Swellnet Dispatch Hurricane Katia Gets Shelved

In: Swellnet Dispatch by Stu Nettle 20 Comments Tue 6th Sep '11

Tags: hurricane katia , typhoon , surf forecasting , Cyclone

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The Quiksilver Pro New York is currently two days into its waiting period without a single wave being ridden. The organisers have called laydays knowing that Hurricane Katia, which is currently moving north-west between Bermuda and the Caribbean, will provide surf in the coming days.

For an Australian site, such as Swellnet, forecasting the surf from Katia has proven to be more difficult than we first imagined. The problem has been that although a hurricane is essentially the same thing as a cyclone (or a typhoon in the north-west Pacific) each has unique patterns of behaviour owing to local conditions.

For instance, the paths of hurricanes off the east coast off the US closely follow the Gulf Stream. They move very slowly and predictably, especially compared to our notoriously unpredictable cyclones.

Another concern, and this one greatly effects wave height, is the size of the continental shelf on the US east coast. The shelf off Long Beach, New York, extends about 200 kilometres offshore. To compare, the continental shelf on Australia's east coast only extends between 20 and 50 kilometres.

The continental shelf reduces wave height by causing waves to 'feel' the ocean floor earlier and lose their energy. The further out it extends the more wave energy – and hence, more size – is lost.

Hurricane Katia is currently a Category 4 system with maximum sustained wind speeds of around 116 knots. Now, if a category 4 cyclone were tracking slowly and perfectly – as Katia is - toward Australia's East Coast we would see a very large and dangerous swell in the 10-15ft range. It would be the type of swell where only Noosa, Kirra and the Pass at Byron Bay would be rideable.

Yet, as mentioned above, Long Island has a large continental shelf and the swell generated by Katia will lose a lot of its energy before it hits the coast. Because of this we only expect the swell at the contest site to be approximately half the size it would be if Katia formed off our East Coast.

Another factor to consider is that as soon as Katia moves out of the Long Island swell window the surf will drop very rapidly. 24 hours after Katia passes the surf at Long Island will lose at least half the size from its peak. The organisers will have to give serious consideration to this point, choosing their days carefully, if they are to avoid running the finals in small waves.

//STU NETTLE & CRAIG BROKENSHA

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