Normal hurricane season forecast

Forecasters from the US National Oceanic and Atmospheric Administration are predicting a near-normal 2009 Atlantic basin hurricane season.

NOAA’s initial outlook for the 2009 Atlantic hurricane season, which runs from 1 June through 30 November, calls for a 50 per cent probability of a near-normal season, a 25 per cent probability of an above-normal season and a 25 per cent probability of a below-normal season.

In terms of the number of storms, the NOAA forecasters said there is a 70 percent chance of having nine to 14 named storms, of which four to seven could become hurricanes, including one to three major hurricanes of Category 3 or above.

Gerry Bell, lead seasonal hurricane forecaster at NOAA’s Climate Prediction Center, urged people who live in areas that could be affected by an Atlantic hurricane to prepare for every season the same way no matter what the forecast outlook.

‘This outlook is a guide to the overall expected seasonal activity,’ he said. ‘Even a near-or-below-normal season can produce land-falling hurricanes, and it only takes one land-falling storm to make it a bad season.’

NOAA’s pre-season outlook is shaped by a number of climate factors. Supporting more activity this season are conditions associated with the ongoing high-activity era that began in 1995, which include enhanced rainfall over West Africa, warmer Atlantic waters and reduced wind shear.

But tropical cyclone activity could be reduced if an El Niño develops in the equatorial Pacific Ocean this summer or if ocean temperatures in the eastern tropical Atlantic remain cooler than normal.

The El Niño weather pattern – also known as ENSO – is a warming of the tropical Pacific Ocean sea surface temperatures that usually hinders tropical cyclone activity because it increases upper atmospheric wind shear, which disrupts hurricane development.

La Niña, on the other hand, is a cooling of tropical Pacific sea surface temperatures that usually supports an active hurricane season in the Atlantic basin.

The La Niña condition that started this year dissipated in April and there is now a neutral ENSO condition in the tropical east Atlantic. A majority of computer forecasting models predict the neutral ENSO conditions to continue for the remainder of the year; others, however, predict an El Niño to begin.

The tropical Pacific is expected to transition into an El Niño eventually, but the timing for that is unsure. Neutral ENSO conditions have sometimes supported active Atlantic-basin hurricane seasons, including the record-breaking season of 2005.

NOAA’s prediction of a near-normal hurricane season echoes a prediction issued last month by Colorado State University scientists Philip Klotzbach and William Gray. The two scientists reduced the number of predicted tropical cyclones in the Atlantic basin this year from their prediction made last December, primarily because of the ENSO transition and cooler than normal sea surface temperatures in the tropical Atlantic.

Global warming and hurricanes

During a recent hurricane preparedness exercise in Cayman, Cayman Islands National Weather Service Chief Meteorologist John Tibbetts gave a slide show presentation that had been prepared by Head of Meteorological Services Fred Sambula.

Although Mr. Tibbetts said there was a ‘fairly good correlation between sea surface temperatures and hurricane frequency’, there could be other factors causing a higher number of storms seen in recent years.

One factor is likely better detection capabilities brought on by satellite imagery.

‘Once we got satellite technology, we started to detect and track storms that were out over the ocean that we weren’t detecting before,’ he said. ‘Some pre-satellite storms just didn’t get counted.’

Mr. Tibbetts spoke about the advantages of modern technology in detecting and tracking tropical cyclones. In addition to satellites, there are the hurricane hunter aircraft and buoy weather stations.

To illustrate the improvements in storm tracking, Mr. Tibbetts said the Galveston hurricane of 1900 only had one observation; in 1954, Hurricane Carol had seven observations; but in 2005, Hurricane Wilma had 280 observations.

‘We’re getting more detailed information about storms,’ he said. ‘And we get more advanced systems coming on line all the time.’