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 websites, including the Bureau of Meteorology’s site. For example, general (low resolution) marine forecasts can be obtained from the following links:
A number of websites offer computer model derived wind forecasts for the Australian region. 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) from the Bureau of Meteorology Aviation website (can be obtained from other sites such as Windy in a decoded format). You will be prompted for access just click "I Accept". On top LHS side, scroll down the list to Aviation Forecasts) click and a map will appear. Select 44 on the map and then press Search. A list of aerodrome forecasts will appear. Scroll down to Hamilton Island (YBHM) and bingo. You will also spot Mackay (YBMK) and Proserpine (YBPN) on the list.
An example of a TAF looks like this:
TAF YBHM 222210Z 2223/2312
14020KT 9999 FEW008 SCT025
FM230100 12020KT 9999 SCT025
T 21 22 22 21 Q 1022 1021 1019 1019
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 SE and NE), the wind experienced away from the Island can be markedly different from the forecast (and measured wind). TAFs are routinely issued every 6 hours but can be amended at anytime if required.
The decode of the TAF above is as follows:
TAF YBHM: Terminal Aerodrome Forecast for Hamilton Island
222210Z: date/time of issue group in Z (UTC) time. In this case issued on 22nd at 2210hr Z (0810hr on 23rd EST)
2223/2312: forecast validity group [valid from 2300hr on 22nd Z (UTC) (0900hr on 23rd EST) to 1200hr on 23rd Z (UTC) (2200hr on 23rd EST).
14020KT: surface wind group (average true wind direction and speed in knots at a height of 10m. In this example, from 140deg T at an average speed of 20 knots)
9999: horizontal visibility group (in this case the visibility is forecast to be 10km or better)
FEW008: cloud amount and height of cloud base group (FEW= 1-2 octas or eights at 800ft).
SCT025: cloud amount and height of cloud base group (SCT= 3-4 octas or eights at 2500ft)
FM230100 12020KT 9999 SCT025: FROM 0100 Z (UTC) on 23rd (1100hr on 23rd EST) the surface wind is forecast to become 120deg T at an average of 20 knots, the horizontal visibility equal to or greater than 10km and the amount of cloud SCT (3-4 oktas) at a height of 2500ft.
RMK: remarks which precedes information on Turbulence (if forecast), Temperatures and QNH
T 21 22 22 21: Forecast temperatures in 3 hourly intervals. In this case commencing at 2300Z at finishing at 0800Z (last 4hr not given)
Q 1022 1021 1019 1019: forecast QNH (roughly the sea level pressure) in 3-hourly intervals. In this case commencing at 2300Z at finishing at 0800Z (last 4hr 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.
The monitoring of actual meteorological conditions (hence very handy for the fine-tuning of forecasts) can be achieved 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 your knowledge 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.
- Shower and/or 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 effect 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.
- 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
- 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.
- You should also be aware that the tidal streams in the area will generally 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 correct side of these streams can make you a winner!
- 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/Tides
- 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.
- 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.