1991 Atlantic Hurricane Season - Season Summary

Season Summary

Before the start of the season, hurricane expert William M. Gray released his forecast for the year's activity, a yearly practice that he began in 1984. In early April, Gray anticipated a "mild" season with fewer than ten tropical storms, of which less than six would become hurricanes. Later that month, the Weather Research Center forecast ten named storms and six hurricanes, of which five would become major hurricanes while three would hit the United States. In early June, Gray released an updated report that predicted the formation of eight tropical storms, four hurricanes, and one major hurricane. The revised June total was very close to the actual season activity, with the exception of forecasting one fewer major hurricane. However, a later revision in August incorrectly anticipated less activity, when Gray predicted seven storms and three hurricanes.

Overall activity in 1991 was below normal. This was partially due to decreased tropical cyclogenesis from African tropical waves, which are troughs that move across the ocean with associated convection. In most seasons, the majority of storms develop from tropical waves. Of the season's twelve tropical cyclones, only five originated from tropical waves; in addition, only three of the eight tropical storms were from tropical waves, and none had the characteristics of a Cape Verde-type hurricane. From late April to late November, there were 73 tropical waves that exited the west coast of Africa. The total was higher than average, although many of them were poorly defined and had little thunderstorm activity. The waves traversed the Atlantic Ocean further south than normal, typically not becoming convectively active until moving across northern South America. Cyclogenesis was also suppressed by higher than normal wind shear, as well as low rainfall amounts across the Sahel. There were also no tropical storms in the Gulf of Mexico for only the third time in the 20th century, after 1927 and 1962. The season produced twelve tropical depressions, which was the lowest in five years. The eight tropical storms was the lowest amount in four years. Four of the storms developed into hurricanes, although for the first time in over 24 years none were from tropical waves.

The season's activity was reflected with a low cumulative accumulated cyclone energy (ACE) rating of 36. ACE is, broadly speaking, a measure of the power of the hurricane multiplied by the length of time it existed, so storms that last a long time, as well as particularly strong hurricanes, have high ACEs. ACE is only calculated for full advisories on tropical systems at or exceeding 34 knots (39 mph, 63 km/h) or tropical storm strength. Subtropical cyclones are excluded from the total.