THE REAL DIRT

The Good Goss on the Gale

May 18th, 2010 · No Comments · Guest Viewpoint

Stuart Kininmonth is a marine ecologist and sailor  based in Townsville. He has a fascination with the perfect breeze, well forecast.  Here is his take on the highs and lows of wind predictions…

There is nothing like good planning to make a cruise enjoyable. You study the tide charts, check the food list, measure the fuel levels and then begin teasing apart the wind forecasts. The forecasts can let you know a week in advance what conditions are likely to be but do they? Our use of the internet has placed the world’s weather forecasts at our fingertips yet we are never sure just how well they might apply to our local cruising ground.

Recently I was all ready to cruise out to our favourite coral reef but the forecasts predicted doom and gloom. I looked at the 3 day then 2 day then next day predictions as the weekend approached. Finally we decided to delay the departure by a day. Waking up on Saturday morning to clear skies and favourable winds prompted me to have a hard look at the forecasts available over the month of April 2010. Every day at 4pm I duly noted the 1 to 3 day upper and lower wind speed predictions for 10 AM and 4 PM time slots. Later I compared these to the real data recorded at two marine weather stations. I was surprised at the results and believe extra knowledge of wind forecasting is essential to cruise planning.

Picture by James Woodford – Stuart Kininmonth at the helm of a tender at Davies Reef on a windy day

The two weather stations were carefully selected to represent the two typical boating areas around Townsville. The first was the Cleveland Bay weather station, located just to the East of Magnetic Island. This represents the sheltered waters that many boats use every weekend. The second was the Davies Reef weather station, located 70 kilometres offshore and is only visited by the more serious boating enthusiasts. Important to note that the Cleveland bay station is sheltered by the jutting Cape Cleveland peninsula while the Davies Reef station is completely exposed to the prevailing winds.

There is a myriad of forecasting services available with some very different approaches. The Bureau of Meteorology, affectionately called BOM, delivers several products that are tailored for the marine community but delivered in dissimilar ways. The ‘Coastal Waters Forecast’ provides a general forecast over a large section of coast while the ‘Forecast Wind for Marine Areas’ (MWF) provides a map with detailed wind forecasts in a manner similar to the popular GRIB files. Commercial suppliers such as Buoy weather, Seabreeze and Windfinder are capitalising on the desire by the water sports fraternity to receive forecasts for a single location. These providers base their forecasts on a very sophisticated United States computer model called the Global Forecast System (GFS). The outputs are not checked before being uploaded to the websites so disturbances, such as cyclones, can render the data useless. I chose the Buoy weather system which has an annual subscription for extended forecasts.
Fact or fiction?
So how did the various forecasts go? None of the forecasts got it right all the time despite the use of large ranges, like 10 to 15 knots. There seems to be a rule that a 5 knot range is acceptable so I calculated how much above or below the stated range each daily forecast over or under estimated. April turned out to be an interesting month with some long periods of little wind and persistent strong winds at other times. The Cleveland bay weather station was exposed to the land effects and the sea breeze changes and consequently predictions here were the most inconsistent. The Coastal Waters Forecast, on average, overestimated the wind by 4.9 knots while the MWF forecast was better with only a 2 knot overestimation. The Buoy Weather estimates for Cleveland Bay were around 0.2 knots over. The Buoy weather and the MWF got the forecast within the stated range 28% of the time while the more general BOM coastal forecast only scored 15%.
Davies Reef was more accurately forecast with Coastal Waters Forecasts, on average, only 2 knots over the recorded wind speeds. The MWF was consistently half a knot over estimated while the Buoy weather underestimated by 0.2 knots on average. The accuracy improvement also was evident with the number of days accurately forecasted. Buoy weather scored an impressive 57% on target while MWF achieved 46% and Coastal Waters Forecast attained 32%. There seems to be a general trend to err on the side of caution and overestimate the wind strength. Importantly, from a safety view, only the Buoy weather provided a forecast that was exceeded by the real wind speeds by more than 5 knots. Surprisingly the accuracy improved only slightly as the outlook shortened. The Buoy weather went from half a knot overestimation for the 3 day forecast to 0.2 knot overestimation for the next day. For most cruisers this would hardly matter.
I also checked the wind direction in case some forecasts are better with a particular direction. There were no appreciable differences but the winds were consistently Easterlies to Southerlies and perhaps Northerlies are less predictable. One noticeable difference was between the morning and afternoon forecasts. The afternoon forecasts were generally more accurate by 1 knot than the morning forecasts.
Perhaps it is possible to supplement the forecasts with local measurements of the barometer or reading cloud patterns? I could not find any patterns in the Barometer readings that would have influenced my wind speed predictions. I did find watching the clouds useful in predicting the sea breeze change and approaching wind changes but there seemed to be little wind speed information attached to the fluffy white sentinels.

Also critical to note that each day has large changes in wind speed that have to be accommodated in forecasts that group each day into several blocks. For this project I was only interested in the 10 am and 4pm forecasts but a night time change would be important especially to those on anchor.

The expert’s view

I caught up with the local BOM meteorologist Dr Gregory Connor to discuss these results and find out what he recommends. He stated that “a good strategy is to look at the Bureau forecast and then also check a web-based forecast. These sites are ideal for getting an idea of the forecast wind at a single point and are popular with mariners and the Bureau is also working towards a similar presentation for its wind forecasts.” Importantly, he suggests that “If you are staying inshore, then it will be necessary to use your local experience to adjust the general forecast winds for local land and sea breezes.” Once again in the cruising world the use of local knowledge is key to avoiding unpleasant surprises. This was very apparent to me when anchored off Gould Island one night. The cruising guide ‘Going Troppo’ stated that night time offshore breezes can be very strong here and so I went back on deck and, in calm conditions, let out extra chain. Later as the boat rocked in strong winds I was very thankful for that piece of local knowledge.

The BOM website states that the Bureau forecasts of wind speed and direction are average (or mean) values over a 10 minute period at a height of 10 metres. Forecasts of gusts are not included as routine, however statistically it is estimated that gusts typically exceed the average wind speed by about one third. All the comparisons here were only based on the average wind speed.

Understanding the weather forecasts and the character of your cruising area is essential to planning a successful trip. When moving outside your local area then keep in mind that complex coast lines will disturb the regular forecasts. Subscribing to a weather service can be a good investment but in times of a cyclone, listen for the local BOM weather updates. The local coast guard broadcasts the Coastal Wind Forecast without alteration but regular radio chatter confirms many people also are happy to share their local observations. The final word comes from the Buoy weather website and states “Forecasting weather is not an exact science though and the forecast will never be exactly what happens. Same with official human made forecasts. They are wrong all the time. There is some skill and experience required for interpreting the models outlook. You are better off doing this yourself than relying on someone else.”

The wind forecasts used in this study were obtained at the flowing sites:
Coastal Wind Forecast; http://www.bom.gov.au/qld/forecasts/northern_qld_coastal_waters.shtml
Marine Wind Forecast;

http://www.bom.gov.au/jsp/marine/wind/index.jsp

Buoy Weather;

http://www.buoyweather.com/index2.jsp

Weather stations;

http://data.aims.gov.au/aimsrtds/faces/latestreadings.xhtml

Stuart Kininmonth lives aboard the 1991 Hunter Legend 433 Orpailleur with partner Olia. He started with Mirror dinghy sailing on the Gippsland lakes then progressed to the Queensland coastal areas in a shared 35 year old Thunderbird.

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