The 2006 Atlantic hurricane season was the least active in the basin since 1997, with nine named storms as well as an additional unnamed tropical storm identified by the National Hurricane Center. 2006 was the first season since 2001 in which no hurricanes made landfall in the United States, and was the first since 1994 in which no tropical cyclones formed during October. Following the intense activity of 2003, 2004, and 2005, forecasters predicted that the 2006 season would be only slightly less active. Instead, it turned out to be a below average season, as activity was slowed by a rapidly forming moderate El Niño event, the presence of the Saharan Air Layer over the tropical Atlantic, and the steady presence of a robust secondary high-pressure area to the Azores High centered on Bermuda. There were no tropical cyclones after October 2.
The season began on June 10 with the formation of Tropical Storm Alberto, which moved from the Caribbean Sea through the east coast of the United States. Hurricane Ernesto caused heavy rainfall in Haiti, and directly killed at least seven in Haiti and the United States. Four hurricanes formed after Ernesto, including the strongest storms of the season, Hurricanes Helene and Gordon. In total, the season was responsible for 14 deaths and $500 million in damage. The calendar year 2006 also saw Tropical Storm Zeta, which arose in December 2005 and persisted until early January, only the second such event on record. The storm is considered a part of the 2005 season, as it developed in that calendar year.
Seasonal forecasts
Forecasts of hurricane activity are issued before each hurricane season by Philip J. Klotzbach, Dr. William M. Gray, and their associates at Colorado State University; and separately by NOAA forecasters.
Klotzbach's team (formerly led by Gray) has defined the average number of storms per season (1950–2000) as 9.6 tropical storms, 5.9 hurricanes, and 2.3 major hurricanes (storms exceeding Category 3 strength in the Saffir–Simpson scale). A normal season, as defined by NOAA, has 6–14 named storms, with 4–8 of those reaching hurricane strength, and 1–3 major hurricanes.
Pre-season forecasts
On December 5, 2005, Klotzbach's team issued its initial extended-range forecast for the 2006 season, predicting an above average of 17 named storms, nine of them hurricanes, and five classified as Category 3 intensity or higher.
As in the 2005 season, the team predicted it was highly probable that at least one major hurricane would directly impact the United States. The forecast suggested an 81% probability that at least one major hurricane would strike the U.S. mainland, a 64% chance of at least one major hurricane striking the East Coast of the United States (including the Florida peninsula), and a 47% chance of at least one major hurricane striking the Gulf Coast of the United States from the Florida Panhandle westward. The team also predicted that the potential for major hurricane activity in the Caribbean was above average. A few months later, on April 4, 2006, CSU issued another forecast confirming its December predictions.
On May 22, 2006, NOAA released its pre-season forecast for the 2006 season. The prediction was for 13–16 named storms, 8–10 of those becoming hurricanes, and 4–6 becoming major hurricanes.
On May 31, 2006, Klotzbach's team released its final pre-season forecast for 2006, confirming its earlier prediction.
Midseason outlooks
On August 3, 2006, Klotzbach's team lowered its season estimate to 15 named storms, with 7 becoming hurricanes and 3 becoming major hurricanes, noting that conditions had become less favorable for storms than they had been earlier in the year. The sea-level pressure and trade wind strength in the tropical Atlantic were reported to be above normal, while sea surface temperature anomalies were on a decreasing trend.
On August 8, 2006, NOAA revised its season estimate to 12–15 named storms, with 7–9 becoming hurricanes, and 3–4 becoming major hurricanes. The reduction was attributed to less favorable environmental conditions, a decrease in La Niña conditions, and the lack of a "very persistent upper-level ridge pattern over the eastern U.S. and western Atlantic".
On September 1, Klotzbach's team also revised its season estimate, to 13 named storms, 5 hurricanes, and 2 major hurricanes, citing a larger volume of the Saharan Air Layer and an El Niño trend in the Pacific. The team again reduced the number of tropical storms expected for the season a month later, on October 3, with an updated forecast of 11 named storms, 6 hurricanes, and 2 major hurricanes, citing the ongoing El Niño.
Seasonal summary
Tropical Storm Zeta formed on December 30, 2005, and lasted until January 6, 2006. Although the majority of its existence occurred in 2006, it is officially a storm of the 2005 Atlantic hurricane season as that is the year in which it formed. Zeta joined Hurricane Alice as only the second North Atlantic tropical cyclone in recorded history to span two calendar years.
The season started on June 1, 2006, and officially ended on November 30, 2006. These dates conventionally delimit the period of each year when most tropical cyclones form in the Atlantic basin. Ten days into the start of the season, Tropical Storm Alberto developed in the Caribbean, and after four months of activity, Hurricane Isaac dissipated on October 3 south of Newfoundland. Compared to the devastating 2005 Atlantic hurricane season, 2006 was not severe in terms of deaths and damage. Three tropical storms made landfall in the United States. The first of them, Tropical Storm Alberto, made landfall in Florida with winds of 50 mph (80 km/h), causing flooding and light damage. Tropical Storm Beryl made landfall on Nantucket, but left little impact. The third and more significant storm was Hurricane Ernesto, which killed two people in Virginia and two in Florida, as well as causing $500 million in damage (2006 USD). During the season, only one tropical cyclone in the Atlantic – Alberto – affected Mexico. Canada was affected by several tropical cyclones during 2006, including Alberto, the unnamed storm, Beryl, Florence, and Isaac.
