The 2011 Atlantic hurricane season was the second in a group of three very active Atlantic hurricane seasons, each with 19 named storms, tied with 1887, 1995, 2010, and 2012. The above-average activity was mostly due to a La Niña that persisted during the previous year. Of the season's 19 tropical storms, only seven strengthened into hurricanes, and four of those became major hurricanes: Irene, Katia, Ophelia, and Rina. The season officially began on June 1 and ended on November 30, dates which conventionally delimit the period during each year in which most tropical cyclones develop in the Atlantic Ocean. However, the first tropical storm of the season, Arlene, did not develop until nearly a month later. The final system, Tropical Storm Sean, dissipated over the open Atlantic on November 11.
Due to the presence of a La Niña in the Pacific Ocean, many pre-season forecasts called for an above-average hurricane season. In Colorado State University (CSU)'s spring outlook, the organization called for 16 named storms and 9 hurricanes, of which 4 would intensify further into major hurricanes. On May 19, 2011, the National Oceanic and Atmospheric Administration (NOAA) issued their pre-season forecast, predicting 12–18 named storms, 6–10 hurricanes, and 3–6 major hurricanes. Following a quick start to the season, NOAA subsequently increased their outlook to 14–19 named storms, 7–10 hurricanes, and 3–5 major hurricanes on August 4; CSU made no changes to the number of cyclones forecast throughout the year.
Many tropical cyclones affected land during the 2011 season; most impacts, however, did not result in a significant loss of life or property. On June 29, Arlene made landfall in Mexico near Cabo Rojo, Veracruz, causing over $223 million (2011 USD) damage and killing 22 people. Tropical Storm Harvey moved into the coastline of Central America in mid-August, and three deaths were reported as a result. During the month of September, Tropical Storm Lee and Hurricane Nate moved into the central United States Gulf Coast and central Mexico, respectively; the former led to 18 deaths, and the latter caused 5 fatalities. As an extratropical cyclone, Lee caused significant damage in the form of flooding across the Northeast United States, especially in New York and Pennsylvania. The deadliest and most destructive cyclone of the season developed east of the Lesser Antilles on August 21. Hurricane Irene caused significant impact across some of the Caribbean Islands and United States Eastern Seaboard, leaving about $14.2 billion in damage and resulting in the name's retirement. Overall, the season resulted in 118 deaths and nearly $17.44 billion in damage.
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's, Philip J. Klotzbach, William M. Gray and their associates at Colorado State University (CSU), Tropical Storm Risk, and the United Kingdom's Met Office. 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. As stated by NOAA and CSU, an average Atlantic hurricane season between 1981 and 2010 contains roughly 12 tropical storms, 6 hurricanes, 3 major hurricanes, and an accumulated cyclone energy (ACE) Index of 66-103 units. 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 or that are particularly strong hurricanes have high ACEs. It is only calculated for full advisories on tropical systems at or exceeding 39 mph (63 km/h), the threshold for tropical storm strength. NOAA typically categorizes a season as either above-average, average, of below-average based on the cumulative ACE Index; however, the number of tropical storms, hurricanes, and major hurricanes within a hurricane season is considered occasionally as well.
Pre-season forecasts
On December 6, 2010, Tropical Storm Risk (TSR), a public consortium that comprises experts on insurance, risk management and seasonal climate forecasting at University College London, issued an extended-range forecast for the 2011 Atlantic hurricane season, calling for tropical cyclone activity roughly 40% above the 1950–2010 average. The team called for 15.6 (±4.3) tropical storms, 8.4 (±3.0) hurricanes, and 4.0 (±1.7) major hurricanes, with a cumulative ACE Index of 141 (±58). Two days later, Colorado State University issued its first extended-range forecast for the 2011 season. In its report, the organization predicted an above-average hurricane season with 17 named storms, 9 hurricanes, and 5 major hurricanes. In addition, the team expected an ACE value of approximately 165, citing that El Niño conditions had little chance to develop by the start of the season. The team also predicted that there was a higher chance of a tropical cyclone hitting the United States coastline when compared to 2010. TSR released an updated forecast on April 4, lowering the number of predicted cyclones by one. On April 4, 2011, CSU revised their December forecast slightly, predicting 16 named storms, 9 hurricanes, and 5 major hurricanes.
