How Active Was 2011?

November 30 was the official last day of the 2011 Atlantic hurricane season—an exceptionally active one from the perspective of basinwide activity, but another below-average season in terms of U.S. hurricane losses. Persistently high sea-surface temperatures and lower-than-average levels of wind shear contributed to the formation of 19 tropical storms (one unnamed)—the third-highest count on record. Hurricane and major hurricane formation, at seven and three, respectively, were slightly above average.

Interestingly, only 37 percent of tropical storms achieved hurricane status in 2011, compared to the long-term average of 55 percent. This hurricane-intensification metric, however, is highly variable from year to year, and many past years have had similarly low proportions. What was unusual about 2011 compared to past seasons with a low rate was the anomalously high count of tropical storms. Because conditions that favor tropical-storm development are typically the same ones that help them intensify to hurricane or even major hurricane status, past years with proportions of less than 40 percent also had average or below average counts of tropical storms.

Part of this apparent anomaly can be attributed to the presence of dry air and limited tropical moisture in the main development region for Atlantic hurricanes during periods of tropical-storm development. Thus, while many past years have demonstrated that a high count of tropical storms does not necessarily mean a high number of landfalls, 2011 has shown that more tropical storms does not even necessarily mean more hurricanes.

Translating Basinwide Activity to Landfalls and Loss

Let's now look at how the season measured up to pre- and midseason forecasts. Figure 1 shows that many forecasting organizations, even later in the year, underestimated the number of tropical storms and overestimated the number of hurricanes. This is partly because most predictions of hurricane counts apply the long-term hurricane intensification rate (55 percent) to the predicted count of tropical storms. So, in a year like 2011 with an intensification rate that differs significantly from the long-term average, the predictions are not likely to verify well compared to observation.