The 2013 Atlantic hurricane season was a well below average Atlantic hurricane season in terms of the number of hurricanes and ACE. It was the first since 1994 with no major hurricanes, Category 3 or higher on the Saffir–Simpson scale, and the first since 1972 where no hurricanes reached Category 2 strength. Altogether, the season produced 15 tropical cyclones, of which all but one became a nameable storm. The season officially began on June 1, and ended on November 30. These dates historically describe the period in each year when most tropical cyclogenesis occurs in the North Atlantic and are adopted by convention. The first storm of the season, Andrea, developed on June 5, while the last, an unnamed subtropical storm, dissipated on December 7. Throughout the year, only two storms, Humberto and Ingrid, reached hurricane strength; this was the lowest seasonal total since 1982.
The season's overall impact was minimal; although 15 tropical cyclones developed, most were weak or remained at sea. Tropical Storm Andrea killed four people after making landfall in Florida and moving up the East Coast of the United States. In early July, Tropical Storm Chantal moved through the Windward Islands, causing one fatality, but minimal damage overall. Tropical storms Dorian and Erin and Hurricane Humberto brought only squally weather to the Cape Verde Islands. Mexico, where Hurricane Ingrid, Tropical Depression Eight, and tropical storms Barry and Fernand all made landfall, was the hardest hit; Ingrid alone caused at least 32 deaths and $1.5 billion (2013 USD) in damage. In early October, Karen brought showers and gusty winds to the central Gulf Coast of the United States, before impacting the U.S. East Coast as a long-lived nor'easter.
All major forecasting agencies predicted an above-average season. All reduced their seasonal predictions in early August; however, even the revised predictions were too high. The lack of activity was primarily caused by an unexpected significant weakening of the Atlantic Ocean thermohaline circulation between winter and spring. This resulted in continuation of the spring weather pattern over the Atlantic Ocean, with decreased mid-level moisture and increased vertical wind shear and atmospheric stability, which suppressed tropical cyclogenesis.
Seasonal forecasts
In advance of, and during, each hurricane season, several forecasts of hurricane activity are issued by national meteorological services, scientific agencies, and noted hurricane experts. These include forecasters from the United States National Oceanic and Atmospheric Administration (NOAA)'s National Hurricane and Climate Prediction Center, Tropical Storm Risk, the United Kingdom's Met Office, and Philip J. Klotzbach, William M. Gray and associates at Colorado State University (CSU). The forecasts include weekly and monthly changes in significant factors that help determine the number of tropical storms, hurricanes, and major hurricanes within a particular year. According to NOAA and CSU, the average Atlantic hurricane season between 1981 and 2010 contained roughly 12 tropical storms, six hurricanes, three major hurricanes, and an accumulated cyclone energy (ACE) index of 66–103 units. Broadly speaking, ACE is a measure of the power of a tropical or subtropical storm multiplied by the length of time it existed. Therefore, a storm with a longer duration, such as Humberto, will have high values of ACE. It is only calculated for full advisories on specific tropical and subtropical systems reaching or exceeding wind speeds of 39 mph (63 km/h). Accordingly, tropical depressions are not included here. After the storm has dissipated, typically after the end of the season, the NHC reexamines the data, and produces a final report on each storm. These revisions can lead to a revised ACE total either upward or downward compared to the operational value. NOAA typically categorizes a season as either above-average, average, or below-average based on the cumulative ACE Index, but the number of tropical storms, hurricanes, and major hurricanes within a hurricane season are considered occasionally as well.
Pre-season forecasts
Multiple agencies predicted above-average activity, citing forecasts for slower-than-average trade winds, warmer-than-normal sea surface temperatures, abnormally low wind shear, and the unlikelihood of an El Niño developing prior to the peak of the season. On December 5, 2012, Tropical Storm Risk (TSR), a public consortium consisting of experts on insurance, risk management, and seasonal climate forecasting at University College London, issued an extended-range forecast. In its report, the organization called for 15.4 (±4.3) named storms, 7.7 (±2.9) hurricanes, 3.4 (±1.6) major hurricanes, and a cumulative Accumulated Cyclone Energy (ACE) index of 134. While no value was placed on the number of expected landfalls during the season, TSR stated that the landfalling ACE index was expected to be above average. Four months later, on April 5, Tropical Storm Risk issued its updated forecast, continuing to call for an above-average season with 15.2 (±4.1) named storms, 7.5 (±2.8) hurricanes, 3.4 (±1.6) major hurricanes, and an ACE index of 131; the landfalling ACE index was once again forecast to be higher than normal.
Meanwhile, on April 8, Weather Services International (WSI) issued its first forecast for the hurricane season. In its report, the organization forecast 16 named storms, nine hurricanes, and five major hurricanes.
On April 10, Colorado State University (CSU) issued its first forecast for the season, calling for a potentially hyperactive season with 18 named storms, nine hurricanes, four major hurricanes, and an ACE index of 165. The probabilities of a major hurricane hitting the Gulf Coast and East Coast were well above average. On May 15, the United Kingdom Met Office (UKMO) predicted 14 named storms, with a 70% chance that the number would be between 10 and 18, and nine hurricanes with a 70% chance that the number would be between 4 and 14. It also predicted an ACE index of 130 with a 70% chance that the index would be in the range 76 to 184. On May 23, 2013, NOAA issued its first seasonal outlook for the year, stating there was a 70% likelihood of 13 to 20 named storms, of which seven to eleven could become hurricanes, including three to six major hurricanes; these ranges are greater than the seasonal average of twelve named storms, six hurricanes and three major hurricanes. On May 30, the Florida State University Center for Ocean-Atmospheric Prediction Studies, FSU COAPS, issued its first and only prediction for the season. The organization called for 12 to 17 named storms, of which five to ten would further intensify into hurricanes; no forecast was given for the number of major hurricanes. In addition, an ACE index of 135 units was forecast.
