Record-setting hurricane season may get more active

August and September typically brings the greatest increase in tropical activity.
Published: Jul. 31, 2020 at 10:15 PM CDT
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WACO, Texas (KWTX) - The 2020 hurricane season is only one-third through but it has already been one of the most active seasons on record. The season got off to an early storm with Tropical Storm Arthur forming on May 16th, but there have been seven more named storms since then, two of which became hurricanes. Cristobal, Edouard, Fay, Gonzalo, Hanna, and Isaias have all set the record for the earliest storm formation for that storm’s letter. Hurricane Isaias, currently lashing the Bahamas with high winds and rain, formed on July 29th. On average, we wouldn’t see the ‘I’ storm (the 9th named storm of the season) until October 4th.

The record-setting start to the season may only get worse over the coming weeks and months. The frequency of tropical storms and hurricanes greatly increases during August and September leading up to the peak of hurricane season on September 10th. The frequency of tropical storms and hurricanes does go down during the latter half of September, but hurricane season’s second peak is in mid-October.

On average, August and September see the biggest increases in tropical storms, hurricanes, and...
On average, August and September see the biggest increases in tropical storms, hurricanes, and major hurricanes leading to the peak of hurricane season on September 10th.(KWTX, National Hurricane Center)

During the months of August and September, nearly the entire tropical Atlantic is active. In August, tropical cyclones most frequently move into the Caribbean Sea, Gulf of Mexico, and near the United States. August storms typically form near the Cabo Verde Islands, an island chain off the northwest coast of Africa. The so-called ‘Cape Verde’ storms are typically strong and long lasting. Some notable Cape Verde hurricanes include Hurricane Dean (2007), Hurricane Frances (2004), and Hurricane Andrew (1992). Cape Verde storms most frequently move into the Gulf of Mexico while other storms in the western Atlantic and the Lesser Antilles most often move toward Florida or turn out to sea. September’s tropical cyclone climatology and tracks are similar to August but with higher frequency.

Although we see tropical storms and hurricanes more often in August and September, meteorologists across the country are worried that the typical most-active time of the year will be especially so this year for two different reasons: high sea-surface temperatures and low wind shear.

Tropical storms and hurricanes require warm sea-surface temperatures to form and to strengthen. The typical threshold for tropical development is 80° and the entire tropical Atlantic is at or above 80°. Unfortunately, the entire tropical basin, with the exception of the Gulf of Mexico, has sea-surface temperatures 1° to 3° above normal for this time of year.

A map of current sea surface temperatures in the tropical Atlantic. The entire tropical...
A map of current sea surface temperatures in the tropical Atlantic. The entire tropical Atlantic has sea-surface temperatures at or above 80° which is the typical threshold for tropical storms to develop.(KWTX, NOAA)

Wind shear is lower than normal across the Atlantic because of La Niña. La Niña, which is the opposite of El Niño, is when sea-surface temperatures along the northwestern coastline of South America are cooler-than-normal. When sea-surface temperatures are cooler than normal, the atmosphere over the Pacific Ocean is typically more stable with lower-than-normal thunderstorm activity. When there’s a lack of thunderstorms, the sub-tropical jet stream is weaker. The sub-tropical jet stream, which does run from west-to-east through the Atlantic Ocean, is a source of wind-shear over the tropics. A weaker jet stream means lower wind shear. Since wind shear can tear into tropical systems and cause them to dissipate, a lack of wind shear is more conducive for them to form.

In May, before the season started, the Climate Prediction Center released the 2020 Atlantic Hurricane Season Outlook which called for an above-normal hurricane season with between 13 and 19 named storms but those projections may need to be revised upward because of the record-pace we’re on. No matter how many storms form over the next four months, the 2020 hurricane season will be

one for the record books and it’s not showing signs of slowing.

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