On May 19, 2011, NOAA released their first forecast for the 2011 Atlantic hurricane season. The organization expected 12–18 named storms, 6–10 hurricanes, and 3–6 major hurricanes would form in the Atlantic during 2011, citing above-normal sea surface temperatures, a weakening La Niña, and the effect of the warm regime of the Atlantic multidecadal oscillation. NOAA also stated that, when looking at climate models, "activity comparable to some of the active seasons since 1995" could occur. On May 26, the UK Met Office (UKMO) issued a forecast of a slightly above-average season. They predicted 13 tropical storms with a 70% chance that the number would fall between 10 and 17. However, they did not – and do not – issue forecasts on the number of hurricanes and major hurricanes. The team also predicted an ACE Index of 151 with a 70% chance that the index would be in the range 89 to 212.
Mid-season forecasts
On June 1, CSU released their mid-season predictions, with numbers unchanged from those published in April. Concurrently, the Florida State University Center for Ocean-Atmospheric Prediction Studies (FSU) issued its third annual Atlantic hurricane season forecast, predicting 17 named storms, 9 hurricanes, and an ACE Index of 163. No prediction for the number of major hurricanes was made. A little over a month later, Weather Services International (WSI) issued their first forecast for the season. A total of 15 named storms, 8 hurricanes, and 4 major hurricanes were expected to develop for the entirety of the season. On August 3, CSU issued a mid-season update, though no change in predictions from their April outlook was made. The following day, NOAA issued their mid-season and final forecast for the season, calling for 14-19 named storms, 7-10 hurricanes, and 3-5 major hurricanes. The increase in numbers when compared to the pre-season forecast was due to the near-record start to the season. TSR also issued their sixth and final outlook on August 4, which increased the number of tropical storms to 16 and the number of hurricanes to 9, but continued to predict that there would be 4 major hurricanes. The final forecast for the 2011 season was issued by WSI on September 21, in which the organization called for 21 named storms, 7 hurricanes, and 4 major hurricanes; the philosophy for the increase in numbers was unchanged from CSU's.
Seasonal summary
The Atlantic hurricane season officially began on June 1, 2011. It was an above average season in which twenty tropical cyclones formed. Nineteen of the twenty depressions attained tropical storm status, tied with 1887, 1995, 2010, and later the 2012 season for the fourth-highest number of named storms since record-keeping began in 1851. Seven of these tropical storms became hurricanes, while four of those hurricanes further intensified into major hurricanes. The season was more active than usual due to lower than average wind shear, warmer than average sea surface temperatures, and the presence of a La Niña. Collectively, the tropical cyclones of this season resulted in nearly $17.44 billion in damage and there were 118 deaths; a majority of it was caused by Hurricane Irene and Tropical Storm Lee. The season officially ended on November 30, 2011. Despite having 19 named storms, the season only generated an average cumulative ACE rating of 126.3.
Tropical cyclogenesis began in the month of June, with Tropical Storm Arlene forming on June 28. After Arlene dissipated on July 1, there was about a two-week lull in activity, before Bret, Cindy, and Don developed in the latter half of July. August was the most active month of the season, featuring eight tropical cyclones – Emily, Franklin, Gert, Harvey, Irene, Tropical Depression Ten, Jose, and Katia. The number of named storms in August was well above the 1944–2010 average of four, and just one short of the record of eight set in 2004 and later in 2012, but the number of hurricanes was below the mean. September was slightly above average, with 5 named storms, 2 hurricanes, and 1 major hurricane and featuring the unnamed tropical storm, Lee, Maria, Nate, Ophelia, and Philippe. Ophelia was the most intense tropical cyclone of the season, peaking as a Category 4 hurricane with winds of 140 mph (225 km/h) and a minimum pressure of 940 mbar (28 inHg). The next two months featured one system each, with Rina developing in October and Sean forming in November. Sean was the final storm of the season and became extratropical on November 11.