Mid-season outlooks
In June, predictions from CSU and TSR were similar to pre-season forecasts due to the a lack of an El Niño and warmer-than-average sea surface temperatures across the Atlantic Ocean. However, by July and August, CSU and TSR all adjusted their forecasts downward because of predictions of cooler-than-average sea surface temperatures and above-average wind shear. NOAA also decreased the amount of activity in its final outlook, despite predicting a wetter-than-average western Africa and above-average sea surface temperatures in its report. On June 3, CSU issued its first mid-season prediction for the remainder of the year. In its report, the organization continued to predict well above-average activity, with eighteen named storms, nine hurricanes, four major hurricanes, and an ACE index of 165 units. CSU stated that there was a 72% chance of at least one major hurricane impacting any stretch of the United States coastline; the chances of a major hurricane hitting the East Coast and Gulf Coast were 48% and 47%, respectively. The following day, Tropical Storm Risk issued its third forecast for the 2013 Atlantic hurricane season, calling for sixteen named storms, eight hurricanes, four major hurricanes, and an ACE of 134 units; this activity was predicted to be roughly 30% above the 1950–2012 long-term mean. TSR gave a 65% probability that the landfalling ACE index would be above-average. A month later, TSR lowered its numbers to 15 named storms, seven hurricanes, and three major hurricanes. On August 2, Colorado State University issued another update for the season, lowering its numbers slightly. However, the organization continued to state that there was an above-average probability of a United States and Caribbean major hurricane landfall. Finally, on August 8, NOAA issued its second and final outlook for the season, predicting 13 to 19 named storms, six to nine hurricanes, and three to five major hurricanes; these numbers were down slightly from its May outlook.
Post-season review
With 14 tropical storms, two hurricanes, and no major hurricanes, activity fell far below most predictions. Brian McNoldy at the University of Miami noted several reasons why NOAA should not cease seasonal predictions, including the variability of hurricane seasons and the value of supplementing climatology with seasonal forecasts. Further, McNoldy argued that forecasting a hurricane season "challenges us to better understand how the atmosphere works." On November 29, Dr. Phil Klotzbach of CSU noted that "[Dr. Gray and I] have been doing these forecasts for 30 years and that's probably the biggest forecast bust that we've had." The program, which had already lost contributions from an insurance company in June, was defunded further following the botched season forecasts. However, Gray and Klotzbach were able to issue predictions for the 2014 season.
Seasonal summary
The Atlantic hurricane season officially began on June 1, 2013. It was a below average season in which 14 tropical cyclones and one subtropical cyclone formed. Thirteen of the fifteen designated cyclones attained tropical storm status. However, only two of those became hurricanes, the fewest since 1982; neither of these intensified into a major hurricane, the first such occasion since 1994. The season's first hurricane did not come until September 11, tying 2002 for the latest formation of the first Atlantic hurricane in a given season in the satellite era (later surpassed in 2026). Also, 2013 was the most active season without a major hurricane. Further, no hurricane generated winds greater than Category 1 strength on the Saffir–Simpson scale for the first time since 1925. Throughout the season, NOAA and the United States Air Force Reserve flew a total of 45 reconnaissance missions over the Atlantic basin, totaling 435 hours; this was the lowest number of flight hours since 1966. One hurricane and three tropical storms made landfall during the season, causing 56 deaths and about $1.51 billion in damage. The last storm of the season dissipated on December 7, a week after the official end of hurricane season on November 30, 2013.
Activity was primarily suppressed by significant weakening of the Atlantic Ocean thermohaline circulation (THC) early in the year, representing the largest reduction in strength of the THC since NCEP/NCAR Reanalysis began. The weakening of the THC was possibly the result of lowered ocean salinity and a decrease in North Atlantic Deep Water formation. Oceanic and atmospheric gyres were able to strengthen in the subtropical Atlantic, allowing southward advection of cold air and water. Consequently, there was a significant cooling of sea surface temperatures over portions of the north Atlantic. This resulted in a continuation of the spring weather pattern over the Atlantic Ocean, with strong vertical wind shear, reduced mid-tropospheric moisture, and high atmospheric stability. The weak THC also resulted in slightly stronger trade winds and less wind convergence and rainfall in the Intertropical Convergence Zone, with an increase in upper-level zonal winds and higher environmental air pressures. Collectively, these factors worked to suppress tropical cyclogenesis.
Tropical cyclogenesis began in early June, with the development of Tropical Storm Andrea in the Gulf of Mexico on June 5. Twelve days later, Tropical Storm Barry formed in the northwestward Caribbean Sea. Two named storms originated during the month of July – tropical storms Chantal and Dorian. Similarly, there were two tropical storms in August, Erin and Fernand. In September, four tropical cyclones formed, three of which strengthened into tropical storms. Two of those reached hurricane status. The first, Humberto, was the strongest system of the season, attaining maximum sustained winds of with maximum sustained winds of 90 mph (150 km/h). The other hurricane, Ingrid, was the most devastating storm of the season and peaked at slightly less intensity. Activity began to slow in October, with the development of only two tropical storms, Karen and Lorenzo. Tropical cyclogenesis then halted for almost a month, until Tropical Storm Melissa formed over the eastern Atlantic Ocean on November 18. This was the only tropical cyclone in the month of November. The final system was an unnamed subtropical storm that developed south of the Azores on December 5. After meandering for two days, it degenerated into a remnant low-pressure area on December 7.
The season's activity was reflected with an accumulated cyclone energy (ACE) rating of 36.1. This was well below the 1981–2010 median of 92, and the lowest value since 1994.