Average hurricane season forecast

Colorado State University scientists Philip Klotzbach and William Gray have reduced their 2009 tropical cyclone activity forecast since their predictions of 10 December, 2008.

Their most recent forecast, which was released this week, calls for a hurricane season that falls closely within parameters considered average during the 50-year period between 1950 and 2000.

The duo’s forecast calls for 12 named storms, which is down from the 14 they forecast in December. Although the number of named storms is still more than 20 per cent above the statistical average of 9.6, the scientists predict only six hurricanes will form. That is one hurricane fewer than they forecast in December, and almost identical to the statistical average of 5.9 hurricanes per season.

In addition, Klotzbach and Gray believe only two major hurricanes of Category 3 or above will form, which is actually below the statistical average of 2.3 major hurricanes per year.

One primary reason for the reduction concerns the El Niño/La Niña conditions in the eastern and central tropical Pacific Ocean, also known as ENSO or EL Niño-Southern Oscillation. La Niña is caused by a cooling of waters in that area of the Pacific and El Niño is a warming of those waters.

‘We expect current weak La Niña conditions to transition to neutral and perhaps El Niño conditions by this year’s hurricane season,’ the duo’s report stated. ‘If El Niño conditions develop for this year’s hurricane season, it would tend to increase levels of vertical wind shear and decrease levels of Atlantic hurricane activity.’

Although a weak El Niño could reduce Atlantic hurricane season activity, there have been very active seasons in the Atlantic Basin when neutral ENSO conditions prevailed in the Pacific. The record-breaking Atlantic hurricane season in 2005 occurred during neutral ENSO conditions.

However, another important variable for tropical cyclone development in 2005, high sea surface temperatures in the tropical Atlantic, are not likely to exist this year.

Klotzbach and Gray note in their report that sea surface temperatures in main development area for hurricanes have cooled more than they usually do in the period between November 2008 and March 2009.

Although the scientists warn that a weaker-than-normal upper-level atmospheric condition known as the Azores High could lead to some anomalous warming of the tropical Atlantic, they still downgraded their tropical activity forecast to reflect the current lower sea surface temperatures.

‘Another reason for our forecast reduction is due to anomalous cooling of sea surface temperatures in the tropical Atlantic,’ the report stated. ‘Cooler waters are associated with dynamic and thermodynamic factors that are less conducive for an active Atlantic hurricane season.’

Klotzbach and Gray look at various atmospheric and oceanic conditions that have existed at specific times of the year for the last 58 years and then comparing that data with the actually tropical activity that occurred the following hurricane season to ‘hindcast’ their forecasts. While they believe the exercise is worthwhile, it comes with a caveat.

‘Everyone should realise that it is impossible to precisely predict this season’s hurricane activity in early April,’ their report states. ‘Our early April statistical forecast methodology shows strong evidence over 58 past years that significant improvement over climatology can be attained.’

After badly under or over-forecasting the previous three hurricane season, the duo tweaked its statistical model and basically nailed the 2008 hurricane season.

‘We would never issue a seasonal hurricane forecast unless we had a statistical model developed over a long hindcast period which showed significant skill over climatology,’ their report stated.