Swellnet Dispatch Wilma Makes Forecasters Look Like Barneys

In: Swellnet Dispatch by Stu Nettle 9 Comments Mon 31st Jan '11

Tags: tropical cyclone wilma , forecasting , steve shearer , Craig Brokensha

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HEY SWELLNET, NEXT TIME YOU GUYS ARE PREDICTING THE ARRIVAL OF A SWELL, HOW BOUT TONING IT DOWN A BIT, SAY IT'S GONNA BE 2-3 FOOT WHEN YOU THINK IT'S GONNA BE 6 FOOT.THAT WAY PEOPLE WON'T BE SO DISAPPOINTED WHEN YOU SAY IT WILL BE 6 FOOT AND IT TURNS OUT TO BE VIRTUALLY FLAT!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!

-Message that appeared on Swellnet's Facebook page on Sunday morning.

Last Friday Swellnet forecasters, Craig Brokensha and Steve Shearer, posted their swell forecasts for the weekend. With TC Wilma in the SW Pacific there was a high likelihood of good waves on the whole east coast. Steve, forecasting for northern NSW and QLD, predicted 4-5 foot waves with the odd larger set. His forecast came in on the mark.

Craig, forecasting for southern NSW, predicted 3-5 feet 'with the possibility of infrequent 5-6 foot sets across swell magnets.' The qualifier was there because of the notoriously unpredictable nature of cyclones; they are erratic and hard to track, and even harder to give accurate swell forecasts for.

Testament to their erratic nature is the Bureau of Meteorology's tracking maps. Even up till 24 hrs out they can have a margin for error of hundreds of kilometres. And if you can't track the cyclone accurately nor can you predict the swell from it.

Another aspect of TC Wilma that made swell prediction difficult was that, unlike other swell-producing cyclones such as Vania earlier this month and Jasper in 2009, it didn't interact with the easterly trade wind belt to create a large band of swell-creating wind (see images attached).

According to Craig, the ideal situation for cyclone swell is, "When we have a sustained fetch of easterly tradewinds. This is because an active sea state is already initiated and the winds wrapping around the cyclone don't have to work as hard to whip up a larger open ocean sea state. In the case of Jasper and Vania, these cyclones moved in over an already active sea state, and although they were weaker systems than Wilma, they produced a larger swell event."

Despite not having the headstart that Jasper and Vania did, Wilma was a very powerful cyclone. As Craig says, "Wilma reached Category 5 status when it moved to a position between New Caledonia and New Zealand. Winds speeds around the core of Wilma reach 210km/h with gusts of 260km/h!"

Another aspect of Wilma that made swell prediction difficult was the diminutive size of the system. While it was an incredibly powerful cyclone it was very small in size, and as it didn't interact with any other weather systems the winds, strong as they were, blew over a relatively small stretch of ocean.

The computer models that forecasters use to predict swell struggle to interpret such small systems as the resolution grid is larger than the system itself. As Craig says, "This is where some intuition has to be used. And with Wilma being such a strong system and tracking favourably through our swell window, there was always the potential for large surf even without an active sea state to move into."

All of which is to say that despite NSW surfers hoping the swell would come in at the upper end of Craig's forecast, they were disappointed when it came in at the lower end. Nor was the size evenly distributed like a classic cyclone swell. Some places registered 4-5 feet waves on Sunday morning, others just 2-3 feet. And we here at Swellnet felt like Barneys.//STUART NETTLE

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