Why are there so many tropical cyclones in late August?

Some of the most impactful storms in U.S. history have developed or made landfall during late August
FILE: A satellite image of Hurricane Katrina from 2005
FILE: A satellite image of Hurricane Katrina from 2005(NOAA)
Published: Aug. 25, 2020 at 8:15 AM CDT|Updated: Aug. 25, 2020 at 8:27 AM CDT
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ALEXANDRIA, La. (KALB) - The last week of August is a historically busy one for tropical cyclones, and August 2020 is shaping up to be no different, with Marco and Laura both impacting the Gulf of Mexico. A tropical cyclone is an organized cluster of storms that rotates counter-clockwise in the northern hemisphere. Tropical depressions, tropical storms, and hurricanes are all tropical cyclones.

Meteorologists consider the peak of hurricane season to be from August through October, with the highest number of tropical cyclones each year typically forming during this three-month timeframe. As history indicates, some of the deadliest and most destructive storms have occurred during these months, but why?

Climatology shows that the peak of hurricane season occurs between August and October.
Climatology shows that the peak of hurricane season occurs between August and October.(KALB)

August is the point in hurricane season where meteorological factors are often the most favorable for tropical cyclones to develop. Temperatures on the surface layer of the ocean, referred to as Sea Surface Temperatures, are rising. This helps fuel the storms. Wind shear- the change in wind speed or direction with height- is typically lower during these months. Too much wind shear will tear a storm apart, but low wind shear will allow the storm to remain organized.

The late summer months also bring more instability. Instability is needed for thunderstorms to thrive and all tropical cyclones begin as a small cluster of storms referred to as a “tropical wave” that can eventually organize into hurricanes. These conditions allow for tropical waves off the coast of Africa to thrive during this time. Earlier in the season, some of these waves encounter Saharan air- dry, dusty air that can cause them to weaken, since tropical cyclones need moist air. However, by August the Saharan air isn’t as prevalent, so tropical waves can go on to strengthen into tropical cyclones.

There are many other more complicated factors that influence the development or lack of development of tropical cyclones, including climate cycles and global ocean-atmosphere patterns like El Nino or La Nina, but the above factors are some of the most basic ones.

The Atlantic basin is typically active during the month of August and tropical cyclones can...
The Atlantic basin is typically active during the month of August and tropical cyclones can affect the Gulf Coast, Florida peninsula, or the East Coast during this time.(KALB)

Here are some of the most impactful tropical cyclones to either develop or affect the United States during the last week of August:

  • Possibly the most infamous hurricane to affect Louisiana was Hurricane Katrina. Fifteen years ago this week on August 25, 2005, Hurricane Katrina made landfall in southeast Florida. Katrina made a second landfall on the Louisiana coast on August 29, and a third on the Mississippi coast. Katrina killed 1,833 people and is among the top 5 deadliest hurricanes on record.
  • On August 28, 2019, Dorian reached hurricane status. Dorian went on to make landfall in the Bahamas on September 1 as a category 5 hurricane, with the second strongest winds in the Atlantic since 1950.
  • On August 30, 2018, a tropical wave formed off the coast of Africa. The wave would go on to become Hurricane Florence and make landfall in North Carolina in early September. Florence is the wettest tropical cyclone on record for North Carolina and caused massive flooding.
  • On August 25, 2017, Hurricane Harvey made landfall in Rockport, Texas as a category 4. Harvey made a second landfall near Cameron, Louisiana. Harvey holds the record for producing the most rainfall from a tropical cyclone with just over 60″ in Nederland, Texas.
  • On August 24, 1991, Hurricane Andrew made landfall in southern Florida as a category 5. It made landfall again in Louisiana as a category 3 several days later.
  • On August 27, 1881, The Georgia Hurricane made landfall near Savannah killing 335 people there and 700 total.
  • On August 27, 1900, The Great Galveston Hurricane of 1900 made it’s first landfall in the Caribbean. The same storm went on to make landfall in Galveston several days later as a category 4, killing 8,000 people. It is still the deadliest hurricane on record.
  • On August 29, 1935, The Great Labor Day Hurricane of 1935 first formed in the Caribbean. By Labor Day, the category 5 hurricane struck the Florida Keys, killing nearly 500 people.

Other notable hurricanes that occurred earlier in the month of August include Charley in 2004, the Last Island Hurricane of 1856, the Sea Islands Hurricane of 1893, and Hurricane Camille in 1969. Many other notable hurricanes have occurred in the months of September and October.

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