Heart of the Hurricane Season Starts Now

Jon Erdman
Published: July 29,2015
Every season has its peak.
For instance, all 162 games "count" in Major League Baseball's six-month regular season.  However, September games tend to take on greater importance, as teams stretch to reach the post-season.
Similarly, the Atlantic hurricane season officially kicks off in June. Certainly the first two months of the season can generate their share of tropical cyclones, some with significant impacts. This year we've already had three tropical storms, including Ana (May), Bill (June) and Claudette (July). Bill was the most significant storm of the three, causing flooding in the southern Plains, Ozarks and Ohio Valley.
However, when it comes to the most impactful tropical cyclones, there's a clear peak to the season.
(MORE: El Nino and Impact on Hurricane Season)

Breaking Down the Stats

This graph shows that named storm frequency is typically the highest August-October.

If there is one reason to pay attention during this time of the season, consider 93 percent of major hurricanes (Category 3 or stronger on the Saffir-Simpson hurricane wind scale), those responsible for the most destruction in any hurricane season, occur from August through October.
All 10 of the costliest hurricanes on record in the U.S. have occurred from August through October, including Sandy (late October 2012), Irene (late August 2011), Andrew (late August 1992), Ike (mid-September 2008) and, of course, Katrina (late August 2005).
(MORE:  Costliest Hurricanes)
Furthermore, 9 of the 10 deadliest U.S. tropical cyclones have also occurred during this three-month peak, including the infamous Galveston Hurricane of 1900.
What is it about these three months that make them so active?

More Real Estate Open

Typical tracks and origins of tropical cyclones in August, September and October.

Atmospheric conditions are more favorable over a much larger expanse of the Atlantic Basin in the peak months of August through October than early or late in the season.
In June, the tropical cyclone formation zones are confined to the Gulf of Mexico, western Caribbean Sea, or just off the Southeast U.S. coast.  This is one reason for the relative lack of early-season hurricanes compared to August through October.  Storms that form in these areas early in the season don't have far to go before reaching land.
From July into August, the formation zone gradually spreads east until the so-called main development region from the Lesser Antilles to just off Africa is in play. This opens up the potential for Cape Verde hurricanes to make the more than 3,500-mile trek from the eastern Atlantic Ocean toward the Caribbean islands or U.S. coast.

Factors: Accentuating Positives, Diminishing Negatives

Several factors play into opening up more of the Atlantic Basin to tropical cyclone formation in the peak months of August, September and October.
First, disturbances called "tropical easterly waves" moving off Africa become more vigorous and well-defined. These are the seedlings responsible for a large majority of all major hurricanes in the Atlantic Basin.
These waves wouldn't have a chance to develop if they are sheared apart.  During the peak of the season, wind shear, or the change in wind speed and direction with height, diminishes in the "main development region" (MDR) mentioned earlier (Lesser Antilles to off the African coast). This adds the MDR to areas of weaker shear in the Gulf of Mexico and western Caribbean Sea from early in the season.
Finally, sea-surface temperatures in the tropics tend to be at their warmest in September, as they have spent the entire summer soaking up the sun's energy.

INFOGRAPHIC: Heart of the Hurricane Season