Hurricane Irma

  • Infrared satellite image of Hurricane Irma making landfall in Cuba as a Category 5 storm. Credit: NASA/NOAA.

    Forecast sea surface temperature anomaly for the subtropical Atlantic Ocean on 04/09/2017. Credit: NOAA.

    Forecast track of Hurricane Irma valid 1500 GMT 09/09/2017. Credit: National Hurricane Center/NOAA.

    ECMWF deterministic surface pressure forecast for Hurricane Irma upon making landfall in Florida. Credit: MeteoGroup.

  • Hurricane Irma
    09.09.2017 17:46

    As the strongest hurricane ever recorded in the Atlantic basin outside of the Caribbean and Gulf of Mexico, and the joint-strongest tropical cyclone at landfall in the Atlantic basin, Hurricane Irma is a storm to be taken very seriously. As this article is being written, Irma has just made landfall in Cuba as a Category 5 storm (the first storm to make landfall there at such intensity since 1924). Upon interaction with the terrain of Cuba, Irma has rapidly weakened to a Category 3, but is expected to strengthen again before the highly anticipated final landfall in Florida.

    The life of Hurricane Irma

    Hurricane Irma’s life began weeks ago over the tropical rainforests of Africa as a tropical easterly wave. These are essentially slight areas of low pressure, often generated by groups of intense thunderstorms that develop over Africa. The easterly flow of the trade winds in the tropics pushes these troughs over the tropical  Atlantic Ocean, where they can provide the residual vorticity or “spin” and convergence in the atmosphere to allow further thunderstorms to develop around a common area of low pressure. This is the birth of a tropical disturbance, which, if conditions are right, can eventually grow into a full hurricane.

    Hurricanes that originate near the west coast of Africa are known as Cape Verde hurricanes, after the group of islands that exists there. Cape Verde hurricanes are often the largest and most intense storms of the season, due to having a large expanse of warm open ocean water to intensify over.

    Why did Irma become so intense?

    Two of the main factors that tropical cyclones need to develop are warm sea surface temperatures (greater than 26.5 °C), and low vertical wind shear (changes of wind direction and speed with height) to develop. In fact, sea surface temperatures were 1.5 - 2 °C above average over most of Irma’s track, reaching 30°C in some places. This, combined with low wind shear over almost the entirety of the tropical Atlantic and Caribbean Sea, resulted in perfect conditions for a major hurricane to develop. As a mature hurricane, high pressure to the north of the system ensured that the storm remained over very warm waters, and even pushed it in an east-south-east direction for several days.

    The combination of these favourable conditions for development, over such a large area and for such a long time is why Hurricane Irma became so intense. Irma holds the record as the second-longest duration a tropical cyclone has remained at Category 5, at 81 hours. Irma has already made landfall several times, but this has been only over small Caribbean islands that have not disrupted the circulation significantly. Only the much larger landmass of Cuba has caused any significant weakening.

    What of Irma’s future?

    By now, it is well know that Irma will make a final landfall in Florida on September 11th, likely as a Category 4 hurricane.  In contrast to Hurricane Harvey, the main hazards from Irma are likely to be the large swath of hurricane-force winds and the storm surge (particularly as much of Florida is close to sea-level). This is because the storm will be moving fairly quickly at landfall, and so the torrential rain will fall over a large area. The tracks from various forecast models have converged in recent days, indicating that a landfall along the Florida Keys and then the east coast of Florida is highly likely. There was initially some uncertainty over where Irma would make landfall in the USA, if at all. However, the National Hurricane Centre has consistently provided accurate, precise and informative warnings that have given the public plenty of time to prepare, whilst also illustrating any forecast uncertainty. 

    By: Richard Martin-Barton