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Does an Active June and July Point to an Above-Average Atlantic Hurricane Season Overall?
Published: July 18, 2017
The 2017 Atlantic hurricane season has already produced five tropical cyclones by the middle of July, a quick start for what is historically a slower time of the hurricane season.
Does this early-season activity say anything about what we can expect in the months ahead? Perhaps; it could point to an active rest of the season, as forecasters already projected.
Arlene (April), Bret (June), Cindy (June) and Don (July) already have been crossed off this year's named storm list. In addition, Tropical Depression Four formed in the Atlantic in early July, but stuck around for a short time and never attained tropical storm strength.
Overall, the activity in June and July has little effect on what happens in the remaining months of the Atlantic hurricane season, based on 1944-1994 data evaluated by the Hurricane Research Division (HRD) of NOAA.
What's more interesting about this early start is that three of those tropical cyclones – Tropical Storm Bret, Tropical Storm Don and Tropical Depression Four – formed in the Atlantic's so-called main development region (MDR). The MDR is a swath of the tropical Atlantic Ocean that stretches from Africa's western coast into the Caribbean.
Having three systems form in that region this early is unusual. In fact, on June 19, Bret marked the earliest a storm has formed in the MDR, beating Ana, which formed June 22, 1979.
Typically, this region of the Atlantic Ocean sees most of its tropical cyclones develop during the peak of the hurricane season – August into October. This is referred to as the Cape (Cabo) Verde Season since the origin of such storms is from tropical waves that pass near the Cabo Verde Islands.
When it comes to just the systems that develop in the MDR in June and July, there is somewhat of a trend.
The HRD says that overall activity in the Atlantic for a given season has been "at least average and often times above average" when a tropical storm or hurricane forms in the MDR during June or July based on 1944-1999 data.
This year marks just the 12th time dating to the mid-1800s that two tropical cyclones have formed in the MDR in a season prior to Aug. 1, according to Dr. Phil Klotzbach, a tropical scientist at Colorado State University. It should be noted, however, that this could have happened in a few other seasons before satellites went into service in the mid-1960s since some tropical cyclones may have gone undetected in the MDR region.
Almost all of those years with two MDR tropical cyclones prior to Aug. 1 ended up being active hurricane seasons overall.
The exception is 2013 when Tropical Storm Chantal formed in the MDR during early July, signaling a potentially active season. While there was a near-average number named storms that year, many of them were short-lived as they succumbed to dry air and/or strong winds aloft.
Only two hurricanes developed in the 2013 season, marking the first time since 1982 that two or fewer hurricanes developed in the Atlantic in a year.
"Other than 2013, all of the other seasons with active June-Julys in the MDR at least ended up with average activity," said Klotzbach in an email to weather.com.
Klotzbach added that having a named storm form in the MDR in June or July is "typically considered a sufficient but not a necessary condition for an active season."
The 2004 hurricane season was cited by Klotzbach as an example because it didn't have a single named storm develop through the end of July. Despite the slow start, the season ended up above average with 15 named storms overall. Four of those storms – Charley, Frances, Ivan and Jeanne – struck Florida as hurricanes during August and September.
Of course, what matters most during hurricane season is not how many storms form, but rather where they track.
There is no strong correlation between the number of storms or hurricanes and U.S. landfalls in any given season. One or more of the additional named storms this season could hit the U.S., or none at all. Therefore, residents of the coastal United States should prepare each year no matter the forecast.
A couple of classic examples of why you need to be prepared each year occurred in 1992 and 1983.
The 1992 season produced only six named storms and one subtropical storm. However, one of those named storms was Hurricane Andrew, which devastated South Florida as a Category 5 hurricane.
In 1983 there were only four named storms, but one of them was Alicia. The Category 3 hurricane hit the Houston-Galveston area and caused almost as many direct fatalities there as Andrew did in South Florida.
In contrast, the 2010 season was active. There were 19 named storms and 12 hurricanes that formed in the Atlantic Basin.
Despite a large number of storms that year, not a single hurricane and only one tropical storm made landfall in the United States.
In other words, a season can deliver many storms, but have little impact, or deliver few storms and have one or more hitting the U.S. coast with major impact.
MORE: Retired Atlantic Tropical Storm and Hurricane Names
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