The 2018 hurricane season: people need to be prepared
For the 2018 hurricane season in the North Atlantic basin, the number of hurricanes is expected to be close to the long-term average of 6.3, and therefore lower than the actual number of hurricanes in the previous year, according to the latest forecasts from various institutes. However, anyone hoping for a respite after last year’s devastating storms should note: even a single severe cyclone can cause extreme losses if it hits a heavily populated conurbation with full force.
The forecasts range from 12 tropical storms with 6 hurricanes to 14 tropical storms with 7 hurricanes. Such forecasts, however, do involve a high degree of uncertainty. In its official forecast, the US weather agency, NOAA, predicts between 5 and 9 hurricanes for 2018. Munich Re expects the actual total to be within the range predicted by the research institutes.
A variety of climatic conditions influence hurricane activity
Predicting the severity of a tropical cyclone season in the North Atlantic is a challenge. Forecasts still have substantial uncertainty because of the difficulty in predicting climate conditions of the upcoming season such as El Niño/La Niña or the sea surface temperatures in the tropical North Atlantic during the boreal spring. Both have an important influence on Atlantic storm activity, besides other factors. And it is even more challenging to predict the number of landfalling storms, not to mention probabilities of a landfall at any specific location.
The year 2017 illustrated just how difficult it is to make accurate seasonal hurricane forecasts during the spring. Last year, leading forecast experts were anticipating a hurricane season that would be slightly below average due to the expected development of weak El Niño conditions later in the year. El Niño conditions are associated with increased vertical wind shear, i.e. the difference in wind speed and direction aloft and at the ocean’s surface, particularly over the Gulf of Mexico and Caribbean Sea. Since increased shear hinders hurricane formation and development, the expectation of El Niño conditions later in the year reduced the seasonal forecast. But as the summer of 2017 progressed, climatic conditions and the associated forecasts changed, and El Niño conditions ultimately did not develop during the peak hurricane months of August to October. As a consequence, hurricane formation during the peak hurricane season was not suppressed by this climate mode.
Anomalously high sea surface temperatures were also an important factor in the development of 17 named storms and 10 hurricanes in 2017 (average since 1950: 11.6 and 6.3 respectively).
Major hurricanes Harvey, Irma and Maria drove overall losses from the 2017 hurricane season to a record US$ 220bn, with US$ 90bn in insured losses.
Sea surface temperatures in the North Atlantic expected to be lower in 2018
Current models predict that sea surface temperatures in the tropical Atlantic will be cooler than in the previous year. In addition, the La Niña phase that developed in 2017 has finished and the equatorial Pacific has returned to neutral conditions, i.e. neither El Niño nor La Niña. There are some indications that an El Niño phase might develop in the later months of 2018, and there is some likelihood that conditions will be on the warm-neutral side during the peak of this year’s hurricane season.
While El Niño conditions normally reduce hurricane activity in the North Atlantic, the neutral conditions expected for 2018 mean that there are no specific effects that either reduce or promote the development of storms. This makes the forecast in neutral years especially difficult because the range of variation is high. For example, the very active and devastating seasons of 2005 and 2017 occurred under neutral-to- weak La Niña conditions.
Researchers are continually working to further improve forecasts and reduce forecast errors. The benefits are obvious: the more reliable seasonal forecasts become, the easier it is for populations at risk to prepare. Apart from seasonal forecasts, the decades-long efforts to improve short-range forecasts on hurricane tracks and landfall location have demonstrated success. In the mid-1990s, the track forecast error with 48-hour lead time was about 370 km. Today, that 48-hour track error is only 130 km. For 72-hour forecasts, track error went down from some 560 km to 230 km.
What does all this mean for the 2018 hurricane season? For US states and the other countries bordering the northwestern Atlantic, Gulf of Mexico and the Caribbean Sea, the current background conditions send the following message: these regions should prepare for a normal tropical storm season with the possibility of a hurricane or even a major hurricane hit in their territory – just as they always should given that it only takes one storm to wreak major destruction.