2015 Atlantic hurricanes, early forecasts suggest average to below season

Early forecasts for 2015 Atlantic tropical storm and hurricane season activity suggest an average to slightly below average season, but there is some divergence of opinion this far out from next years tropical storm season.

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As ever the first forecast for 2015 Atlantic hurricane season activity comes from Tropical Storm Risk, who say that their extended range forecast suggests another below average season next year.

Tropical Storm Risk says that based on current and projected climate signals, Atlantic basin tropical cyclone activity is forecast to be around 20% below the 1950-2014 long-term norm and around 30% below the recent 2005-2014 10-year norm.

TSR says that its main predictor for this extended range forecast is expected July-September trade wind speed over the Caribbean Sea and tropical North Atlantic. TSR notes that this can influence cyclonic vorticity (how storms spin up) as well as vertical wind shear in the main hurricane track region. Right now TSR believes that the trade wind predictor will have a small suppressing effect on activity.

For the 2015 Atlantic hurricane season, TSR forecasts 13 tropical storms, 6 hurricanes and 2 intense hurricanes, with accumulated cyclone energy ACE of 79.

The 65 year climate norm is 11 tropical storms, 6 hurricanes, 3 intense hurricanes and ACE of 102, while the 10 year average is 15 tropical storms, 7 hurricanes, 3 intense hurricanes and ACE of 113.

TSR notes that in its opinion there is a 24% probability that the 2015 Atlantic hurricane season ACE index will be above-average, a 32% likelihood it will be near-normal and a 44% chance it will be below-normal.

TSR does note that its December forecasts have a low-level of precision, but they do provide a useful first outlook based on current and expected climatology and weather conditions. You can read the whole forecast from TSR here.

Second to come out with a forecast or outlook for the 2015 hurricane season is the Colorado State University Dept of Atmospheric Science forecast team of Klotzbach and Gray. They do not provide a forecast for numbers, but instead provide qualitative factors that suggest different levels of activity.

The Colorado State forecasters assess the 2015 Atlantic hurricane activity based on two factors, the strength of the Atlantic thermohaline circulation (THC) and the phase of ENSO.

They believe that we remain in an active era for Atlantic basin tropical cyclones since 1995 (despite the quiet seasons that occurred in 2013-2014), and say that they expect typical conditions associated with a positive Atlantic Multi-Decadal Oscillation (AMO) and strong thermohaline circulation (THC) will return in 2015.

They note that it is challenging to forecast for 2015 whether or not the currently developing weak El Niño will persist through the 2015 hurricane season. While significant weakening of the Atlantic Multidecadal Oscillation (AMO) and thermohaline circulation (THC) was noted during the spring of 2014, North Atlantic SST and sea level pressure patterns have since rebounded to conditions characteristic of an active era.

They anticipate that the 2015 Atlantic hurricane season activity will be primarily determined by the strength of the THC/AMO and by the state of ENSO.

For the 2015 hurricane season, four possible scenarios are anticipated with the probability of each as indicated below:

1. THC circulation becomes unusually strong in 2015 and no El Niño event occurs (resulting in a seasonal average net tropical cyclone (NTC) activity of ~ 180) – 10% chance.
2. THC continues in the above-average condition it has been in since 1995 and no El Niño develops (NTC ~ 140) – 40% chance.
3. THC continues in above-average condition it has been in since 1995 with the development of a significant El Niño (NTC ~ 75) – 40% chance.
4. THC becomes weaker and there is the development of a significant El Niño (NTC ~ 40) – 10% chance.

So that leaves a large range of uncertainty in the forecast, which seems to suggest anything from slightly above average to below average, based on the extended range qualitative outlook. Typically, hurricane seasons with the those NTC values have tropical cyclone activity as follows:

• 180 NTC – 14-17 named storms, 9-11 hurricanes, 4-5 major hurricanes
• 140 NTC – 12-15 named storms, 7-9 hurricanes, 3-4 major hurricanes
• 75 NTC – 8-11 named storms, 3-5 hurricanes, 1-2 major hurricanes
• 40 NTC – 5-7 named storms, 2-3 hurricanes, 0-1 major hurricane

So the greatest probability is thought to be from 8 to 15 named tropical storms, with from 3 to 9 hurricanes and 1 to 4 major hurricanes. The middle of that forecast would be slightly above the long-term averages.

You can access the full qualitative discussion from these forecasters here.

The 2015 Atlantic hurricane season is of course extremely relevant to the reinsurance, catastrophe bond and insurance-linked securities (ILS) marketplace, with it providing the single highest peril exposure to all of these marketplaces.

At the moment it is too far out to have any certainty in the level of activity we can expect to see, these forecasts suggest something around the long-term average. However these early extended range outlooks begin to provide some colour as to the expected climatological conditions and how they could potentially affect tropical storm formation, intensification and hurricane activity across the Atlantic basin.