Saturday, December 15, 2012

2012 Season Summary

The 2012 Atlantic hurricane season was above average, with

19 cyclones attaining tropical depression status
19 cyclones attaining tropical storm status
10 cyclones attaining hurricane status
1 cyclones attaining major hurricane status

Before the beginning of the season I predicted that there would be

15 cyclones attaining tropical depression status
13 cyclones attaining tropical storm status
5 cyclones attaining hurricane status
3 cyclones attaining major hurricane status

The number of tropical depressions, tropical storms, and hurricanes was well above my predictions prior to the start of the season, though there was only 1 major hurricane relative to the 3 I predicted.

Much of the discrepancy in the forecast can be explained by the status of the El Nino over the course of the season. Early in 2012, the NOAA predicted the development of a moderate El Nino event by midyear, which was to inhibit the development of cyclones in the latter half of the season. However, the development of the El Nino was delayed and inconsistent, allowing for periods of explosive cyclonic formation.

In spite of the very large number of cyclones, there was only a single major hurricane, Michael, during the season. Broader cyclones such as Ernesto, Isaac, and Leslie struggled mightily in strengthening and grossly underperformed in intensity relative to short-term forecasts. Leslie, for example, was predicted to reach strong Category 2 strength, but its peak intensity was only as a minimal Category 1. Smaller cyclones, though, did the opposite, often surviving or intensifying more than expected. Michael, the strongest cyclone by winds during the season, intensified in an only marginal environment, even doing so in relatively close proximity to the much larger Hurricane Leslie. These data are indicative of the prevalence of dry air throughout the basin during the season and also of differences in the direction of upper- and lower-level winds over the season.

Another interesting trend that emerged around early September was the development of unusual blocking patterns and lack of steering currents over the northern part of the Atlantic basin. This was due to an anomalous shift of the jet stream to the north over the Atlantic, as this allowed subtropical ridges to remain in place, and prevented cyclones from being caught in the flow of troughs, which followed a more poleward path. The trend became evident with Leslie and Michael, and was exemplified by Nadine, which survived for a very long time over the north Atlantic, and finally by Hurricane Sandy, which was able to recurve westward due to the unusual shape of the jet stream.

Some notable cyclones and facts about the season include:

  • 2012 was the third Atlantic hurricane season in which two preseason storms (in this case Alberto and Beryl) formed, after 1887 and 1908
  • Beryl was also the strongest Atlantic preseason cyclone since 1972
  • Debby, becoming a named storm on June 23, surpassed the previous record for earliest fourth tropical cyclone formation of July 7 set during the 2005 Atlantic Hurricane Season
  • Though Gordon developed into a tropical storm before Helene, the tropical depression that became Helene formed before that which became Gordon, and thus the names are reversed on the chronological listing
  • Michael was the strongest storm of the season by wind speed, achieving 115 mph winds, or minimal Category 3 intensity, the lowest highest wind speed for an Atlantic hurricane season since 1994
  • Hurricane Nadine spent 21.75 days as a tropical cyclone meandering around the northeast Atlantic, making it fifth on the all-time list of longest lasting tropical cyclones
  • Hurricane Sandy recorded the season's minimum pressure of 940 mb as it transitioned to an extratropical cyclone and made landfall in New Jersey
  • The 2012 season overall was tied for the third most active in terms of number of named storms
Overall, the 2012 Atlantic hurricane season was quite active, and included several hurricanes, but was not very intense. Hurricane landfalls included Ernesto in the Yucatan Peninsula and Isaac in Louisiana, both of which were not particularly strong, and caused relatively little damage. The Caribbean Sea was relatively quiet, as wind shear and dry air mostly kept cyclones in the region to tropical storm intensity, with the exception of Hurricane Sandy, the most notable storm of the season. Sandy, which made landfall in Jamaica, Cuba, and the Bahamas as a hurricane, went on to develop into a hybrid system and slam into the northeast coast, causing widespread damage and unprecedented storm surge along the New Jersey and New York coastlines.

1 comment:

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