2006 Atlantic hurricane season

An average year after three extreme ones

The archive's counts for 2006 match the National Hurricane Center's own: ten tropical storms, five hurricanes and two major hurricanes, none reaching Category 5. The NHC's season summary noted that these totals sat very close to the 1966 to 2005 averages of about 11 storms, 6 hurricanes and 2 major hurricanes, and put the season's energy at 90 percent of the long-term mean, the lowest since 2002. Measured against 2003, 2004 and 2005, though, the year felt quiet: nine storms received names, and a tenth, off New England in July, was identified only in the postseason review, so the named-storm count was the lowest since 1997.

The ocean itself favoured an active season. Sea surface temperatures in the tropical Atlantic and Caribbean between 10 and 20 degrees north were the second warmest on record since 1871 during the peak months, behind only 2005, and wind shear was relatively low. What tipped the balance was an El Niño that developed unexpectedly in late summer. It did little to raise shear across the basin, but it reinforced a broad area of sinking air over the western Atlantic, leaving the atmosphere there warmer, drier and more stable and limiting the formation of new storms. Dry Saharan air over the tropical Atlantic added to the drag.

Forecasters had not seen this coming. In May, NOAA called for 13 to 16 named storms, 8 to 10 hurricanes and 4 to 6 major hurricanes, and Colorado State University had forecast 17 named storms, 9 hurricanes and 5 major hurricanes since December 2005. As the El Niño emerged, the university team cut its outlook to 13 storms, 5 hurricanes and 2 major hurricanes on 1 September, close to what actually happened.

Slow early months, then a compressed burst

Tropical Storm Alberto opened the season on 10 June, forming south of the western tip of Cuba and crossing the eastern Gulf of Mexico. Over warm Loop Current waters its winds jumped to 60 kt on 12 June, before dry air and shear weakened it ahead of landfall in Florida's Big Bend. July brought two short-lived storms spun off the same stalled front along the US East Coast: an unnamed tropical storm southeast of Nantucket on 17 and 18 July, first treated as a frontal low and upgraded after the season, and Tropical Storm Beryl, which moved north parallel to the mid-Atlantic coast and crossed Nantucket on 21 July.

The deep tropics stayed hostile into August. Tropical Storm Chris peaked at 55 kt near Anguilla on 2 August before northwesterly shear and dry air broke it down north of Hispaniola, and Tropical Storm Debby, born near the Cape Verde Islands on 21 August, never got past 45 kt in dry, stable air over marginally warm water.

Every hurricane of the year then formed in a 37-day window between 27 August and 2 October. Ernesto briefly reached hurricane strength in the central Caribbean on 27 August, and Florence, Gordon, Helene and Isaac all became hurricanes in September. A strong mid-level trough over the eastern United States that month steered all four out to sea near or east of Bermuda, in contrast with the high pressure that had pushed so many storms westward toward land in 2004 and 2005. Isaac faded near Newfoundland on 2 October, and the season produced nothing more. Since 1966, only 1983 and 1993 had ended earlier.

Gordon and Helene

The season's two major hurricanes peaked within four days of each other and reached identical intensities, one-minute winds of 105 kt and a central pressure of 955 hPa, in the central Atlantic. The site lists Hurricane Gordon as the strongest because it reached that peak first. Gordon formed east of the Leeward Islands on 10 September and, once an upper trough stopped shearing it, intensified rapidly, its winds climbing 50 kt in 30 hours to peak around 500 nautical miles east-southeast of Bermuda on 14 September. It then stalled, weakened over its own upwelled cooler water, and regained 90 kt on 19 September despite sea temperatures near 25°C.

Hurricane Helene formed south-southeast of the Cape Verde Islands on 12 September and strengthened steadily on its way west-northwest, reaching its peak on 18 September. At that moment the two hurricanes sat at nearly the same longitude, with Gordon about 875 nautical miles to the north. Helene soon developed concentric eyewalls and weakened, turned north well east of Bermuda, and stayed a hurricane until 24 September northwest of the Azores.

The other hurricanes were modest. Hurricane Florence grew from two merged tropical waves into a large, slow-to-organize system, peaking at 80 kt south of Bermuda on 10 September. Hurricane Isaac struggled over water already cooled by Gordon and Helene before reaching 75 kt east of Bermuda on 1 October, and Hurricane Ernesto held hurricane strength only briefly at 65 kt.

Landfalls and impacts

Hurricane Ernesto was the season's most damaging storm. After its brief hurricane peak south of Haiti, it made landfall as a tropical storm in southeastern Cuba, on Plantation Key and the South Florida mainland, and finally at Oak Island, North Carolina, early on 1 September with 60 kt winds, near the hurricane threshold. Its rains killed five people in Haiti, the NHC's count of direct deaths, and flooded more than 400 homes in Santo Domingo. In the Carolinas and Virginia, rain totals topped 250 mm in several places, with 371 mm at Wrightsville Beach, and rivers north of Wilmington stayed in flood for days. Ernesto's extratropical remnant caused two further deaths in Gloucester, Virginia, when a tree fell on a house. The NHC estimated total US property damage at roughly $500 million in 2006 dollars, about double the insured losses of $245 million.

Tropical Storm Alberto came ashore near Adams Beach, Florida, on 13 June with 40 kt winds. It dropped up to 445 mm of rain on western Cuba and caused minor surge flooding along Florida's Gulf coast, including in Citrus and Levy counties. The NHC recorded no direct deaths; after Alberto became extratropical, a boy drowned in a drainage system near Raleigh, North Carolina. Tropical Storm Beryl brought only gusty winds to Nantucket, and Tropical Storm Chris, which passed north of the Greater Antilles, still caused flooding and mudslides around Santo Domingo without loss of life.

Most other impacts came from storms far from the US coast. Hurricane Florence passed about 50 nautical miles west of Bermuda on 11 September, bringing hurricane conditions, gusts up to 100 kt and minor damage and power cuts, and as an extratropical storm it carried hurricane-force winds to Newfoundland. Hurricane Gordon crossed the Azores between São Miguel and Santa Maria on 20 September, the first hurricane to affect the islands since 1991, then as an extratropical storm brought gusts to 99 kt in northwestern Spain, where about 100,000 customers lost power, and cut power to some 126,000 homes in Northern Ireland. Helene's remnant brought gales to Ireland and Scotland, and Isaac passed just southeast of Newfoundland's Avalon Peninsula.

A break for the United States

After twelve hurricanes struck the United States in 2004 and 2005, none did in 2006, the first such year since 2001. Three tropical storms, Alberto, Beryl and Ernesto, reached the US coast, and the NHC recorded no direct tropical cyclone deaths in the country. Wikipedia's season article also notes a June system that moved ashore near Morehead City, North Carolina, and fed deadly Mid-Atlantic flooding; it was never classified as a tropical cyclone and is not part of the season's counts.

The quieter year was kind to track forecasters: the NHC's official track errors from 12 to 72 hours reached all-time lows. Intensity forecasts fared worse. Only 3.3 percent of 24-hour intensity changes counted as rapid strengthening, against 7.1 percent in 2005, and forecasts that kept expecting storms to strengthen ended up with negative skill against a simple climatology and persistence benchmark.