The Unusually Quiet Atlantic Hurricane Season of 2013 Ends

The end of the unusually quiet Atlantic hurricane season of 2013 is at hand. The final tally of thirteen named storms was above the average of eleven for a season, but the two hurricanes (Ingrid and Humberto) and zero major hurricanes were well below the average from 1950 - 2012 of six and three, respectively. The 2013 season ranked as the sixth-least-active Atlantic hurricane season since 1950, in terms of the collective strength and duration of named storms and hurricanes (ACE index), which was just 33% of the 1981 - 2012 average. The 2013 hurricane season was the first time since 1994 no major hurricanes formed, and was only the third below-normal season since the high-activity period for Atlantic hurricanes began in 1995. NOAA and the U.S. Air Force Reserve flew 45 hurricane hunter aircraft reconnaissance missions over the Atlantic basin this season, totaling 435 hours--the fewest number of flight hours since at least 1966, said NOAA in a press release summarizing the 2013 hurricane season.



Worst storm of the season: Ingrid
Mexico took a severe beating in 2013, with eight landfalling storms: one hurricane (Ingrid) and two tropical storms (Barry and Fernand) from the Atlantic side, and two hurricanes (Manuel and Barbara), and three tropical storms from the Pacific side. The deadliest and most expensive Atlantic storm of 2013 was Hurricane Ingrid, which weakened to a tropical storm with 65 mph winds before hitting Mexico about 200 miles south of the Texas border on September 16, 2013. Ingrid's heavy rains triggered flooding that killed 23 and did $1.5 billion in damage, making the storm the 7th costliest tropical cyclone in Mexican history. Barry and Fernand, which both hit the Mexican coast in the Gulf of Mexico between Tampico and Veracruz, dumped torrential rains and triggered floods that killed five and fourteen people, respectively. The first storm of the season, Tropical Storm Andrea, was the only named storm to make landfall in the United States this year. Andrea brought tornadoes, heavy rain, and minor flooding to portions of Florida, eastern Georgia and eastern South Carolina, causing one fatality and damage less than $25 million. No other deaths were recorded from Atlantic named storms in 2013. Tropical Storm Chantal did minor damage on Dominica and Martinique in the Lesser Antilles, and Tropical Storm Gabrielle did minor damage on Bermuda.


Figure 1. The strongest Atlantic hurricane of 2013, Category 1 Hurricane Ingrid, lays siege to Mexico on September 15, 2013. Ingrid killed 23 and did $1.5 billion in damage to Mexico. On the Pacific side, we see Tropical Storm Manuel, which killed 169 people and did $4.2 billion in damage to Mexico. Image credit: NASA Earth Observatory.

A preseason forecast bust
It was a bad year to be in the seasonal hurricane forecast business. All of the pre-season forecasts called for at least 7 hurricanes, 3 major hurricanes, and an ACE index at least 30% higher than average. With the actual numbers being 2 hurricanes, 0 major hurricanes, and an ACE index of just 33% of average , these forecasts were a major bust. The only pre-season forecast that one could deem successful was issued by a team at Penn State, led by Dr. Michael Mann, who only attempted to predict the number of named storms (they said 12 - 20, with a best estimate of 16.) The preseason forecasts largely failed because many of the factors that usually lead to active seasons that we can look at months beforehand all pointed towards an active season:

1) No El Niño was present. When El Niño conditions are not present in the Eastern Pacific, wind shear tends to be low over the tropical Atlantic, favoring hurricane formation.

2) Ocean temperatures were above average.

3) Sea level pressures were lower than average.

4) Wind shear was near average.

5) The African Monsoon was active, with many strong tropical waves emerging from the coast of Africa. These disturbances form the nucleus for about 85% of all major hurricanes.

However, these factors tell only roughly 50% of the story. The other 50% is not predictable more than a week or two in advance: the large-scale atmospheric circulation. This summer and fall, an unusually strong trough of low pressure over the Central Atlantic brought large amounts of dry, sinking air to the tropical Atlantic. Large amounts of dry air also invaded from the Sahara, and from Northeast Brazil, which had suffered the most expensive drought in Brazil's history ($8 billion) earlier in the year. The combined onslaught of dry air from these multiple sources was enough to overwhelm the otherwise favorable conditions for development, leading to one of the quietest Atlantic hurricane seasons on record. According to Phil Klotzbach of the Colorado State seasonal forecast team, the relative humidity at the 700 mb level (roughly 10,000 feet) in the Main Development Region of the tropical Atlantic (7.5- 22.5°N, 20-75°W) in August was the lowest observed in the past 35 years, and was the 8th lowest during September. The strength of the sinking motion of the air in this region during August and September was the second greatest of the past 35 years. It's tough to sustain a thunderstorm updraft when there is so much dry, sinking air at middle levels of the atmosphere.



Special Characteristics of the 2013 Hurricane Season
The 2013 hurricane season had the following special characteristics, as summarized by Phil Klotzbach of the Colorado State seasonal forecast team:

• Thirteen named storms occurred during 2013. This is the most named storms to occur in a year with two or fewer hurricanes in the historical record. The 1931 hurricane season had thirteen named storms but only three hurricanes.

• 35.75 named storm days (NSD) occurred during 2013. This is the fewest NSD since 2009 (30 NSD).

• Two hurricanes formed in 2013. This is the fewest hurricanes since 1982, when two hurricanes also formed.

• No major hurricanes formed in 2013. The last year with no major hurricane formations was 1994.

• ACE in 2013 was only 30 units. This is the lowest ACE for an Atlantic hurricane season since 1983 (17 ACE units.)

• No major hurricanes made U.S. landfall in 2013. The last major hurricane to make U.S. landfall was Wilma (2005), so the U.S. has now gone eight years without a major hurricane landfall. Since 1878 when relatively reliable landfall data became available, the U.S. has never had an eight-year period without a major hurricane landfall.

• The maximum intensity reached by any hurricane this year was 85 mph (Humberto and Ingrid). This is the weakest maximum intensity achieved by the most intense hurricane of a season since 1968 (Gladys, 85 mph.)

• Humberto reached hurricane strength early on September 11. It became the second latest forming first hurricane of the year, developing into a hurricane just hours before the previous record latest forming first hurricane of the year (Gustav, 2002.)

• Two tropical cyclones (TCs) formed in the Main Development Region (south of 23.5°N, east of 75°W) prior to 1 August. The last year with two TCs forming in this region prior to 1 August was all-time busiest hurricane season on record, 2005. The median ACE of the 10 years with two TCs in the MDR prior to 1 August was 174 ACE units. The 2013 season clearly defied many of the typical pre-season climate signals.


Video 1. Wunderground member CycloneOz put together this animation of all the named storms of the 2013 Atlantic hurricane season.

Have a great weekend, everyone, and I'll be back Monday with a new post.

Jeff Masters