Higher probabilities of hurricane landfall
The annual April forecast of three organisations all predict considerably above-average tropical cyclone activity during the 2011 Atlantic Basin hurricane season.
In their forecast issued Wednesday, Colorado State University scientists Phil Klotzbach and William Gray reduced their December forecast number of named storms by one, to 16, but again predicted nine hurricanes and five major hurricanes.
UK-based Tropical Storm Risk’s forecast was issued Monday, predicting 14.2 named storms, 7.5 hurricanes and 3.6 major hurricanes. AccuWeather.com issued its forecast last week, predicting 15 named storms, eight hurricanes and three major hurricanes.
Historical averages in the Atlantic Basin are around 10 named storms, six hurricanes and two major hurricanes.
Mr. Klotzbach and Mr. Gray stated that both their statistical and analog forecasts call for a very active Atlantic hurricane season.
“Our seasonal forecast has been reduced slightly from early December, since there is a little uncertainty about ENSO and the maintenance of anomalously warm tropical Atlantic [sea surface temperature] conditions,” stated their forecast report.
ENSO refers to the El Niño/La Niña-Southern Oscillation, a cyclical warming and cooling of sea surface temperatures of the equatorial Pacific Ocean. When an El Niño is present, warmer sea surface temperatures cause higher upper atmosphere wind shear in the areas of the Atlantic Ocean and Caribbean Sea where most hurricanes form. Higher wind shear inhibits the formation, continuation and strengthening of tropical cyclones. Conversely, the cooler sea surface temperatures of La Niña cause less wind shear in the tropical Atlantic and Caribbean, as do ENSO neutral conditions.
Mr. Klotzbach and Mr. Gray noted in their report that the moderate-to-strong La Niña conditions that were in place through the winter of 2010/11 have weakened rapidly. However, the scientists do not believe it will transition into an El Niño by late summer and early fall, the peak part of the hurricane season.
“There is a very wide spread in the model guidance for the August-October period, with several models calling for either El Niño or La Niña conditions and the rest calling for neutral conditions,” the scientists’ report states. However, Mr. Klotzbach and Mr. Gray point out that only one of the European Centre for Medium-Range Weather Forecasts’ ensemble models – which are known to be the most accurate with El Niño/La Niña-Southern Oscillation forecasts – predict sea surface temperatures approaching El Niño by September, with the rest showing a La Niña persisting or ENSO neutral conditions.
Based on that and other climatology information, the scientists’ best estimate is that there will be neutral ENSO conditions during the 2011 hurricane season.
“Since we expect to continue to see a warm tropical Atlantic, we believe that ENSO will not be a significant detrimental factor for this year’s hurricane season,” they stated. “However, there remains a need to closely monitor these conditions in the next few months. We should be more confident about ENSO conditions for the upcoming hurricane season by the time of our next forecast on June 1.”
Mr. Klotzbach and Mr. Gray stated that a significant focus of their recent research involved forecasting probability of hurricane landfall along the US coastline and in the Caribbean.
“Whereas individual hurricane landfall events cannot be accurately forecast months in advance, the total seasonal probability of landfall can be forecast with statistical skill,” the scientists wrote.
The scientists estimate considerably higher probabilities of a landfall of a tropical storm, hurricane and major hurricane along the US coast and in the Caribbean during the 2011 hurricane season.
With regard to the Caribbean, Mr. Klotzbach and Mr. Gray estimate a 95 per cent probability of the landfall of a tropical storm; a 77 per cent chance of a land-falling hurricane; and a 61 per cent chance of the landfall of a major hurricane. Average probabilities for those occurrences in the Caribbean are 82 per cent, 57 per cent and 42 per cent, respectively. For the Cayman Islands specifically, it is estimated that there is a 53 per cent chance of a tropical storm tracking within 50 miles in 2011. The figure is 33 per cent for a hurricane and 13 per cent for a major hurricane. When expanding the track to within 100 miles, the percentages are 68, 44 and 13, respectively.
Tropical Storm Risk also forecasts a 25 per cent above-normal chance of a land-falling tropical cyclone in the Atlantic Basin in 2011.
The National Hurricane Center in Miami does not issue its first hurricane season forecast until late May. Tropical Storm Risk will also update its forecast in late May. Mr. Klotzbach and Mr. Gray will issue their next forecast on 1 June.
The Atlantic Basin hurricane season runs from 1 June through 30 November. However, Atlantic Basin hurricanes have occurred since 1851 in every month except April, and tropical storms have occurred every month of the year.