Systems
Tropical Storm Arlene
A tropical wave emerged into the eastern Atlantic on June 13. Tracking westward, the disturbance remained disorganized prior to reaching the western Caribbean. Cyclonic rotation became increasingly evident on satellite imagery, though organization was halted by the disturbance's passage over the Yucatán Peninsula on June 26. After emerging into the Bay of Campeche, favorable environmental conditions allowed for the development of Tropical Storm Arlene by 18:00 UTC on June 28. Moving generally westward due to the influence of high pressure to the cyclone's north, Arlene gradually intensified, reaching its peak intensity with maximum sustained winds of 65 mph (105 km/h) and a minimum barometric pressure of 993 mbar (29.3 inHg) at 12:00 UTC on June 30; the strong tropical storm moved ashore in Mexico near Cabo Rojo, Veracruz, about an hour later. Once inland, the center of circulation became increasingly diffuse, and the storm dissipated over the Sierra Madre Mountains early on July 1.
Numerous tropical cyclone watches and warnings were issued for the coastline of Mexico shortly after the formation of Arlene; once inland, all were discontinued. Despite its relatively weak strength, Arlene produced isolated rainfall totals in excess of 9 in (230 mm). High amounts of precipitation led to numerous mudslides and flooding, causing an estimated $223.4 million in damage. A total of 22 deaths were reported in association with Tropical Storm Arlene. Although the center of circulation remained over Mexico, the storm's far-reaching effects brought minimal relief to drought-stricken portions of southern Texas and Florida.
Tropical Storm Bret
An area of low pressure formed on the southern extent of a stalled cold front just off the East Coast of the United States on July 16. Tracking south-southeast, the low-pressure center was initially baroclinic in nature, but quickly transitioned into a warm-core low over the warm waters of the western Atlantic. Decreasing vertical wind shear allowed for the development of convection – shower and thunderstorm activity – atop the low-level circulation, and Dvorak satellite classifications were initiated early on July 17 given the organization on satellite imagery. Following an aircraft reconnaissance flight into the disturbance that afternoon, the National Hurricane Center (NHC) subsequently classified the system as Tropical Depression Two at 18:00 UTC on July 17 about 100 mi (160 km) northwest of Great Abaco Island in the Bahamas. After about six hours, the depression intensified into Tropical Storm Bret.
As the system turned northeastward, the formation of an anticyclone atop Bret's center provided favorable conditions for intensification; accordingly, an eye-like feature and an eyewall – a ring of thunderstorms around the eye where typically the most intense convection associated with a tropical cyclone is located – were noted on microwave satellite imagery during the afternoon hours of July 18. At 18:00 UTC, the system attained its peak intensity with winds of 70 mph (115 km/h) and a minimum barometric pressure of 995 mbar (29.4 inHg). After moving over increasingly cool sea surface temperatures, the cyclone began to gradually weaken. Bret weakened to a tropical depression at around 00:00 UTC on July 22 and no longer met the criteria of a tropical cyclone by 12:00 UTC, while positioned approximately 375 mi (605 km) north of Bermuda. Bret left minimal impact in the Bahamas, with rainfall alleviating drought conditions. Precipitation was also generally beneficial on Bermuda, though minor flooding affected some local businesses in poor-drainage areas.
Tropical Storm Cindy
On July 17, an area of showers and thunderstorms, associated with the same frontal system that spawned Tropical Storm Bret, consolidated around a developing area of low pressure about 345 mi (555 km) west-southwest of Bermuda. Tracking east-northeastward, the system gradually organized and became better defined. The disturbance produced moderate rains while passing south of the territory, peaking at 1.16 in (29 mm); gusty winds were also observed. At 06:00 UTC on July 20, the low developed into a tropical depression east of Bermuda. Embedded within the mid-latitude westerlies, the depression moved northeast and maintained this general direction for the remainder of its existence.