Wilma Makes Forecasters Look Like Barneys
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
Comments
Hahah.....now onto Yasi.
And lets not forget the Bianca swell for the SW of WA.
That tricky beast came right in under the radar.
I too have to say that Wild Wilma under performed and made me look like Fred and Barney put together.
SE Qld didn't come close to 4-5ft with 6ft sets.
Given Wilma was so far offshore, I'm still very puzzled how just down the road at Steve's local came in at 4-5ft with 6ft sets and the Goldy was lucky to be 4-5ft tops on the biggest sets of Saturday afternoon? Wave height disparity like this from a system/fetch that was so far offshore is just very strange indeed.
And I won't mention the email I got from my Sydney mate surfer this morning. He wasn't happy at all, claiming very small surf on Sunday morning.
Biggest question now is who's gonna be gun shy with forecasting Yasi!!!
I thought the wave heights came in pretty much as expected. Probably solid 4 at my local, easy enough to say 4-5. Biggest issue was the 'when'. It certainly wasn't here on Friday. Was definitely here on Saturday and Sunday.
As for Yasi, it's unlikely to do anything for Sydneysiders by my reckoning, probably only the qlder's getting anything from it. There, stuck my neck out. Now I'll go and check what Steve is saying in the TC Yasi thread.
I don't reckon anyone should blame the poor surf forecaster if the swell came in under expectations. After all it's a free service being provided. What ever happened to looking at the synoptics and raw swell data and working it out for yourself based on your knowledge of the area? Surfers have got it too easy these days and in my opinion that's what leads to the crowds. Before the internet we used to take a punt on the swell and sometimes we hit and sometimes we missed. When we hit we usually had it to ourselves. I think surf forecasters should just tell us the forecast swell and wind ie 2-3 metre swell with westerly winds and let the surfers work out for themselves what this means at their local. Would lead to a lot less crowded sessions I reckon.
The guy was spot on, it is better to under estimate as it removes any chance of disappointment. I am addicted to swell forecasting sites because it raises my spirits to know there is some joy on the way. To be fair the other two sites (coastalwatch & stormsurf) both ramped it up but at the end of the day Mother Nature runs it's own show.
I like when you get it wrong the other way round and a pre dawn surf check reveals waves that were never in the equation.
At the end of the day synoptics are the true source, not to down play written interpretation but Lows spin clockwise, Highs anti, the closer the iso bars the more wind the more swell.
Captured fetch on active sea state sound far more appealing so I'll keep checking.
I don't agree with the "better to underestimate as it removes any chance of disappointment." That's not surf forecasting, that's sitting on the fence.
I say give it your best shot and the reasons why you think it's gonna be that and then if you're wrong so be it. At least you attempted to forecast the surf. Any mug can say it's gonna be 3-4ft when it's actually gonna be 5-6ft but this ain't forecasting in my opinion. It's called being conservative and not putting your balls on the line.
Fair Call, that's the beauty of opinions everyones got one.
But I don't see it as being less ballsy to be conservative, it can be argued that any mug can call 5-6 foot when it's actually gonna be 3-4.
It could be argued that that this in turn creates extra interest in the source of the forecast(of which I am not) so it increases traffic. When it's all said and done forecasting is interpreting weather and as a result is subject to conjecture and if your wrong so be it dissapointingly.
I wasn't suggesting over calling it either. Merely stating call a spade a spade.