A Memorable Below Average Hurricane Season

18 12 2015

Dale C. S. Destin |

The 2015 hurricane season came to a quiet end November 30. Overall, it was a below average year, based on NOAA’s criteria. However, for us in the Caribbean, it will live on in everlasting memory, due to the catastrophic impact of Erika on Dominica.

Tropical Storm Erika Over the Eastern Caribbean

Tropical Storm Erika Over the Eastern Caribbean Aug 27, 2015. Credit NASA

Before Erika, Danny “scare the living daylights” out of many when it rapidly and unexpectedly intensified into a major hurricane, less than 800 miles east of the Caribbean. However, as predicted, it just about died before reaching the islands.

Less than three after Danny, Erika caused horror across Dominica. In less than 12 hours, the storm deluged much of Dominica with over 320 mm (12 in) of rain, 229 mm (9 in) of which fell in less than 6 hours. Peak rain rate in excess of 36.5 mm (1.44 in) were observed leading to catastrophic flooding and mudslides.

24-hr Radar Rainfall Estimates ending 2 am Sun, 28 Aug, 2015

24-hr Radar Rainfall Estimates ending 2 am Sun, 28 Aug, 2015. Rainfall over Dominica is off the chart. At most, southwest Antigua had 30 mm (1.18 in)

There were villages flattened, around 890 homes were destroyed or left uninhabitable while about 14,300 people were left homeless. Erika killed at least 31 persons in Dominica, the deadliest disaster in the country’s history since Hurricane David in 1979.

Flood water in Dominica from Tropical Storm Erika - Aug 27, 2015

Cars being washed away in flood waters in Dominica from Tropical Storm Erika – Aug 27, 2015

The system caused around half a billion dollars in damage to Dominica, around 100% of GDP, setting the country’s development back by at least 20 years.

Erika also caused major flooding and landslides in Puerto Rico and Hispaniola. She also took the lives of five persons in Haiti.

Jun-Nov 2015 Standard Precipitation Index. Reds indicate low rainfall, blue indicate high rainfall

Jun-Nov 2015 Standardized Precipitation Index (SPI). The darker the reds (negative values), the drier the weather; the darker the blues (positive values), the wetter the weather. Most of the Caribbean basin, has had near record or record low rainfall (red), this is at least a 1 in 40 year event.

This hurricane season will also be remembered for being the record driest or among the driest ever on record for much of the Caribbean basin. Antigua, for example, had record low rainfall for the season; the rainfall was 48% below normal. Much of the rest of the region had at least 25% less rainfall than normal.

The season produced 11 named storms. Of the 11, 4 became hurricanes and 2 reached major hurricane status – category three (3) or higher. The strongest was Joaquin which had peak winds of 155 mph. The total ACE was 63% of the median or 56% of average (1981 – 2010). This is the fourth lowest since 1995.

2015_HurricaneSeasonSummary

Our ensemble forecast did quite well in predicting the eventual outcome of the season. It called for nine named storms (we got 11) of which four were predicted to become hurricanes (we got four) and one to become a major hurricane (we got two). We also predicted an ACE of 52 and we got 59.

The below average season was due largely to suppressing effect of El Nino on rainfall processes over the tropical Atlantic and Caribbean basin. Secondarily, a positive North Atlantic Oscillation caused cooler than normal sea surface temperature during the early part of the season.

Other notable facts of the 2015 Atlantic hurricane season are:

  • Combined, 2013-2015 totals 12 hurricanes. This is the lowest three-year figure since 1992-1994 (11 hurricanes).
  • The total number of major hurricanes for the period 2013-2015 is four. No other three-year interval has had fewer major hurricanes since 1992-1994 (2 major hurricanes).
  • Joaquin is the first Category 4-5 hurricane to impact the Bahamas during October since 1866.
  • The ACE for September was only 11. The Atlantic three-year (2013-2015) summed ACE for September was only 44, the lowest since 1912-1914 when only an ACE of 29 was recorded during September.
  • June-October-averaged 40,000-5,000 feet vertical wind shear (hostile to tropical cyclone) in the Caribbean (10-20°N, 90-60°W) was 28.5 knots (33 mph), the strongest on record since 1979.
  • Hurricane Fred is the farthest east that a hurricane has ever formed in the Atlantic basin.

This season reminded us all of the need to be prepared for every hurricane season, below average or not. Again, a below average season does not mean you are guaranteed a cyclone-free year. The past two years have had below normal activity, yet the region had major damage from Erika and Gonzalo (2014).

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