Ironically, some of the most important information the scientists rely on in predicting the Atlantic hurricane season isn't even taken from the Atlantic Ocean. Scientists lean heavily on measurements from El Niño and La Niña, a combined weather phenomenon in the central and east-central Pacific around the equator. (In the climate community, El Niño and La Niña also are known as ENSO, for El Niño/Southern Oscillation.)
El Niño is a water warming trend in the Pacific Ocean, especially those waters in the eastern Pacific, on the west side of Central America; La Niña is a cooling trend. The two are important because, generally, cooler water in the Pacific suggests warmer water in the Atlantic (and vice versa).
Monitoring the temperatures of the Pacific matters. Hurricanes are partly fueled by warmer water, so cooler water in the Pacific (a result of La Niña) generally means warmer water — and more hurricanes — in the Atlantic.
"When we're talking climate predictions, the Pacific tends to be the basin that has the most predictability," Belanger says. "We can make a prediction more skillfully in that region than anywhere else."
NOAA's prediction is based on no El Niño this season. "The outlook reflects our expectation of a weak or non-existent El Niño, near- or above-average sea-surface temperatures across the tropical Atlantic Ocean and Caribbean Sea, and average or weaker-than-average vertical wind shear in that same region,” Gerry Bell, Ph.D, lead seasonal hurricane forecaster with NOAA’s Climate Prediction Center, said in a statement.
Another well-known tracking tool that goes into the computer models for predicting hurricanes is known as the Atlantic Multidecadal Oscillation (AMO).
"El Niño and La Niña affect the hurricane season for a year at a time, while the AMO affects the season strength for decades (25 to 40) years at a time," Bell told an "Ask Me Anything" Reddit last year. "By predicting the combined impacts of these climate patterns, we can often predict what the hurricane season will bring. This is because they strongly control an entire set of key conditions such as vertical wind shear, trade winds, wind patterns coming off of Africa, ocean temperatures, and air pressure, all of which come together to make the season either more or less active."
The people and machines who predict hurricanes have a varying degree of success, depending on the year, of course. In May 2017, the CPC (which is a part of the National Weather Service, under the NOAA, all under the U.S. Department of Commerce) predicted an "above normal" season, with 11 to 17 named storms, five to nine hurricanes with two to four being major (Categories 3, 4 or 5) hurricanes for 2017. The '17 season produced 17 major storms, 10 hurricanes and six major hurricanes.