NOAA’s 2011 Atlantic Hurricane Season Outlook

May 19, 2011

Like a college football fan who thinks preseason polls are meaningless yet gets excited when his team is in the top ten, I write about the hurricane season predictions even though I find them to be of dubious utility.

Today, the Climate Prediction Center branch of NOAA released their 2011 Hurricane Season Outlook.

Before getting into the numbers, it is worth noting that NOAA has changed their baseline numbers for what constitutes a normal/average season. Previously, they were based on averages from 1950-2000. They now use averages from 1980-2010 (The practice of using a 30 year average to define normal is standard when talking about rainfall, temperature, etc). This has the effect of increasing the number of named storms in a normal season from 10 to 12 while fractionally increasing the number of hurricanes and major hurricanes to 6.4 and 2.7 ,respectively.  Median Accumulated Cyclone Energy is  increased by a few points as well, to 92.8.  As their background note states, the increase in the number of named storms is likely a function of modern technologies allowing more marginal storms to be detected than would have been in the past.

Thus defined, the forecast calls for a 65% chance of an above normal season, 25% chance for a near-normal one, and a 10% chance of the season being below normal.  It predicts a 70% chance of  12-18 named storms, 6-10 hurricanes, 3-6 major hurricanes, and ACE ranging from 97-186.  If we take the means of these ranges we find they are anticipating 15 named storms, 8 hurricanes, 5 major hurricanes, and  ACE to be 142.

On these numbers, the forecast is similar to the ones issued from 2005-2008.  Only one of those forecasts was in the ballpark across the board. In 2005, along with everyone else, NOAA grossly under-forecast the record-breaking season. Like most they incorrectly forecast 2006 to be above average.  In 2007, they over-estimated the number of severe storms.  Along with everyone else, NOAA had a better performance in 2008.

Season NOAA Named Storms/Hurricanes/Majors Hurricane Forecasts Actual NOAA Accumulated Cyclone Energy Forecast Actual
2005 14/8/4 28/15/7 136 248
2006 15/9/5 10/5/2 149 79
2007 14/8/4 15/6/2 147 72
2008 14/8/4 16/8/5 136 145
2009 12/6/2 9/3/2 85 51
2010 19/11/5 19/11/5 186 166

Ironically, among those four years, the big 2006 over-forecast came when NOAA was most confident of an active season and gave a 80% chance of the season being so (contrast to 2008, when they gave only a 65% of the season being active).  The primary cause of the bust was the onset of El Niño conditions that was not anticipated in the forecast.

It seems the ghost of 2006 continues to haunt the forecasters as they have kept the possibility of sudden El Niño conditions in mind. While neutral conditions are expected to prevail, they are not a given:

The present uncertainty in the model forecasts is indicated by the large spread in predicted SST departures in the east-central equatorial Pacific during August-October 2011. Most models predict ENSO-Neutral conditions (defined by the CPC as SST departures between -0.5oC and +0.5oC) during this period. However, a few models predict weak La Niña conditions (SST departures cooler than -0.5oC), or even weak El Niño conditions (SST departures above +0.5oC).

This spread in the model forecasts, combined with the limited predictive skill exhibited by all such models at this time of the year, is a main reason why we are presently indicating only a 65% chance of an above-normal season. If El Niño does not develop, the probability of an above- normal Atlantic hurricane season will be even higher and the actual seasonal activity will more likely be toward the upper end of our predicted ranges.

The next forecast to be released that I use in my forecast comparisons is from the Colorado State team. It will be released the day hurricane season begins, June 1.


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