The National Oceanic and Atmospheric Administration updated its hurricane forecast Thursday, saying it’s more likely that the season will be unusually active.
The NOAA Climate Prediction Center anticipates 12 to 17 named storms this year, with five to eight becoming hurricanes and two to three becoming major hurricanes, Category 3 or above. The Atlantic hurricane season started June 1 and peaks in mid-August through October.
In May, NOAA forecasted nine to 15 named storms, four to eight hurricanes and one to three major hurricanes this season. Those numbers are considered normal; the 30-year average for named storms is 12.
The higher forecast resulted from a later-than-expected start to El Niño, the rise of Pacific sea surface temperatures that causes a number of weather phenomena. For hurricanes, El Niño’s main impact is creating wind shear over the Atlantic Ocean, which tends to suppress hurricanes. The horizontal force of the shear rips the vertical cyclones apart, or prevents their formation.
“We don’t expect El Niño’s influence until later in the season,” Gerry Bell, a forecaster at the Climate Prediction Center, said in a statement.
But the start of El Niño is uncertain, and if it comes sooner, the hurricane season could end up below average.
“It may come in time to put a damper on it,” said Rick Luettich, the director of the UNC Institute for Marine Sciences.
No matter the forecast, a storm only matters to most people if it hits land, and NOAA does not make landfall predictions.
But meteorologists at Colorado State University predicted this week that the coasts of the Carolinas have a 38 percent chance of being hit by a named storm, a 27 percent chance of a hurricane, and an 8 percent chance of a Category 3 or above hurricane.
The hurricane category refers to the wind speed. A named storm needs winds of at least 39 mph. A Category 1 hurricane has winds of at least 74 mph, while a Category 5 is at least 156 mph.
But the category of a storm doesn’t always indicate its severity, particularly when it comes to coastal flooding, said Luettich. “At various times it’s been thought that such-and-such category corresponds to such-and-such storm surge,” Luettich said. “This has been debunked.”
Storm surge, the rise of ocean water caused by winds and tides, causes the greatest threat to life and property. Especially vulnerable are those living in low-lying coastal areas, where the surge can travel several miles inland.
“Winds can be damaging, but there’s a high survival rate in strong winds,” said Luettich. “But with the flooding and inundation, the survivability diminishes.”
Hurricane Katrina had weakened from a Category 5 to a Category 3 storm when it hit Louisiana in 2005. But the associated storm surge caused levees around New Orleans to breach, flooding 80 percent of the city to depths of 20 feet. Storm surge and its flooding aftermath were responsible for most of the 1,577 Louisiana deaths.
Luettich is one of the developers of the computer model used by many agencies, including NOAA, to predict storm surge and flooding. The model, known as ADCIRC, is mostly a prediction tool that helps FEMA establish insurance rates and is also used to guide development planning in southern Louisiana.
But in extreme events, it can switch to forecasting mode. Before Hurricane Irene last year, ADCIRC recommended the Coast Guard move its Portsmouth, Va., command center inland. They moved, and it was a good thing.
“They did get flooded out,” said Luettich.