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Atlantic hurricane season may be less active than some predict
May 23, 2018 | 2:50 PM
by Karen Braun
Debris and a damaged vehicle lay on a roadside damaged by Hurricane Maria near the municipality of Caguas, outside San Juan, Puerto Rico, October 13, 2017. Photo - Reuters file for illustrative purpose
 
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Last year featured the most active Atlantic hurricane season since 2005, and some forecasters believe 2018 could end up the third straight above-average season.

These predictions rely on expected atmospheric and oceanic conditions throughout the next several months, so they are certainly subject to change. However, recent trends, including the potential onset of El Niño later in the year, could derail the active forecast.

The 2017 season was packed with powerful hurricanes, including four that reached Category 4 or 5 status - the highest ratings on the Saffir-Simpson scale.

One of these storms, Harvey, jolted the oil and energy markets as its Texas landfall took an estimated 23 per cent of US refining capacity offline at one point. Harvey also stalled the transport of grains and other commodities to southeast Texas ports as well as to Mexico since the storm left rail lines inundated with flood waters.



Additionally, the deluge displaced cattle and damaged cotton fields in the area. Atlantic hurricane season officially runs from June 1 through Nov. 30, peaking on Sept. 10, though storms can occur outside of this window. This year, there are already signs of early activity, though this is not necessarily indicative of things to come.

As of Tuesday morning, the National Hurricane Centre saw a 40 per cent chance that an area of storms off the east coast of Belize would form into a tropical cyclone within five days, with likely movement into the Gulf of Mexico.

The US agency had been monitoring another system in the Gulf early last week that also had potential, albeit slim. The National Oceanic and Atmospheric Administration (NOAA) will release its first official outlook for the 2018 Atlantic hurricane season on Thursday.

Various commercial and academic weather groups have forecasted this year’s Atlantic season to be at least average with a strong lean toward above-average. These outlooks take several things into account such as Atlantic Ocean surface temperatures, pressure and wind patterns, and large-scale climate indicators such as the El Niño-Southern Oscillation (ENSO).

But there may be more factors working against hurricane activity than were present last year. Three main ingredients must come together for hurricane formation: warm sea surface temperatures, minimal to no wind shear, and an area of organized, long-lasting thunderstorms.

Wind shear characterizes how wind speed and direction change with height. Strong thunderstorms, and thus hurricanes, rely on strong vertical motion in the atmosphere. Too much wind shear prevents the vertical development of the storms, reducing their intensity.

Atlantic hurricanes are more likely in the presence of La Niña, the cold phase of the ENSO cycle in the eastern equatorial Pacific Ocean. La Niña tends to decrease the wind shear over the tropical Atlantic Ocean.

The last several months featured the strongest La Niña cycle observed in six years, though it has since faded. Two weeks ago, the US Climate Prediction Centre placed a near 50 per cent chance that the warm ENSO cycle, El Niño, would emerge by the 2018-19 Northern Hemispheric winter. The US agency also indicated that ENSO-neutral conditions are favored for the next four to six months. This is more supportive to hurricane development than the appearance of El Niño would be, so the ENSO tendency will be very important to watch over the next couple of months.

Since 1950, only one distinctly above-average Atlantic hurricane season (2004) has been observed while El Niño conditions dominated during the peak of the season. (https://tmsnrt.rs/2J4DjZY)

The Atlantic Multi-Decadal Oscillation, or AMO, may also play a role. This index, which deals with sea surface temperature variability in the North Atlantic Ocean, has mostly been in a positive phase for the last 20 years. This tends to favor hurricanes.

But recently, the AMO index has crossed into negative territory, and this could suppress hurricane development - and spoil early forecasts - if the trend continues. Atlantic Basin hurricanes generally form as a wave off the west coast of Africa, meaning that moisture anomalies in that region play a big part in creating the necessary instability.

The West African monsoon has been more active than usual as of late, which is conducive to thunderstorm development. However, current temperature variability across the Indian Ocean would also indicate better chances of dry air infiltration in the tropics, which could limit the onset of activity in the Atlantic Ocean if this temperature pattern holds.

In an average season, the Atlantic Basin will experience 12 tropical storms, six hurricanes, and two major hurricanes. In 2017, there were 17 tropical storms, 10 hurricanes, and six major hurricanes. (https://tmsnrt.rs/2IYXsR9)

The worst of 2017’s storms were Harvey, Irma and Maria, causing devastation all over the basin including Texas, Florida, Puerto Rico, and many Caribbean islands.

The World Meteorological Organization retired these three names plus Nate from future use due to their destruction and associated loss of life. With Harvey and Irma, last year also marked the first time that two Atlantic hurricanes of Category 4 strength or greater made US landfall in the same season. Storms of this intensity pack sustained winds up to 156 miles per hour (251 km/hour).

The 2016 season was also above average, but nowhere near the intensity of last year. The worst storm was Hurricane Matthew, which made landfall in the Carolinas in early October. Matthew significantly reduced hog slaughter and up to 5 million poultry birds may have perished in the resulting floods. The 2013, 2014 and 2015 seasons were relatively inactive by comparison. - Reuters

* The opinions expressed here are those of the author, a market analyst for Reuters

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