Dr. William Gray, a noted expert in long-range hurricane forecasting, issued his revised forecast for the remainder of the 2009 hurricane season, and it is good news: A below-average season, about 85% of the long-term average in terms of activity, is expected. This is primarily due to a developing El Nino, which increases vertical wind shears which are hostile to hurricane growth.
Here is a portion of the text from his report, posted on the Colorado State website August 4, 2009:
Information obtained through July 2009 indicates that the 2009 Atlantic hurricane season will be less active than the average 1950-2000 season due largely to the development of an El Niño. We estimate that 2009 will have about 4 hurricanes (average is 5.9), 10 named storms (average is 9.6), 45 named storm days (average is 49.1), 18 hurricane days (average is 24.5), 2 major (Category 3-4-5) hurricanes (average is 2.3) and 4 major hurricane days (average is 5.0). The probability of U.S. major hurricane landfall and Caribbean major hurricane activity is estimated to be below the long-period average. We expect Atlantic basin Net Tropical Cyclone (NTC) activity in 2009 to be approximately 85 percent of the long-term average. We have decreased our seasonal forecast slightly from early June.Complete report (38 pages): (Link)
This forecast is based on an extended-range early August statistical prediction scheme that utilizes 106 years of past data. Analog predictors are also utilized.
We have witnessed the development of an El Niño event over the past couple of months. These conditions are expected to intensify to a moderate El Niño over the next few months. El Niño events tend to be associated with increased levels of vertical wind shear and decreased levels of Atlantic hurricane activity. Tropical Atlantic sea surface temperatures anomalies have warmed somewhat since our early June prediction and surface pressures have fallen somewhat. But, the negative influences of El Niño-induced strong Caribbean Basin and Main Development Region vertical wind shear typically dominate over surface pressure and sea surface temperature in the tropical Atlantic.
Although we have been in an active multi-decadal Atlantic Basin hurricane era since 1995, it is not unusual to have a few below-average years within an active multi-decadal period. Likewise, it is not unusual to have a few above-average years within an inactive multi-decadal period. We expect the active Atlantic hurricane era that we have been in since 1995 to continue for the next 10-15 years.