Tropical storm Henri gives us a total of eight named storms thus far in the Atlantic hurricane season. This is getting us close to the 100+ year average of about 10 named storms. We’ve only had two hurricanes form from those eight named storms – one of which became a “major” hurricane with 115 mph winds, Danny. Seems like the season is a little busy? Yes and no.
Let’s take a closer look at the real measure of the hurricane season from an energy output perspective. We call this ACE or Accumulated Cyclone Energy. It’s a score, like a college basketball team achieves in a single game. Each player is like a named storm. Some will score big and lead the way while others may get a lot of playing time but only add a few points to the overall total. In the end, you may have 7 or 8 players who rotated in and out for the night but only a few truly had an impact on the final score. This is how the hurricane season works, in a general sense anyway.
ACE is simply a way to measure how long each storm lived and how much wind energy it generated. It has nothing to do with landfall, dollars cost or lives lost. It’s all math and is totally objective. In short, it is a great way to sum up the quality of a particular hurricane season, not just the numbers of named storms.
A typical Atlantic hurricane season will yield a final ACE score of around 100 points (give or take a few). Some have been hyper-active with 20o+ points in a single season (2004 and 2005 come to mind).
For this season, Colorado State University’s hurricane seasonal forecast predicts a final ACE score of around 35 points, well below the average. Right now, the score is roughly 25.
Why does this matter? For one, it helps to better define how a particular hurricane season behaved. We could have 12 to 15 named storms but if most are weak and short lived, then the energy output is relatively low and thus you can have a seemingly busy season with a fairly low ACE score.
Conversely, a season like 1961 which featured “only” 11 named storms racked up an ACE score of 211. This is because there were seven, yes seven, MAJOR hurricanes that season. The stronger the hurricane, the faster the ACE accumulates over time.
The largest single ACE event for the Atlantic was for a hurricane in 1899 which scored 73 points of ACE – all by itself. This is higher than many entire hurricane seasons from just ONE hurricane!
Then you have a season like 2015. So far, eight named storms and things have seemed pretty busy since about the end of August. In fact, we’ve had something to talk about or track every day since about August 20. Might seem like an active season to some. The ACE argues otherwise. Only Danny really amounted to creating any significant ACE points (around 9) but it was fairly short-lived as shear coming out of the Caribbean put a quick end to Danny’s run.
Had Erika lived up to the various computer models’ forecasts of being an intense hurricane, it would have certainly piled on perhaps 20 ACE points, maybe more. Instead, Erika died out and scored very low: 3 total ACE points.
This tells us a lot about how the season is behaving. The low quality of the storms and the short-duration hurricanes can be looked at as the reason why the ACE points are so low even though we’ve had plenty to track.
In the end, ACE says nothing for impact and we need only look to Dominica for that fact. Heavy rain took a devastating toll as Erika moved through and it is likely, in my opinion, that it will be retired from the list of names due to the damage and loss of life it caused, despite it’s poor ACE showing.
I bring all of this up as we are now at the climatological peak of the Atlantic hurricane season. Henri is churning out over the open water well away from Bermuda. It will score only a few ACE points since it is not likely to become a hurricane. We’ll probably see two or three more named storms before all is said and done which will be above what most experts predicted for the season as a whole. However, unless there is a particularly intense hurricane at some point, lasting for a few days, it is doubtful that the season will be memorable from an ACE perspective, at least not from the high end of things. You never know, all it takes is for a hurricane to form and just roam around the Atlantic for a week and we could end up with an ACE score closer to the long-term average.
As far as the tropics go, Henri is a non-issue except for shipping lane interests in the North Atlantic. Elsewhere, there continues to be talk of a possible western Gulf of Mexico storm or hurricane next week. Right now, it’s just that, talk and speculation. The global models have not been much help this season with predicting genesis of storms and then their subsequent intensity. So let’s just wait and see after this cold front comes down and quickly lifts out what happens. It’s peak time of the hurricane season, everyone should be paying attention anyway – if something develops, we’ll deal with it then.
I’ll have more tomorrow morning.
M. Sudduth 11:20 AM Sept 10