The US National Oceanic and Atmospheric Administration released its annual pre-hurricane season forecast for the Atlantic Basin last week, predicting a 65 per cent chance of above-normal tropical activity this year.
NOAA, which operates the National Hurricane Center in Miami, predicted between 12 and 16 named storms; six to nine hurricanes; and two to five major hurricanes with winds of at least 111mph. An average hurricane season has 11 named storms including six hurricanes, two of which become major hurricanes.
Gerry Bell, NOAA’s Climate Prediction Center’s lead seasonal hurricane forecaster said the main factors influencing this year’s forecast include the multi-decadal signal and the anticipated lingering effects of La Niña.
The multi-decadal signal is a cyclical combination of oceanic and atmospheric conditions that have increased hurricane activity since 1995.
‘One of the expected oceanic conditions is a continuation … of warmer-than-normal temperatures in the eastern tropical Atlantic,’ he said.
La Niña is a cooling of the waters in the Eastern Pacific Ocean that creates upper atmosphere conditions conducive to tropical system development in the Atlantic Basin. Its climate cousin – El Niño – is a warming of those same waters which has historically created upper atmospheric conditions like vertical wind shear that inhibit hurricane formation.
These climate conditions in the Eastern Pacific Ocean, also referred to as ENSO, are creating uncertainty in the long-range Atlantic hurricane forecast. Last year’s strong La Niña has weakened and is transitioning into either a neutral ENSO or an El Niño condition.
‘El Niño and La Niña forecasts are presently the biggest source of uncertainty for the hurricane outlook,’ Mr. Bell wrote in his forecast report. ‘The period between March and July is referred to as the springtime forecast barrier, a period when predicting these phenomena can be difficult because the atmosphere is in a state of transition.’
Even though La Niña seems to be waning, its atmospheric impacts often persist after Pacific Ocean temperatures return to normal, Mr. Bell wrote.
In addition, there have been some very active Atlantic Basin hurricane seasons in the past during neutral ENSO years, including the record-breaking year of 2005, when 28 named storms formed.
Long range forecasting models differ as to weather there will be neutral ENSO conditions or El Niño conditions for the peak hurricane season months of August through October.
Even with atmospheric and oceanic conditions favorable as they are now for an above-normal hurricane season, Mr. Bell noted it won’t necessarily happen.
‘Historically, seasons with climate patterns similar to those expected this year have produced a wide range of activity, and have been associated with both near-normal and above-normal seasons,’ he wrote in his report. ‘This outlook considers the historical distribution of activity for these climate factors, uncertainties in the La Niña impacts, and the possibility of other unpredictable factors also influencing the season.
‘While an above-normal season is most likely… there is a significant 25 per cent chance of a near-normal season and a 10 per cent chance of a below-normal season.’
The Colorado State University forecasting team of Phil Klotzbach and William Gray issued their last long-range forecast on 3 April. Their predictions of 15 named storms, eight hurricanes and four major hurricanes fall within the range – albeit on the upper end – of the NOAA forecast. The next CSU forecast is due out on 3 June.
The UK-based Tropical Storm Risk forecast from 7 April also has a similar prediction of tropical activity in the Atlantic this year, as they estimated a 63 per cent probability of an above-average season, a 23 per cent chance of a normal season and a 14 per cent probability of a below-normal season.
Even during the most active of hurricane seasons, the probability of the centre of a hurricane – where the winds and wave surge are greatest – making landfall in one particular place is very low. However, hurricanes are often large weather systems that can cause damage and death even when the eye passes many miles away.
NOAA’s hurricane forecast will be updated on 7 August, just as the peak hurricane season is beginning.