2017 Hurricane Season Begins

The National Hurricane Center’s climatological progression of tropical cyclone activity as measured by Number of Storms per 100 years. The peak of hurricane season can be seen as from mid-August through mid-October. For more official climatology, see here.

The Atlantic Hurricane Season is each year from the 1st June through the 30th November, with tropical cyclone activity peaking in the first half of September. Over the past three seasons, Bermuda has seen disruptive impacts from four October hurricanes; Fay (2014), Gonzalo (2014), Joaquin (2015), and Nicole (2016).

Early indications suggest that this hurricane season will be more active than normal. On 25th May, the National Hurricane Center released their forecast for the upcoming hurricane season. They expect a near normal or above normal season with 11-17 named storms, with 5-9 storms becoming hurricanes, and 2-4 hurricanes becoming major hurricanes. An average season has 12 named storms, of which 6 become hurricanes and 3 of those hurricanes become major hurricanes.

Factors like the phase of the El Niño-Southern Oscillation (ENSO) and Atlantic sea surface temperatures (and many others) in their current and forecast states are used to make educated assessments of whether a season will be more or less active than normal. This year, returning weak El Niño phase may act to limit hurricane activity while warmer than normal Atlantic sea surface temperatures may act to enhance activity: this conflict is one of the reasons that this season’s forecast comes with quite a bit of uncertainty. Should the effects of El Niño not materialize, or should Atlantic sea surface temperatures cool through the season, the final assessment of overall activity could be dramatically different.


Screen Shot 2017-05-31 at 1.34.19 PM
Sea Surface Temperature Anomalies for Aug-Sep-Oct of the 1997 El Niño event. Warmer than normal sea surface temperatures across the central and eastern Pacific are key characteristics of El Niño.
Screen Shot 2017-05-31 at 1.34.58 PM.png
500 hPa Relative Humidity Anomaly for Aug-Sep-Oct of the 1997 El Niño event. Higher than normal mid-tropospheric relative humidity over the central-eastern Pacific is indicative of enhanced deep convection there. Suppressed convection, indicated by abnormally low relative humidity, is seen over South America and the Maritime Continent in the western Pacific.

The enhanced deep convection over the central and eastern Pacific results in vertical wind shear across the Western Atlantic. Wind shear acts to weaken tropical cyclones by keeping their convection disorganized and allowing dry air into their circulation, suppressing further convection.

[Watch wind shear push deep convection (greens) to the south while the low level swirl (grays) of 2006’s tropical storm Chris continues to the west]

The Atlantic hurricane season peaks in September. We see that for inactive hurricane seasons, there is an El Niño signal across the eastern and central Pacific. But for active hurricane seasons, that signal is more reminiscent of La Niña with cooler than normal sea surface temperatures in the central and eastern Pacific. However, for active seasons, there is the additional ingredient of warmer than normal sea surface temperatures across the tropical Atlantic – deep convection (and therefore supporting tropical cyclone development) there.

[Compare SSTs for Active and Inactive seasons]

It is important to remember that seasonal forecasts make no skillful assessment of where the storms form or track. Just one tropical storm or hurricane can make for a bad season, and so it is important to prepare for each and every season by topping-up supplies and having an emergency plan.

It is important to exercise constant vigilance during hurricane season by regularly checking for updates by the Bermuda Weather Service and National Hurricane Center.


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