gain a level of confidence in your forecast either through persistence or knowledge of the prevailing large-scale climate (i.e. ENSO, AMM, NAO, etc.) as NOAA, Gray and Klotzbach, and other long-range climate forecasting outfits.
Interestingly, the past 3-years have seen less than a quarter of their yearly output past September 11, the climatological peak of the season according to the past 60-years of data. Keep track of global tropical cyclone ACE at Ryan Maue’s FSU page.