Today is September 3rd which is pretty much right in the middle of the Atlantic hurricane season. The peak of the season climatologically is Sept 10th but we are close enough. Most people are wondering why the season has seemed so quiet so far? The answer is complex it’s both part science but also in large part a communication failure of how we focus too much on Hurricane Seasonal Forecasts of named storms. For a long time I have not been a fan of all the mass exposure the seasonal forecasts get. The number of named storms is just a horrible metric of seasonal severity and intensity. The general public does not care how many storms there are in the entire Atlantic they simply care whether or not they will affect them. So 15-20 “fish” storms are irrelevant to the public if none of them impact the U.S. Especially when all the news coverage seems to be about a “bad” hurricane season based on the high number of storms forecasted. The problems arise when people think something is going to happen and it doesn’t they lose their sense of being prepared.
There are countless examples of seasons where we get few storms but 1-2 are impactful. Case in point is in 1992 when Hurricane Andrew was the 2nd most devastating storm to hit the U.S. It was just the 1st of 6 named storms that entire season.
Then in 2010 we had 19 named storms which is the 3rd most active on record in the Atlantic. Though that year just 1 weak tropical storm made a U.S. landfall that season.
So to the public which season was worse? Andrew with a devastating impact and only 6 storms or 2010 with 19 storms and no impacts? I think the answer is pretty simple but we continue to focus on the numbers of storms. The build up of the seasonal forecasts by the media further highlights the problems. Often missing in the coverage is the perspective on how big the Atlantic is and how the number of storms doesn’t equal intensity or severity of impacts on the U.S. Also missing is communicating the unknowns of seasonal forecasting and how they are still not proven accurate enough to gauge how hurricanes could or will impact you.
Inflation of Named Storm Numbers:
Another issue that is arising is the inflation of the number of named storms. It’s kind of a running joke that we seem to be naming any thunderstorm that swirls for a few minutes in the Atlantic. This isn’t some grand conspiracy by the government to meet their seasonal forecasts for named storms. It’s a simple matter of us having better tools to diagnose tropical systems then we have ever had before. With the modern satellite observations & coverage tools like Scatterometers along with GPS Dropsondes, Doppler Radar, Hurricane Drones, etc. We seem to have an ever growing bevy of tools to name a storm system. We had a similar thing happen with tornadoes with the advancement in Doppler radar technology. The number of low-end tornadoes being reported increased tremendously. The Storm Prediction Center wisely knew there was an observational bias now and adjusts their numbers for this inflation. I think it’s time for the NHC (National Hurricane Center) to maybe think about something similar . If you think I’m crazy just look at this season so far. We have had 6 named storms which seems like pretty normal until you look at their average minimum pressure. For the 6 storms so far this season their average minimum central pressure has been 1003.8 mb. Anything above 1000mb is a fairly weak tropical system in terms of central minimum pressures. It appears to me low end storms are getting named far more frequently than ever before.
I went back and looked at just the past 10 years and about 26% of all the storms that have formed in the Atlantic over that time period have had a minimum central pressure above 1000 mb. Remember this is the lowest pressure in the life span of the storm so many times the pressure were actually much higher.
This isn’t meant to be disparaging of the naming of any of these storms. It just seems logically that we are naming more weak storms than we have in the past due to our advancements in both observations and meteorological understanding of tropical meteorology.
I’ve always been a fan of ACE or Accumulated Cyclone Energy to gauge how a season is going. The ACE of a season is calculated by summing the squares of the estimated maximum sustained velocity of every active tropical storm (wind speed 35 knots (65 km/h) or higher), at six-hour intervals. (Yeah Math) It’s really a measure of how strong each storm is and how long it sticks around. Where are we this year?
So far this season ACE has been 8.3 when 37 is average year to date. ( Graphic via Dr. Ryan Maue)
So what’s going on this year?
It’s been odd to say the least and many forecasters are stumped. I personally think the large and persistent 500 mb ridging early in the season over the Atlantic has warmed the mid levels of the atmosphere deterring vertical instability. At one point in June there was 600 dm ridge over the Atlantic which is more often seen over land than the ocean. For whatever reason vertical instability has been well below the long-term average. This vertical instability allows for thunderstorms to grow and maintain themselves.
The other big factor which is not completely independent of the instability problem has been the dry air and dust that has been so persistent. I don’t recall a year with such frequent and large amounts of dust and dry air coming off Africa into the tropical Atlantic. This is called the SAL(Saharan Air Layer) and while it’s normal to have some this year it’s been excessive. This dust and dry air serves to limit tropical development when it’s present.
So what going to happen the rest of the season?
Things will take a while longer to get going but it’s very likely we could have the latest forming hurricane on record this season. I just don’t see things getting active until around or after Sept 11th which is when in 2002 we got Gustav the 1st hurricane that season. We will have a long way to go and it only takes one storm hitting your location for a bad season. I think the Cape Verde season will continue to struggle but development close to the U.S. is more likely. Far removed from the SAL and the stable air in the central Atlantic. The good news is hurricanes never sneak up on you and you should always prepare for the worst and hope for the best.