Atlantic Category 5 Hurricane Drought Reaches Eight Years

You may have heard about Florida's record hurricane drought. You may have also heard that the U.S. is in a record streak without a Category 3 or stronger hurricane landfall.

This week the Atlantic basin reaches another hurricane milestone. It has been eight years since a Category 5 hurricane has formed there.

Hurricanes attain Category 5 status, the highest level on the Saffir-Simpson hurricane wind scale, when their maximum sustained winds are at least 157 mph (137 knots or 252 kph).

These winds are capable of destroying many framed homes. The sheer magnitude of downed trees, power lines and power poles may lead to power outages lasting weeks, even months in some areas.

This, by the way, only considers the Category 5 hurricane's wind impact. If the landfall area is remotely surge-prone, as we saw with Hurricane Katrina, storm surge would wipe away buildings near the immediate coast, sometimes penetrating 5-10 miles inland along bays and inlets."Most of the area will be uninhabitable for weeks or months" after a Category 5 landfall, according to the National Hurricane Center. 

On Sept. 2, 2007, Hurricane Felix rapidly intensified to a Category 5 hurricane in the Caribbean Sea, then made landfall at that intensity in northeast Honduras two days later, destroying thousands of homes and claiming 130 lives in Honduras and Nicaragua.

While a hurricane's size is as important for impacts – if not sometimes more so – than its maximum winds (e.g. Ike, Sandy), there are several reasons there hasn't been an Atlantic Category 5 since Felix.

Category 5 Hurricanes Are Rare

From 1950-2014, there have been 25 Atlantic Category 5 hurricanes. These have occurred in 19 of the 65 hurricane seasons since 1950. On average, you can expect one such intense Atlantic hurricane every three to four years, so multiyear gaps are the norm.

An eight-year gap has happened before.

Hurricane Gilbert on Sept. 13-14, 1988, put to end a more than eight-year Category 5 drought, starting after Hurricane Allen reached Category 5 intensity three separate times along its track through the Gulf of Mexico in August 1980. 

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This stretch in the 1980s included several seasons with reduced numbers of hurricanes, including 1982 (two hurricanes) and 1983 (three) during and just after one of the strongest El Nino events on record. Atlantic hurricane activity can be suppressed during a stronger El Nino.

Only four seasons dating to 1950 had multiple Category 5 hurricanes, led by the record-smashing 2005 hurricane season's quartet of Emily, Katrina, Rita and Wilma. Two Category 5 hurricanes formed in 2007, 1961 and 1960.

Category 5 Hot Spots

Nineteen of the 25 Category 5 hurricanes since 1950 reached that intensity in either the Caribbean Sea or Gulf of Mexico. This includes the last seven Category 5 hurricanes, from Ivan (2004) through Felix (2007). 

Only six hurricanes during that time reached Category 5 intensity outside of that area, including Andrew and Hugo, among others.

So the key for formation of a Category 5 Atlantic hurricane most often is a track through either the Caribbean Sea or Gulf of Mexico, where a long track over warm water with minimal land, wind shear or dry air  allows an intense tropical cyclone to develop. 

Since 2007's Hurricane Felix through 2015's Hurricane Fred, 54 consecutive Atlantic hurricanes have failed to attain Category 5 status.

Despite the Category 5 drought, there have been intense Atlantic hurricanes.

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Interestingly, only two of those 12 – 2008's Gustav and Paloma – were in the Caribbean Sea or Gulf of Mexico when they were Category 4 hurricanes. Hurricane Ike weakened after a Category 4 landfall in eastern Cuba, while both Earl (2010) and Omar (2008) reached Category 4 status near the northern Leeward Islands.

It's Not Sea-Surface Temperatures

While a sufficiently deep layer of warm water is important as a starting point for tropical cyclones and certainly plays a significant role as the storm intensifies, sea-surface temperature is not the sole factor determining the ultimate intensity of a tropical cyclone.

It's often more complicated than that.

During the 2008 through 2014 Atlantic hurricane seasons, sea-surface temperatures were generally near or above average in the Atlantic basin, including the Caribbean Sea and Gulf of Mexico, according to an analysis from NOAA's Earth System Research Laboratory. 

In fact, the least warm anomalies were in a strip from eastern Cuba and the Bahamas into a part of the southwest Atlantic south of Bermuda. 

So there's no cooler-than-average water that would work against a hurricane during the Category 5 drought.

Wind shear, namely the change in wind speed and/or direction with height, effectively rips apart the structure of a tropical cyclone, pushing convection away from the center of circulation.

Given the prevailing roughly east-to-west surface winds in the tropical Atlantic basin, anomalous west winds aloft can be used as a proxy for wind shear. 

In the same seven hurricane seasons – 2008 through 2014 – the NOAA/ESRL analysis found anomalously strong westerly winds aloft in the Gulf of Mexico, much of the Caribbean Sea and near the Bahamas, roughly the same area lacking in even Category 4 hurricanes.

Dr. Philip Klotzbach, a tropical weather expert from Colorado State University (Wunderblog), found that August 2015 wind shear values over the Caribbean Sea were the highest on record dating to 1979.

Dry air is another inhibitor both for developing tropical cyclones and those attempting to reach Category 5 intensity.

Once ingested into the circulation, dry air encourages the development of stronger thunderstorm downdrafts, which then either keep nearby thunderstorms from forming or push them away from the center of circulation.

This dry air is also stable, meaning it suppresses upward vertical columns of air needed to maintain or form new thunderstorms.

The past three hurricane seasons have shown a particularly propensity for dry air over the Caribbean Sea and over parts of the belt between Africa and the Windward Islands.

So the combination of wind shear and dry air have done a number on the ability of Atlantic hurricanes to reach Category 5 status since 2007. 

Even the two Category 4 Caribbean hurricanes over the Caribbean Sea since 2007 – Gustav and Paloma, both in 2008 – were victimized by tracks over land, wind shear, and, in Gustav's case, dry air. 

History says this Category 5 drought can't last much longer.

However, as long as wind shear and dry air persist in the Caribbean Sea and Gulf of Mexico particularly during the peak months of the hurricane season, there's no telling how long this drought may last.

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