Musings: Updated 2011 Hurricane Forecast Still Calls for Active Year

The latest forecast update from Professors Philip Klotzbach and William Gray of the Department of Atmospheric Science at the Colorado State University (CSU) says the upcoming hurricane season is expected to see above-average activity. The April 6th forecast is slightly lower than their December 2010 forecast largely due to uncertainty about the sea surface temperatures in both the South Pacific and South Atlantic oceans that can assist or retard the development and strengthening of tropical storms. 

The forecasting team has developed a new April methodology based on a data collected from 1982-2010. There are four predictors employed in the model with two of them based on sea surface temperatures in the Atlantic and Pacific oceans. The Pacific Ocean for most of the past year has been cooler than normal, which helped contribute to the Atlantic basin’s storm activity last year mostly turning north before reaching the U.S. In general, sea surface temperatures in the eastern and central tropical Pacific Ocean have been 0.50C-1.00C below average.

Exhibit 21.  Pacific Ocean Sea Surface Temperatures Low
Pacific Ocean Sea Surface Temperatures Low
Source:  Colorado State University

On the other side of the globe, Atlantic Ocean sea surface temperatures remain at or above average levels. They have cooled recently but most likely that has been caused by a shift from a negative phase for the North Atlantic Oscillation to a positive phase. Atmospheric conditions currently are conducive for an active hurricane season as wind shear, a force that can limit the development and strengthening of tropical storms, across the basin has been well below average over the past two months.

Exhibit 22.  Atlantic Ocean Temperatures Near Normal
Atlantic Ocean Temperatures Near Normal
Source:  Colorado State University

With these conditions, the CSU forecasting team began looking for analog years to help fine-tune their forecast. They were looking for years generally characterized by weak to moderate La Niña conditions and above-average tropical Atlantic and far North Atlantic sea surface temperatures during February and March. They found five seasons that met these conditions. Four of them had either neutral or La Niña conditions during the hurricane season and all four of them were very active years. Those four years were 1955, 1996, 1999 and 2008.

Exhibit 23.  Analog Years For Hurricane Forecast
Analog Years For Hurricane Forecast
Source:  Colorado State University, PPHB

The forecasters also found 2006, which had the same February-March conditions. That year, however, experienced an unexpected El Niño, which greatly reduced hurricane activity. The CSU team anticipates 2011 to be slightly more active than what was experienced in the average of these five analog years due to the very active season predicted by their new statistical model. It is interesting that there were only two analog years that fit the parameters for both the December 2010 and April 2011 forecasts, and those years were 1999 and 2008.

Exhibit 24.  Latest Hurricane Forecast Down Slightly
Latest Hurricane Forecast Down Slightly
Source:  Colorado State University, PPHB

The CSU forecast calls for a total of 16 named storms, down one from the December 2010 forecast total. It also expects there to be nine hurricanes and five major hurricanes. The total number of storm days will be down by five, from 85 to 80, with a similar reduction for each of the other storm categories. 

In terms of landfall probabilities, the forecast calls for a 72% probability of a storm hitting the entire U.S. coastline compared to the 52% average for the past century. There is a 48% chance of a landing on the East Coast including the Florida peninsula compared to the historic 31% rate.  For the Gulf Coast from the Florida Panhandle to Brownsville, Texas, there is a 47% chance of a tropical storm landfall versus the historical average of 30%. Despite the higher probabilities, nature is such that it is impossible to forecast with any degree of accuracy until a storm is on its way whether it will reach land.  For the offshore energy industry, it will need to be on alert this hurricane season, although if the current pace of permitting continues, there won’t be too many offshore rigs to have to worry about this fall. Is that a backhanded positive?

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