The updated outlook released Thursday calls for a total of 19 to 25 named storms (winds of 39 mph or greater), of which 7 to 11 are expected to become hurricanes (winds of 74 mph or greater), including three to six that could become major hurricanes (winds of 111 mph or greater). This update covers the entire hurricane season, which ends Nov. 30, and therefore includes the nine named storms to date.
About 95 percent of hurricanes and major hurricanes form from August to October, National Weather Service director Louis Uccellini said during a news conference call. In more than two decades of issuing these forecasts, NOAA has never predicted that as many as 25 named storms would form in a single season.
It would take only 21 named storms before all the names on the Atlantic list, determined ahead of time by the World Meteorological Organization, are exhausted and forecasters would turn to the Greek alphabet. This happened only once before, in 2005, a devastating season that was the most active on record.
The updated outlook is a sharp increase from NOAA’s initial prediction, released in May, which called for a 60 percent likelihood of an above-average season, with a 70 percent chance of 13 to 19 named storms, six to 10 of which would become hurricanes. Three to six of those could become major hurricanes of Category 3 intensity or higher, NOAA predicted.
Now the chance of an above-average season has increased to 85 percent, with a 5 percent chance of it being below average. An average season produces 12 named storms, including six hurricanes, of which three become major hurricanes.
The new projections come a day after meteorologists at Colorado State University released their updated hurricane outlook, which also projects so many storms that by the end of the season names would have to be pulled from the Greek alphabet to keep track of them all.
The CSU team predicts the 2020 season will see 24 named storms, including 12 total hurricanes and five major hurricanes. The forecast was a significant increase from its previous outlook, issued in early July.
The season has the potential to be one of the busiest on record, NOAA said.
Historically, only two named storms form on average by early August, and the ninth named storm typically forms on Oct. 4.
Ocean temperatures, La Niña and other factors are focus of forecaster concerns
“Weather and climate models are all indicating an even higher likelihood of an extremely active season,” said NOAA’s chief hurricane seasonal forecaster Gerry Bell.
Driving the forecast is a series of factors that point to an unusually active season. These include widespread above-average sea surface temperatures in the tropical Atlantic Ocean, particularly in the main development region between West Africa and the Leeward Islands. In addition, there are above-average temperatures in the Caribbean Sea, and sea surface temperatures are also running well above average off the East Coast and in the Gulf of Mexico. The warm waters are the result, in part, of long-term, human-caused climate change, as well as natural climate cycles.
Also drawing forecasters’ attention is a developing La Niña event in the tropical Pacific Ocean. Such episodes, which feature lower-than-average water temperatures in the central to eastern tropical Pacific, tend to slacken upper-level winds over the tropical Atlantic. This removes an impediment to tropical storms and hurricanes, since they are highly vulnerable to wind shear, which occurs when winds blow at different speeds and/or directions with height. Shear can tear a nascent storm apart.
Another major factor this year is a cycle known as the Atlantic Multidecadal Oscillation (AMO). It sets the Atlantic up for above-average odds of years’ worth of particularly active storm activity. We are in such an active phase right now, though that doesn’t mean that every season will set records. The current active phase of the AMO has lasted since 1995.
In addition, Bell said, an unusually active West African monsoon season is leading to more storm-forming opportunities, as thunderstorms emerge off the west coast of Africa and have a chance to develop into a tropical storm or hurricane. Years that feature unusually dry conditions in central Africa tend to be less active.
The new outlook is unwelcome news for a country already reeling from the coronavirus pandemic, which complicates storm evacuation and shelter logistics.
During Hurricane Isaias, for example, many people were told to shelter in place, go to a hotel or stay with relatives to prevent crowding in shelters with social distancing requirements. The Federal Emergency Management Agency is having to deal with an unprecedented circumstance in which all 50 states have been declared disaster areas because of the virus, while also gearing up for responding to tropical storms and hurricanes, which is among the toughest challenges the agency faces.
The country has already been hit by two hurricane landfalls, both of which were Category 1 storms.
To determine whether a season is “extremely active,” NOAA uses a measure of the overall hurricane season activity, known as the Accumulated Cyclone Energy (ACE) index, which incorporates both the intensity and duration of all named storms during the season. “This year, we expect more, stronger, and longer-lived storms than average, and our predicted ACE range extends well above NOAA’s threshold for an extremely active season,” Bell said in a news release.
Climate change’s role
Studies have shown that while warming sea and air temperatures are not leading to greater numbers of tropical cyclones, they are influencing their intensity in a detectable way. A new study, published in June, provides observational evidence that the odds of major hurricanes around the world — Category 3, 4 and 5 storms — are increasing because of human-caused global warming.
The study, by a group of researchers at NOAA and the University of Wisconsin at Madison, builds on previous research that found a trend, though not a statistically robust one, toward stronger tropical cyclones.
In addition to seeing stronger, wetter storms, there is also a shift toward hurricanes that make sudden leaps in their intensity, known as rapid intensification. This process can be difficult to predict.