BATON ROUGE, LA (WAFB) - September 10.
From a historical perspective, it is the "climatological peak" of the hurricane season and roughly the statistical mid-point for seasonal activity. The National Hurricane Center (NHC) indicates that, on average, we could expect six 'named' storms, three hurricanes and one 'major' hurricane in the Atlantic Basin by this point in the season.
However, remember that the NOAA experts - and virtually every other credible group that issues seasonal forecasts - have been calling for "below-average" activity since the spring and continued with that chant as recently as last month.
These forecasts for reduced activity were largely driven by the expected presence of El Niño - signaled by warmer-than-normal waters over the eastern equatorial Pacific - through the heart of the hurricane season. Indeed, the latest projections for the current El Niño suggest that we could be headed towards one of the "strongest" El Niño events in the modern record (records back to 1950).
During El Niño, we tend to see an increase in mid- and upper-level wind shear over the Atlantic Basin - think of it as a "downstream" response in the upper-level west-to-east atmospheric flow from the eastern Pacific. The presence of the Pacific's El Niño phase is also linked to a tendency for sinking air (subsidence) in the mid- and upper-levels of the atmosphere over the tropical Atlantic. Both wind shear and subsidence are well-known inhibitors for tropical development and intensification.
In addition, sea-surface temperatures (SSTs) over the tropical Atlantic have been and are expected to remain a little cooler-than-normal. While SSTs over the tropical Atlantic are well above the 'rule-of-thumb' threshold of 78 to 80 degrees Fahrenheit for tropical cyclone support, cooler-than-normal SSTs suggests a little less energy for developing systems (warm waters are the "fuel" for tropical cyclones). Plus, cooler SSTs in the tropics are linked to stronger low-level trade winds over the basin. Like wind shear and subsidence, stronger (faster, more persistent) trade winds can also reduce the ability for storms to form and strengthen.
Lastly, we've been seeing a fair amount of "dry" air entrainment (ingesting) this season with many of the storms. Subsidence and lower SSTs can result in "drier" (less humid) air at the low levels of the atmosphere. That drop in atmospheric moisture means that there is less water vapor for tropical thunderstorms to "feed" on.
Yet, even with these inhibitors in place, storm counts are running above-average through Thursday.
Henri is the eighth 'named' storms of the 2015 Atlantic Season. Danny and Fred both achieved hurricane strength, with Danny briefly becoming a Category 3 ('major') hurricane on August 21. So, from a purely numbers perspective, the season appears to be running not just ahead of the forecasts but above-normal, too.
However, if we dive a little deeper into the data, we find that high numbers do not always mean "active" in terms of total tropical energy during a hurricane season. The ACE - Accumulated Cyclone Energy index - gives us a way to measure the total storm energy accumulated across all of the 'named' storms during a season.
While storm counts are above-average through September 10, the ACE for the season is only about half of the norm at this stage.
How can that be? Above-average storm numbers, including a Category 3, yet the total combined energy of these eight 'named' storms is only half of the normal for mid-September?
Think of it as if the tropical inhibitors we mentioned earlier are doing half of their expected work. While storms are still managing to form, and we've even had two hurricanes in the basin already, the "life-span" of most of the 'named' storms so far have been on the short-side of normal. Shorter life-spans means less total energy produced for the ACE.
Obviously, the life-span of a 'named' storm can be highly variable, ranging from just a few hours to well beyond 15 days (at least seven Atlantic systems have lasted for 20 days or more at tropical-storm or hurricane strength). As a rough guide, 'named' storms in the Atlantic typically last for seven or more days, on average. (This includes storms whose life-span has been cut short due to landfalls.)
This season, only Danny (seven days) and Fred (eight days) have lasted that long. The average life-span for all eight storms thus far is just five days - and that assumes that Henri retains tropical storm strength into Saturday. So, while numbers are up for the season at this point, the atmospheric inhibitors (El Niño, etc.) have been cutting storm durations short. And, we've no complaints there.
Here are a couple of other interesting points on the current season. May's "pre-season" Ana got the 2015 season off to an early start and was the first US tropical landfall since Arthur in July of 2014.
June's Tropical Storm Bill made landfall in Texas, the Gulf Coast's first landfall since June 2013's Andrea hit the 'Big Bend' of Florida. In addition, Bill remains the only 'named' system in the Gulf so far this season, yet on average, roughly one-in-three Atlantic tropical systems are Gulf storms. Gulf Coast residents have no complaints about that either!
What about the rest of the 2015 season? Does the Gulf stay quiet? Do El Niño and the other inhibitors deliver an early end to the season after the quick start?