August hurricane analysis shows where Atlantic storms form and which US states are most vulnerable
Peak hurricane season in the Atlantic Basin unfolds in mid-August and lasts through October.

August is known for several notable, destructive hurricanes in the United States.

From Katrina to Harvey, August hurricanes have caused billions of dollars of damage across the southern and eastern United States.
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Storms in the early part of the season, May, June and July, tend to form due to interactions with weather systems and other non-tropical features.

However, sea surface temperatures begin to increase in August, causing the entire tropical belt to become more active, according to AccuWeather Hurricane Expert Dan Kottlowski.

Therefore, the ability for tropical systems to form from tropical features tends to increase.

"Areas of low pressure develop across Africa, as [they] move off the coast, they become tropical waves," Kottlowski said.

Tropical waves cause most tropical development in August, September and October.

Winds also impact tropical development, as wind shear helps to determine the intensity.

All the variables become very favorable for tropical development across the Atlantic basin in August, September and October.

Water temperature is the biggest factor in tropical development.

"When the water temperature rises, it causes the surface pressure to lower. When the surface pressures are lower, it makes it much easier for storms to spin up," Kottlowski said.

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Tropical waves moving off the coast of Africa already have a slight rotation. Warmer ocean water and lower surface pressure cause the rotation to tighten up, which is why there is greater development in August.

As the season continues into September, systems are more likely to form in the central and eastern Atlantic than in the Gulf of Mexico. During this time, ocean temperatures are also climbing toward their peak.

In the last 20 years, 81 named tropical systems have developed. Sixty of these storms developed in the Atlantic, seven formed in the Caribbean, and 14 formed in the Gulf of Mexico, according to AccuWeather analysis of National Oceanic and Atmospheric Administration (NOAA) data.

Seventeen of these storms made their first direct landfall in the U.S.; some made secondary U.S. landfalls, such as Hurricane Katrina.

Harvey, Bret and Bonnie are a few of the storms that made their first landfall as a hurricane.

U.S. states and territories that were hit most frequently include Florida, Texas, Louisiana and Puerto Rico.

Eleven of the 81 named storms strengthened to at least major hurricane status. Charley, Katrina, Bret, Earl and Frances are among those most relevant to the U.S.

Most of those stronger hurricanes strengthened in the Atlantic. However, the Caribbean basin held the longest sustained major hurricane in the 20-year period, which was Hurricane Dean, according to the AccuWeather analysis.
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The majority of hurricanes formed in the Atlantic basin in the last 20 years, which is largely because the Atlantic basin has been warmer than normal.

The Atlantic basin has been warmer because it is in the warm phase of the Atlantic Multidecadal Oscillation (AMO), according to Kottlowski.

The AMO is a climate cycle that affects the sea surface temperature of the North Atlantic Ocean. The current warm phase began in 1995. The result is warmer sea surface temperatures in the Atlantic basin over the last 23 years, leading to an increase in tropical storm activity in the Atlantic basin.

"Sea surface temperatures are not quite as high as they were about 10 years ago, so the question is whether the AMO cycle going back to a cooling trend," Kottlowski said.

However, the transition is gradual, so a conclusion cannot yet be determined.

Sea surface temperatures have been higher off the Carolina and Northeast coast of the U.S. However, temperatures have been cooler in the North Atlantic off the coast of Africa, in the main tropical developmental region.

Water temperature has been averaging about 1.5 degrees below normal in that developmental region in the late spring of 2018. However, in late July and early August of 2018, the trade winds have dropped off and the ocean water is no longer as cool. Sea surface temperatures across that region are now only half a degree below normal.

"This shows how quickly the water can warm up, and that’s what we see in August all the time," Kottlowski said. "It’ll be interesting to see what happens in the next few weeks with that water temperature."

These destructive August storms wreaked havoc when they hit the U.S.:

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