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Weather Forecasts

Weather Forecasting in the Whitsundays by Kenn Batt

Two questions are regularly asked by yachties heading north: “Where do I get detailed weather forecast information from for the Whitsunday area from?” and “How can I best fine-tune the broader weather forecasts to factor in local effects?”. Firstly, apart from the very detailed daily weather forecast that is normally prepared for each race day, one can find a lot of useful information from a number of web-sites, including the Bureau of Meteorology’s site. For example, general (low resolution) marine forecasts can be obtained from the following links:

A number of web-sites offer computer model derived wind forecasts for the Australian area. The results from these have to be treated very carefully as they are firstly, untouched or massaged by human hands, and secondly will generally not factor in local effects as they run at fairly coarse horizontal resolutions. They are nevertheless a good guide, more particularly when used in conjunction with the written forecast (from links above).

Some useful sites (amongst many) are:

From a higher resolution weather forecast angle, one can access an aviation weather product known as a TAF (Terminal Aerodrome Forecast) for the Whitsunday area from the Bureau of Meteorology web-site under the Aviation banner. You will be prompted for access details (Username: bomw0007, Password: aviation. If you forget, it’s given on the Aviation page anyway) then scroll down the list to Hamilton Island (YBHM).

An example of a TAF looks like this:

TAF YBHM 030418Z 030618
10013KT 9999 SCT025
RMK
T 24 24 23 23 Q 1013 1014 1015 1015

It’s a forecast for a 5nm radius around Hamilton Island Airport couched in “aviation-speak”. Please bear in mind that the topography around this Airport is quite complex. As such, in certain sectors (particularly the NE), the wind experienced away from the Island can be markedly different from the forecast wind. TAFs are routinely issued every 6 hours and can be amended at anytime. For your racing, the TAF valid 2008 (2000 to 0800Z or from 0600 to 1800 EST) would normally be the one that you will be using.

The decode of the TAF above is as follows:

030418Z date/time of issue group in UTC(Z)
030618 forecast validity group [valid from 0600(1600EST) 3rd to 1800UTC(0400EST) on 4th.
10013KT wind group (average true wind direction and speed in knots at a height of 10m. In this example, from 100deg T at an average speed of 13knots)
9999 visibility group (in this case visibility is forecast to be 10km or better)
SCT025 cloud amount and height of cloud base group (SCT= 3-4 octas or eights at 2,500ft)
T 24 24 23 23 Forecast temperatures in 3 hourly intervals. In this case commencing at 0600Z at finishing at 1500Z (last 3hr not given)
Q 1013 1014 1015 1015 forecast QNH (roughly the sea level pressure)in 3-hourly intervals. In this case commencing at 0600Z at finishing at 1500Z (last 3hr not given)

Other useful TAFs in the area, would be Mackay (YBMK) and Prosperpine (YBPN)

More in depth information on TAF decoding is available here.

Monitoring of actual meteorological conditions, and hence very handy for the fine-tuning of forecasts, can be performed via the following links:

Once you’re happy with all this, you’re then ready to factor in the very high resolution, local scale stuff. This should incorporate the following:

  • Wind-flow can be markedly affected by the presence of headlands and islands.
  • Wind can be channelled or funnelled between islands or between the mainland and an island.
  • Wind speed tends to be enhanced when blowing around headlands and the direction can be quite erratic. Beware of the wind bullets.
  • On the other hand, wind flow can be very much impeded in the lee of any land mass. The general rule of thumb is to stay roughly 10 to 15 times the height of any obstacle downwind to be assured of a steady wind. This, however, will have to be fine-tuned by the user.
  • Any shower and thunderstorm activity as these can markedly affect the local winds in the very short term.

Tidal currents can play a big role in determining the state of the sea and can also have a marked affect on the surface wind flow. For example, the situation of tide opposing wind can set up a very nasty sea-state and this should be factored into any race strategy. It doesn’t stop there though, you will have to be prepared to weather watch and update your forecast if required as the race moves along.

Other Bits:

  1. When south-west or southerly changes make it to the Whitsundays, they bring cooler conditions coupled with gusty winds and at times rain showers. It does pay to keep some warm clothes and wet-weather gear onboard to cater for these situations.
  2. Watch the weather maps regularly because these surges in trade winds may arrive in the tropics a day or two after the cold front has passed through well to the south. There will seldom be a well defined change marked on the weather maps, but you should notice the isobar spacing tighten to the south of you before the surge arrives.
  3. You should also be aware that the tidal streams in the area flood to the south and ebb towards the north. Variations, including eddies, will occur locally due to many factors including the effect of headlands, convergence between islands, etc. These tidal streams can run at speeds up to four knots. Check out the tides for the area. Do this well before you go racing. Getting on the right side of these streams can make you a winner!
  4. Apart from using the BoM’s Marine site for tidal data. The Queensland Government web site also has some great information on tides at www.msq.qld.gov.au/Home/Tides
  5. Be aware that the tide times are 25 minutes earlier at Hayman Island than at Shute Harbour (the secondary port). Dent is five minutes earlier than Shute and Coppersmith Rock is 15 minutes earlier. The BoM’s tide link also offers time offsets for various Whitsunday locations based on Shute Harbour.
  6. If required, a glossary of meteorological terms to aid you in your weather endeavours can be found at http://www.bom.gov.au/lam/glossary/

Above all else, remember the Eye Ball Mk 1 method. Keep looking up and around you for clues to wind and weather changes. Good Racing.

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