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Florida Historically Becomes More Vulnerable To Hurricanes In September and October

General Tropical Cyclone Formation Zones in the Month of September
General Tropical Cyclone Formation Zones in the Month of September

It's already been an active hurricane season. Here's why it could get even more intense in the next couple of months.

Fifteen tropical storms or hurricanes have already come and gone in 2020, all before the traditional peak of hurricane season even arrives. Florida has escaped a direct hit so far, but is historically more vulnerable to one during the months of September and October.

The fifteen named storms to date would normally already account for a full season of activity, given that there are typically twelve named storms on average, six of which become hurricanes, and three that reach major status (Category 3 or higher). Also during an average year, four named systems, two hurricanes, and one "major" would have occurred by the beginning of September.

This year to date, a total of fifteen named storms - almost four times as many in an average year - have formed, four of which have become hurricanes, and one that was a major hurricane.

Laura became the fourth hurricane and the first major hurricane of the 2020 Atlantic Hurricane Season just last week. This marks the first time four hurricanes have formed in the tropical Atlantic basin by August 25 since the the record-breaking 2005 season.

However, this does not break the record for the most hurricanes seen so far up to this point. The record still stands at seven hurricanes, which occurred during the seasons of 1886 and 1893.

Hurricane Laura followed a typical path which is often seen during late August and early September. The warm sea surface temperatures and minimal vertical wind shear continue to dominate the entire Gulf of Mexico region which favors cyclone development. Tropical trade winds help to push these storms closer towards western coastlines of the Gulf of Mexico.

Louisiana and Texas, both of which sustained heavy damage from Hurricane Laura, statistically experience the majority of landfalling storms in the months of August and especially September. The atmospheric pattern begins to change though during the end of September.

Once late September and October arrive, upper-level westerly winds become stronger, cold fronts begin to dip farther south towards the Gulf of Mexico, and cool air masses aid in decreasing ocean temperatures. These factors hinder tropical cyclone formation and prevent many from tracking far north into the United States. Cyclones during this transition period become less likely to develop in the Gulf of Mexico or in the far eastern Atlantic.

Instead, October storms begin to favor other locations for formation including the western Caribbean, the Bay of Campeche, and the southern Gulf of Mexico, all of which are located much closer to land masses.

While northern latitude prepare to transition from summer to winter, the Caribbean is still home to warmer sea surface temperatures and minimal vertical wind shear, often creating ideal conditions which allow tropical systems to form. September may statistically be the most active month of hurricane season but October ranks as the most active month for hurricanes across South Florida due to this pattern change.

Some of the deadliest and most costly hurricanes to impact the United States have occurred in October including Hurricane Wilma (2005), Sandy (2012) and Michael (2018), with other notable storms such as Hurricane Matthew (2016) and Opal (1995).

October is also the month that has produced the strongest hurricanes recorded. Hurricane Opal intensified to reach a minimum barometric pressure of 916 mb, which remained the record until Hurricane Wilma reached an intensity of 882 mb, becoming the strongest tropical cyclones worldwide outside of the West Pacific. Both of these storms impacted during the month of October and both came ashore in Florida.

“Florida is still a big target in October,” said Colorado State University hurricane expert Phil Klotzbach. “The odds for Texas and the Carolinas get lower, but Florida is still vulnerable.”

In records dating back to 1851, 10 major hurricanes of Category 3 or higher have made a Florida landfall in October. August yielded only 6 total major hurricanes. September still reigns supreme with 19 major hurricanes impacting the coastlines of the Sunshine State but comparing these stats to western Gulf states will show that Florida remains very vulnerable during October.

From 1851 through 2008, 68 hurricanes directly impacted the state of Texas. The majority of them took place during the months of August and September with 22 (32.4%) and 23 (33.8%) landfalls, respectively. Comparing this to a significant decrease in the month of October where only five hurricanes impacted the Lone Star State (7.4%).

Louisiana is similar, in the same time period a total of 66 hurricanes impacted Louisiana. The month of August yielded 16 (24.2%) total hurricanes; September had the maximum at 29 (44%); and October showed less at 12 (18.2%).

Laura did become a strong Category 4 hurricane with maximum sustained winds near 150 mph, just shy of Category 5 strength as it came ashore near Cameron, Louisiana. However, Laura was not an outlier with regards to its track. Nevertheless, it made its mark in the history books as becoming one of the top ten strongest hurricanes (based on barometric pressure) to impact the United States.

Laura is now the seventh major hurricane to hit the state of Louisiana since records began in 1851 and, based on wind speeds, it ties for the strongest hurricane ever to make landfall in Louisiana. The last hurricane with wind speeds of 150 mph was known as the “Last Island Hurricane” which made landfall in 1856.

According to the National Hurricane Center, an average hurricane season would produce another seven named storms, four of which become hurricanes and two of which become major. If there are more than eight additional named storms, the Greek alphabet would have to be utilized for the second time on record. The last time storm names such as Alpha and Beta were used was during the historic hyperactive season of 2005.

Utilizing the Accumulated Cyclone Energy Index (ACE), which takes into account not just the frequency of tropical systems, but also the intensity and longevity of these storms, the 2020 hurricane season is the  ninth-most-active season-to-date since 1966, when full satellite coverage of the Atlantic Basin began, according to Klotzbach. Using the ACE index as a better proxy for seasonal activity, 76 percent of an average hurricane season's activity happens after August 28.

The North Atlantic has been trapped in an active period for an extended amount of time. The last time the basin experienced a near-normal hurricane season was in 2015 where eleven named storms formed, four of which were hurricanes, and two of those reaching major classification. This would make 2020 the fifth consecutive hurricane season that has experienced above-normal activity which is a record for the longest active stretch going back to 1900.

The peak of the Atlantic hurricane season normally runs from mid-August until late October, with the majority of tropical systems developing within this period. This is also typically when most tropical cyclones form in the Gulf of Mexico as well as the tropical Atlantic and eastward towards Africa.

Wind shear, which can tear tropical systems apart, tends to be strong in May but gradually begins to lessen in June and July before reaching a minimum by mid to late August. Additionally, ocean temperatures in the tropics begin to increase during the summer along with an increase in air temperatures and atmospheric moisture.

All the prime ingredients come together during the peak months of hurricane season which increases the frequency and strength of tropical cyclones. The statistical peak day of the hurricane season – the day you are most likely to find a tropical cyclone somewhere in the Atlantic basin – is September 10th.
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