The 2021 hurricane season is expected to be 'above normal' yet again - and that's based on a new official definition of normal

• Tropical Storm Elsa made landfall in Florida on Wednesday, bringing 65-mph winds and heavy rain.
• Elsa was briefly a hurricane - 2021's first. Experts forecast an "above-normal" hurricane season.
• As Earth and its oceans continue to warm, storms are getting stronger, wetter, and more destructive.

Tropical Storm Elsa made landfall on Florida's Gulf Coast on Wednesday, bringing 65-mph winds and heavy rain.

Storms get names when their cyclonic winds reach 39 mph, and Elsa is already the year's fifth named storm. It's the earliest point in the Atlantic hurricane season that we've ever seen the fifth named storm form.

That's not wholly unexpected, though: Two months ago, ahead of hurricane season, the National Oceanic and Atmospheric Administration reported a 60% chance that 2021 will be an "above normal" hurricane year, just like 2020.

Last year's hurricane season broke two records - it had the highest number of named storms ever, 30, and the most ever to make landfall, 11.

"The NOAA May forecasts for 2020 and 2021 are very similar," Astrid Caldas, a senior climate scientist with the Union of Concerned Scientists, told Insider. She added that the agency will update its forecast in August.

For now, NOAA predicts between six and 10 hurricanes in 2021. That includes three to five major hurricanes - meaning a hurricane that's a Category 3, 4, or 5, with winds at least 111 mph. There were seven major hurricanes in 2021.

Overall, the average number of predicted storms per season has increased. NOAA recently updated its seasonal hurricane predictions based on data from 1991 to 2010, so a season that's considered average now would now have 14 named storms, seven of which are hurricanes. That's up from 12 storms in total, with six being hurricanes.

The number of predicted storms per season is increasing

By definition, any cyclone-shaped storm with winds faster than 39 mph is a tropical storm. Above 74 mph, it's a Category 1 hurricane on the Saffir-Simpson scale. Typically, hurricane season runs from June 1 to November 30, with activity peaking around September 10. But Caldas thinks the season may be getting longer over time.

"2021 is the seventh year in a row that a storm formed before the official start of the season," she said. The first storm of the season, Ana, formed in May.

A key factor in determining whether a hurricane season will be above or below normal is the El Niño/La Niña cycle. El Niño suppresses hurricane formation because it produces strong westerly winds across the Atlantic that shear the tops off of tropical storms as they build. La Niña, by contrast, weakens those same winds, allowing more storms to form.

In 2020, there was no El Niño to suppress hurricane activity, Caldas said. That, combined with warmer water in the Atlantic and an enhanced monsoon season in western Africa, increased the likelihood of an active hurricane season.

Conditions are similar this year, she said, though La Niña might emerge later in the season.

Hurricanes are getting stronger, wetter, and slower

Earth's warming increases the chance that a hurricane will be more devastating. Rising ocean temperatures in particular play a major role, since the temperature of the water below a storm influences its wind speed. A 1-degree-Fahrenheit rise in ocean temperature can increase a storm's wind speed by up to 20 miles per hour, according to Yale Climate Connections.

That also enables storms to intensify and develop into powerful hurricanes in less time. There's a term for this: Rapid intensification refers to an increase of at least 35 mph in a storm's wind speeds within 24 hours. Last year, this happened with Hurricane Laura - its winds jumped from 75 to 140 miles per hour on August 26, catapulting the hurricane from Category 1 to Category 4.

These intensification events have gotten more frequent over the last 40 years. One study found that the chance a hurricane will undergo rapid intensification went from 1 in 100 in the early 1980s to 1 in 20 by mid-2005.

"Hurricanes are becoming stronger, wetter, slower, and more destructive, and all these trends have been linked to anthropogenic global warming in one way or another," Caldas wrote in a blog post.

Over the past 70 years or so, the speed at which hurricanes and tropical storms move has also dropped about 10%, according to a 2018 study. That's a problem, since it means any given storm has more time to pummel an area with wind and rain.

To make matters worse, a warmer atmosphere can hold more moisture, so a 10% slowdown in a storm's pace could double the amount of rainfall and flooding that an area experiences. The peak rain rates of storms have increased by 30% over the past 60 years. That translates to up to 4 inches of rain in an hour.