James Waller, PhD, Research Meteorologist
Atlantic Sea-Surface Temperatures (SSTs)
Observed SSTs have been cooler than average for areas of the far northern and eastern Atlantic, as well as areas of the Gulf of Mexico. Meanwhile, some areas of the western Atlantic have been trending warmer than average. The pattern could well change over the coming months, but with all else being equal this indicates the potential for near-normal activity in the basin, and some potential for development of tropical storms or hurricanes adjacent to the mainland. Seasonal outlook providers have variable views on the influence for 2018 hurricane activity.
For perspective, the Atlantic Multidecadal Oscillation (AMO) index accounts for Atlantic SSTs, and remains in a generally positive phase over recent months. This index has been largely positive since 1995, consistent with the general period of elevated hurricane activity that began in 1995. There were some indications prior to 2017 of a possible shift in the AMO, and an associated shift to a less-active phase of hurricane activity. However, the state of transition remains unclear given the especially active season of 2017, and the ongoing positive state of the AMO.
Basin to Landfall Ratio
The basin to landfall ratio is highly volatile on an annual basis, and history reminds us of impactful hurricanes even during quiet years. Hurricane Betsy (1965) rendered severe impacts to the Northern Gulf during a season with only four observed hurricanes in the basin. Meanwhile, the 2010 season was very active with 12 hurricanes in the basin, and not a single U.S. landfall. Hurricane landfall is influenced by how close the hurricane is to the mainland and steering currents in play during the hurricane lifecycle; neither of these are reliably predictable on long timescales.
Seasonal outlook providers predict near- to above-normal hurricane activity for the Atlantic Basin in 2018. Variable SSTs and the potential for development adjacent to the mainland lend some uncertainty to predictions. Some forecast revisions can be expected in late May, along with the official NOAA outlook for early June. Seasonal outlooks tend to show improved skill in the late-spring period.
Link to Part I >>
Sources: Tropical Meteorology Project at Colorado State University (https://tropical.colostate.ed/media/sites/111/2018/04/2018-04.pdf); The Weather Company (https://weather.com/storms/hurricane/news/2018-04-19-2018-hurricane-season-forecast-the-weather-company-ibm-april); and North Carolina State University (https://news.ncsu.edu/2018/04/2018-hurricane-prediction/)
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