

Why did seasonal hurricane forecasts bomb this year?




A year for wimps: The 2013 Atlantic hurricane season. (National Hurricane Center)

Probably the best way to measure a season’s activity is to sum up the number, strength and duration of its storms, a value known as Accumulated Cyclone Energy. For the 2013 Atlantic season this value was 31.1, or about 30 percent of normal levels. Put another way, it was among the five lowest ACE values recorded since 1950.

What’s remarkable about this is that seasonal forecasters predicted a busy season. NOAA even predicted a possibly “hyper-active” season.


So what happened?

Here are some thoughts from Chris Hebert, the excellent hurricane forecaster with ImpactWeather, a company based in Houston.


There are a number of parameters that are used to predict seasonal hurricane activity. Parameters such as the location of the warmer water, average pressure in the deep tropics and the presence (or absence) of an El Niño. Last spring, the forecast was for warmer-than-normal water temperatures and lower-than normal pressure in the deep tropics (the main development region). In addition, it was predicted that water temperatures in the Nino 3.4 region of the Tropical Pacific would remain in the “cool-neutral” range through the season. The general thinking was that the warm water and low pressure from the Caribbean Sea to the coast of Africa combined with the cooler water in the Tropical Pacific (Nino 3.4 region) would all act to enhance hurricane activity in the Atlantic Basin this season. Thus, the predictions of a very active Atlantic hurricane season.

It is interesting to note that the predictions of all three parameters above verified very well. The warm water was located farther south than in 2012, pressures were quite low in the deep tropics all season and the Tropical Pacific remained cool-neutral through the season. But if the forecast of these predictive parameters verified so well, what happened to the hurricanes? It turns out that there is an additional parameter that was not generally considered when making seasonal predictions.

Back in September, once it became obvious that Atlantic activity would remain well below normal (no hurricanes yet), I began searching for the reason behind the general lack of tropical cyclones in the Atlantic. What I discovered was that moisture levels in the mid-level of the atmosphere (18,000 to 25,000 ft.) in the Atlantic was well below normal. Lack of moisture can certainly limit tropical cyclone activity. Looking further, I determined that not only was the atmosphere in the Atlantic basin dry, the entire Northern Hemisphere south of 30N was much drier than normal in the mid levels.

Anomaly for levels of specific humidity, in the mid-levels of the atmosphere, for July through September. (NOAA)

Anomaly for levels of specific humidity, in the mid-levels of the atmosphere, for July through September. In short, it was quite dry. (NOAA)

But why was the atmosphere so dry aloft? The cause of the dry air in the Atlantic appears to be sinking air produced by high pressure in the upper atmosphere. This sinking air reduced the available moisture and made the atmosphere across the basin more stable than normal. This was why hurricane activity was so limited in the Atlantic this season. To date, only two Category 1 hurricanes formed in the Atlantic this season, making the 2013 season the quietest in over 75 years in terms of the number and duration of hurricanes. The next question to ask would be about the predictability of such dry, sinking air months in advance. I was discussing this with Dr. Phil Klotzbach recently and he thinks such predictions are possible at least out for the next month or two. Whether or not such conditions are possible to predict for peak season (August and September) as early as April or May remains to be seen.

To further the theory that dry mid-level air was the inhibiting factor, let’s take a look at the West Pacific activity. I mentioned that tropical cyclone activity was well below normal in all basins as of September. The West Pacific Accumulated Cyclone Energy (ACE) was about 50% of normal through the end of September. Dry air aloft appeared to be inhibiting devleopment there as well. But in late September and early October, the West Pacific suddenly sprung to life. A record 8 typhoons formed in October, many of them quite intense. What had changed there? Checking the moisture levels in the West Pacific in late October, the dry air was gone. But which came first, the moisture increase or the enhanced typhoon activity? Digging deeper, I determined that there was a dramatic increase in mid-level moisture in the West Pacific just prior to the start of the enhanced typhoon activity. There was the answer. The typhoons didn’t cause the moisture increase, the moisture increase led to the increased typhoon development.

I’ll be attending the 31st Conference on Hurricanes and Tropical Meteorology in San Diego next spring. This is a once in every two years meeting of all the top researchers and forecasters from around the world (many from Asia/Australia, which is the reason for the west coast location). I’m hopeful that there will be some discussions of seasonal predictions at this and other conferences next spring. We learn something each year when we make seasonal hurricane forecasts. This year we learned of another parameter that can significantly impact seasonal activity. I’m hopeful that we will find a way to predict this new parameter far enough in advance to have confidence in the seasonal forecast in the coming years.


My thanks to Chris for taking the time to share this detailed and thoughtful analysis.

TL/DR: In the battle between tropical cyclones and dry air, dry air wins.

Eric Berger

7 Responses

  1. bradman says:

    We kept hearing about the presence of Saharan dust most of the season and how that was keeping activity down, but Mr. Hebert doesn’t mention it. Is the dust something that occurs at the mid-levels, and thus contributes to the dry air? Or is it an unrelated factor?

  2. jason says:

    I think that the reason the hurricane forecast bombed is that every year, the prediction is normal to above normal hurricane activity. Simply because the forecasters are so certain that “climate change” has brought more storms and more turbulent storms.

    the reality of most hurricane seasons is normal, and the past few have been less than normal.

  3. DerAdlerSteigt says:

    Isn’t sinking air also COOLER? Funny how that wasn’t mentioned.

    I wonder what hurricane season looked like during the Little Ice Age. Probably nonexitent.

  4. Mike says:

    I agree with jason 100 percent, except that I can’t remember the last time they even said “normal.” I remember telling somebody a couple of years ago who was panicked because he had heard on the radio that it was going to be an above-average hurricane year, “Oh, they say that every year; some years they are correct and other years they are not.” Sort of a variation on the “stopped clock is correct twice a day” meme. Oh, well, I guess it sells newspapers…

  5. rts says:

    The reason for no storm activity in this area is real simple. I need to replace my roof. I decided to wait out the Hurricane season before replacing it. Figured, if we got a strong storm I might get some insurance money to help with the cost. But, next year will most likely be the worst storm season in decades, as I can’t wait any longer to replace the roof.

  6. Ann says:

    I have a feeling I’m not going to win the prediction contest this year…yet again. :-)

  7. steve sokol says:

    A quick look at this theory and the map; would hint that the red areas
    (very low relative humidity in the mid-levels)between 40W and 50 W;
    and 5N and 15 N are also downstream of the Saharan Dust areas coming off Africa. Perhaps a combination of the author’s theory and the Saharan Dust
    theories is a possible explanation.