Before we examine those factors, consider that we find very little relationship between storms forming before the end of June and overall activity for the year.
For example, in the seasons since 1950 with two or more named storms forming by the end of June, an average of 6.1 overall hurricanes were observed, while in seasons since 1950 with zero or one named storm by the end of June, an average of 6.3 overall hurricanes were observed.
If anything, Alex, the season’s first storm, which formed in January, was more related to climate conditions present in late 2015 than what is currently present.
A large variety of groups now issue Atlantic basin seasonal hurricane forecasts, but for simplicity’s sake, we display in the table below four of the longest-running entities issuing these predictions.
The forecasts fall into two camps, with both the National Oceanic and Atmospheric Administration (NOAA) and Colorado State University (CSU) calling for a near-average Atlantic hurricane season, while the United Kingdom (UK) Met Office and Tropical Storm Risk predict an above-average Atlantic hurricane season.
The two primary areas of uncertainty with this forecast continue to be the future status of El Niño and La Niña and the configuration of Atlantic basin sea surface temperatures.
El Niño and La Niña
El Niño has weakened considerably over the past few weeks, with cold sea surface temperature (SST) anomalies starting to emerge along the equator. While El Niño is still present according to NOAA, it is likely to be declared over in the next few weeks.
Waters averaged over the upper 300 meters of the ocean in the central and eastern tropical Pacific have cooled dramatically since January, also pointing towards a breakdown of El Niño conditions and the development of La Niña.
There are a large variety of dynamical and statistical computer models that predict the future state of El Niño and La Niña. Most of the dynamical models are calling for La Niña conditions in the tropical Pacific for the peak of the Atlantic hurricane season from August to October. If this forecast verifies, upper-level winds should be much more conducive for hurricane formation and intensification in 2016 than were present in 2015.
Why are both NOAA and CSU calling for an average hurricane season given the likelihood of La Niña developing over the next few months? The primary reason is that the far North Atlantic as well as the subtropical northeast Atlantic remain colder than normal. Cold anomalies in these regions tend to force high pressure and strong low-level winds that then force cold SST anomalies in the tropical Atlantic by the peak of the Atlantic hurricane season.
Typically, colder-than-normal water in the far North Atlantic is indicative of a negative phase of the Atlantic Multi-Decadal Oscillation (AMO). Negative phases of the AMO tend to be associated with weaker overall Atlantic hurricane seasons. In addition, cold water in the far North Atlantic as well as off of the west coast of Portugal tend to force higher pressure and stronger winds across the tropical Atlantic. Surface pressures observed across the tropical and subtropical Atlantic were much higher than normal in May.
The big question that will be answered over the next few months is how much these cold SST anomalies impact conditions in the tropical Atlantic. Both NOAA and CSU think that it will have a moderate to strong influence, counteracting the more conducive hurricane formation conditions generated by La Niña, while both Tropical Storm Risk and the UK Met Office believe that these cold SST anomalies will play less of a role, leading to an overall active Atlantic hurricane season.
Why issue Atlantic seasonal hurricane forecasts?
The question has often been asked as to what the utility of Atlantic basin seasonal hurricane forecasts is. These forecasts are provided as an informational tool, as there is inherent curiosity amongst the general public as to how active the upcoming season is likely to be. There is significant skill in these predictions, based both upon historical data as well as upon real-time forecast skill.
However, these forecasts are not a preparedness tool. Very active Atlantic hurricane seasons can have no hurricane landfalls (such as 2010), or weak seasons can have significant hurricane landfalls (Alicia-1983, Andrew-1992). Statistically speaking, more active Atlantic basin seasons have more landfalls, but the general public is reminded that it only takes one hurricane landfall near you to make it an active season.