On June 20, an upper-level disturbance formed east of the Bahamas and moved westward across the islands. Between June 24 and 26, areas of convection developed occasionally, and a low-level disturbance formed. The system turned northward and upon reaching the Gulf Stream on June 27, it began to mature. It made landfall near Morehead City, North Carolina and moved northeastward along the U.S. East Coast. The storm contributed to severe and deadly flooding in the Mid-Atlantic states. While the NHC did not operationally classify it, data from reconnaissance aircraft, NEXRAD weather radar, and surface observations suggest it may have met the criteria for a tropical cyclone.
The National Hurricane Center's pre-season activity outlook predicted 13–16 named storms, 8–10 hurricanes and 4–6 major hurricanes.
They also predicted a high risk of at least one major hurricane strike to the Southeastern United States. In the event, only ten storms formed during the season, the lowest number since the 1997 season, when there were seven. Five of the ten storms developed into hurricanes— the lowest number since 2002– only two attained major hurricane status, tying with 2002 for the fewest since 1997. Only one named storm was observed during October, the lowest number since 1994, when none were seen during that month. Additionally, only three named storms made landfall in the United States, the fewest since 2001. Because of several factors, including a rapidly forming El Niño event, the Saharan Air Layer over the tropical Atlantic and the presence of a high-pressure area to the Azores High situated near Bermuda, it contributed to a below average season. Also, sea surface temperatures in the western Atlantic were just at or slightly below average. In contrast, sea surface temperatures during the 2005 season were well above average.
Overall, the season's activity was reflected with a low cumulative accumulated cyclone energy (ACE) rating of 78.5. 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.
Systems
Tropical Storm Alberto
On June 10, an area of disturbed weather associated with a broad low-pressure area off the coast of Belize organized over the warm waters of the Caribbean into the first tropical depression of the season. It dropped light rainfall in Mexico, with a 24-hour total peaking at 4 inches (100 mm) in Peto, Yucatán. Southwesterly vertical wind shear initially prevented significant development, but as it moved closer to Florida, the depression strengthened into a tropical storm on June 11. Passing over the warm, deep water of the Loop Current allowed accelerated development, and the cyclone reached its peak winds of 70 mph (110 km/h), just shy of hurricane strength. Subsequent weakening occurred as it moved over the cooler waters of the continental shelf, and Alberto made landfall near Adams Beach, Florida, on June 14 with winds of about 45 mph (72 km/h). Losing its tropical characteristics, Alberto moved northeastward and produced heavy rainfall in South Carolina and North Carolina. The remnants tracked off the U.S. East Coast and transitioned into a powerful extratropical storm which affected Nova Scotia with high winds, heavy rain, and rough surf, leaving four fisherman missing offshore.
Alberto caused record rainfall in North Carolina, peaking at 8 inches (200 mm). In Florida, two people died, and damage was estimated at $250,000 (2006 USD). Later, the storm left four sailors missing about 230 miles (370 km) south of Nova Scotia.
Unnamed tropical storm
A cold front exited the eastern United States on July 13 and subsequently stalled over the western Atlantic Ocean. It decayed and dissipated, leaving behind two areas of low pressure. The southern area near North Carolina became Tropical Storm Beryl, and the northern system became an extratropical low on July 16, south-southeast of Cape Cod, Massachusetts. The system separated itself from the dissipating front later that day while moving northeastward over warm waters. Convection developed near the center, and the system transitioned into a tropical depression early on July 17, about 240 miles (390 km) southeast of Nantucket, Massachusetts. Accelerating northeastward, the depression intensified into a tropical storm six hours later. Banding features became prominent, and after continued strengthening, the storm attained peak winds of 50 mph (80 km/h) late on July 17, while located about 245 miles (394 km) south of Halifax, Nova Scotia. Shortly thereafter, the storm encountered much cooler water temperatures after leaving the Gulf Stream. The storm quickly weakened as the convection rapidly diminished, and on July 18, degenerated into a non-convective remnant low. The remnants crossed Newfoundland, before turning to the east-northeast and dissipating on July 19. The storm greatly weakened prior to moving across Newfoundland, and as a result, its impact was minimal.
Operationally, the storm was considered as a non-tropical gale, connected to a cold front. However, a post-season analysis provided enough evidence of tropical characteristics, indicating no frontal features and no cold air intrusion at the time of peak winds. Observations analyzed the storm as having a symmetric warm core, whereas in real time it was considered subtropical. The National Hurricane Center officially re-classified the system as an unnamed tropical storm on December 15, 2006.
Tropical Storm Beryl
The same frontal system that developed the previous storm also produced another low-pressure area east of North Carolina. The system was broad and ill-defined at first, although the low-level circulation became sufficiently organized to be designated a tropical depression on July 18, about 290 mi (465 km) east-southeast of the coast of North Carolina. Later that day, the depression developed organized rainbands over the eastern periphery of the circulation, and as a result, the NHC upgraded it to Tropical Storm Beryl. The nascent storm moved generally northward at first along the western periphery of a subtropical ridge, parallel to the East Coast of the United States. With low wind shear and favorable outflow, Beryl intensified to reach a peak intensity of 60 mph (97 km/h) early on July 20. Around that time, the storm developed an eye-like feature in the center of the convection. Cooler waters caused Beryl to weaken, as it turned northeastward ahead of an approaching trough. Early on July 21, the storm struck the island of Nantucket with winds of 50 mph (85 km/h). Shortly thereafter, Beryl transitioned into an extratropical cyclone, which moved through Nova Scotia before merging with another extratropical cyclone on July 22 over Newfoundland.