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NOAA Predicts "Big" 2006 Tropical Season, But Guesses Low Most of the Time

The National Oceanic and Atmospheric Administration issued their 2006 Atlantic Hurricane Outlook today. According to NOAA, this season stands an 80% chance of being an above normal season, a 15% chance of being a normal season, and a 5% chance of being a below normal season. Their definition of normal comes from a formula known as the Accumulated Cyclone Energy (ACE) index. This calculation is basically a function of how intense storms become and how long they last. The longer and more intense a storm is, the more it contributes to the ACE. Personally, I think this is the best forecast NOAA could could give. Rather than naming a specific number of storms, they say, to paraphrase, we think that given the current atmospheric conditions, the odds are this will be an above average hurricane season.

Of course, they also follow that up with a numerical forecast: 13-16 named (tropical) storms, 8-10 hurricanes, and 4-6 major (Category 3, 4, and 5 on the Saffir-Simpson scale) hurricanes. The average number of storms for the last 11 years, since we entered the active phase of the Atlantic multidecadal oscillation (AMO), is 15 named storms, 8.5 hurricanes and 4 major hurricanes. So, while this year's forecast is above the climatological average (since consistent records have been recorded in 1950s), it is just a tick above the average year since the tropics in the Atlantic became active again.

So, what does this mean to you? Let's take a look at how NOAA has perfomed recently in forecasting the number of named storms:

Year Forecasted Actual
2005    12-15     28
2004    12-15     15
2003    11-15     16
2002     9-13     12
2001     8-11     15
2000      11+     14
Forecasted and actual numbers of named storms (tropical storms and hurricanes) for the Atlantic Basin since 2000 Source: NOAA

So, it appears that NOAA has been a little conservative with their estimates of the number of named storms since the turn of the century, with last year being a glaring outlier. This year, NOAA made their highest prediction yet. However, I think NOAA may have once again been a touch conservative with their estimate, which is right around the average of named storms since 1995.

The first reason that pops into everyone's minds about hurricane season is El Niño. As you might recall if you are a reader of this blog, back in February my colleague, Bryan Woods, wrote about an impending La Niña event in the East Pacific. While it did appear to form in the late winter months, by April the conditions were back to a neutral state. There should be no (anomalous) effects from the Pacific's ocean temperatures on Atlantic tropical storm frequency this season.

That leads us to the pink elephant in the room: sea-surface temperatures (SSTs). No, I'm not talking about global warming. One of the major reasons for last season's improbable season was very high SSTs in the region between the Lesser Antilles and the west coast of Africa, especially early in the season.

SST_May05_a.gif
Figure 1. SST anomalies (differences from the average) for May 21, 2005. The warmer colors indicate warmer than average SSTs and the cooler colors indicate cooler than average SSTs (Source: NOAA)

Notice how much warmer than average it was over the Atlantic at the onset of hurricane season last year. Compare that image to this year:

SST_May06_a.gif
Figure 2. SST anomalies for May 19, 2006. Coloring as in Figure 1. (Source: NOAA)

While it is still warmer than average in the tropical Atlantic, it's not quite as warm as last year. So, while I don't think we will have a season as bad as last year's, I do think this year will be a little bit above what NOAA is predicting, due to the warmer than average SSTs in the Atlantic. If I had to throw out a number (which I'm not necessarily excited about, but since it's a blog, I guess I have to), I'd predict between 15 and 19 storms for this coming season. Oh, and if anyone tells you that we are going to have 13 of those make landfall and that the East Coast will be destroyed, they are just trying to scare you.


Posted by Adam Moyer | Permalink