Waves crash against the seawall in Puerto Rico as Hurricane Irma slams across islands in the northern Caribbean
Hurricane Irma made its first U.S. landfall 30 miles east of Key West, Florida on September 10. The storm was rated as a category 4, with sustained winds of 130 miles per hour at the time. The monster storm had previously battered Cuba and several Caribbean islands for most of a week at wind speeds of up to 185 miles per hour before turning north and creeping slowly into Florida with heavy rain and hurricane-force winds battering both the east and west coasts of southern Florida causing seas to rise with storm surges.
Hurricane Harvey after making landfall in Texas, with two potential new disturbances that may seed new tropical storms, one off Florida, the other in the Atlantic Ocean off the west coast of Africa.
Although the ferocity of the storm was quite shocking warnings had been posted by the USA’s National Oceanic and Atmospheric Administration (NOAA) Atlantic Hurricane Season Outlook warning to expect a much more active season than was originally predicted. The mid season update issued on August 9. 2017, predicts a total of 14-19 named storms, including 5-9 hurricanes and 2-5 major hurricanes. This compares to long term averages of 12 storms, six hurricanes and three major storms each year. The outlook indicates a 60 per cent chance of an above-normal season, a 30 per cent chance of a near-normal season, and only a 10 per cent chance of a below-normal season. There’s also a possibility that the season could be extremely active, with the cumulative energy in storms reaching 170 per cent of the long term average.
Satellite image on August 18, showing the disturbance in the eastern Caribbean that would become Hurricane Harvey.
This contrasts with the original seasonal outlook, published on May 25, that indicated a slightly less active season than average. However, developments in the atmosphere since then point to more conducive conditions for tropical storm formation, including less vertical wind shear, weaker trade winds, a more conducive African Easterly Jet (AEJ) in association with a stronger west African monsoon system, and a stronger upper-level subtropical ridge. According to the agency this set of conditions allows for stronger African easterly waves, from which tropical storms and hurricanes can more easily develop
Coastal devastation in Texas in the aftermath of Hurricane Ike in 2008. Photo NOAA
In addition, sea surface temperatures are well above average, which helps to promote the evaporation of surface water that’s required to feed a developing storm. There’s also much higher confidence that a neutral phase of the El Niño/ Southern Oscillation will persist for the rest of the season, and therefore the possibility of El Nino developing and supressing the formation of Atlantic tropical storms is reduced.
The Atlantic hurricane season officially runs from June 1 to November 30 each year, although there are occasional storms outside of these dates.
Prior to Irma the eighth cyclone of the 2017 Atlantic season, Hurricane Harvey, had become the first hurricane to make landfall in Texas since 2008 and the most active to hit the state since 1961.