Weather and the web

Kenn Batt from the Bureau of Meteorology looks at forecasting tools with particular emphasis on the Sydney-Hobart Race.

The Sydney-Hobart course is renowned for its ability to produce some of the roughest water in the world. It is therefore important from a safety as well as a race strategy point of view to understand the weather that you may be faced with during the race. A typical weather map sequence for the race would be as follows:

Day 1: A cold front moves through the fleet sometime during the first day. One of the worst case scenarios would be for the front to develop into a “Southerly Buster.” In this typical scenario the race would start in a NE sea breeze and turn southerly along the NSW coast behind the front. The winds would blow from the west to south west in Bass Strait and further south.

Day 2: Southerly winds easing along the NSW coast and turning more SE. Winds would be SW further south. The passage of a second cold front through Bass Strait is possible. In this situation the wind would be more westerly over southern parts of the race course.

Day 3: Winds E to NE (sea breeze effects) along much of the NSW coast, becoming SW to southerly further south. Another cold front can move across Bass Strait and Tasmania. Should the cold front develop, westerly winds would be experienced in Bass Strait and the western Tasman Sea.

Day 4: S to SE winds moderating over the day. Coastal sea breezes are possible in the afternoon.

Wind frequency analyses (average winds) for a number of coastal locations can be obtained from this link: http://www.bom.gov.au/climate/averages/wind/selection_map.shtml

Be aware that the wind velocity (both direction and speed) at coastal locations can be very different to those further offshore, particularly when the wind direction is offshore. Treat coastal data with great caution.

In recent times the Bureau of Meteorology’s (BoM) website has undergone a number of big face-lifts. The most exciting new feature is the availability of the ACCESS Numerical Weather Prediction (NWP) system. The model which is essentially driving this system is the UK Met Office model, one of the best NWP models in the world today.

Along with US Wave Watch 3 model, this UK model forms the backbone of the BoM’s new Next Generation (NextGEN) weather forecast and warning system. Part of this system is the Graphical Forecast Editor (GFE) which will drive a Forecast Explorer. The GFE also allows for accurate graphical as well as text-based forecast products produced by BoM meteorologists.

More information can be had from this link

http://www.bom.gov.au/weather-services/about/forecasts/about-graphical-forecasts.shtml and from here

http://www.bom.gov.au/australia/charts/bulletins/apob80.pdf

So how can you better utilise the BoM website information?

At least 3-4 months before the race, have a good look at the website. Get to know it backwards. The categories of most interest are: Weather and Warnings; Marine Services; Charts; Satellite; Radar; Learn about Meteorology; and Practise your Skills.

You should start practising well before Boxing Day. A good starting point would be to work down the following list:

• Perform and/or obtain a comprehensive analysis including the latest observations for a wide area around the race track. These links should assist you:

http://www.bom.gov.au/australia/charts/synoptic_col.shtml – Latest surface weather map as well as those for the last six days.

http://www.bom.gov.au/nsw/observations/nswall.shtml – NSW observations

http://www.bom.gov.au/vic/observations/vicall.shtml – Victorian observations

http://www.bom.gov.au/tas/observations/tasall.shtml – Tasmanian observations

http://www.bom.gov.au/weather/radar/ – Weather Watch Radar

http://www.bom.gov.au/satellite/
– Satellite images

• Diagnose the situation. Why am I getting the current weather? Can you explain why from the current observations and surface analysis? Note that there can be many variations to the idealised weather assigned to the different weather systems. These variations can be due to a number of reasons mostly related to moisture content, intensity and speed, just to name a few.

More information can be obtained by working through the following links: http://www.bom.gov.au/info/weathmap/weathmap.htm and from http://www.bom.gov.au/info/ftweather/contents.shtml

• Prognose the future. Work down the “Forecast funnel”, from large to small scale:
1. Synoptic scale: utilising regional NWP forecast charts. Check out;

http://www.bom.gov.au/australia/charts/viewer/index.shtml – ACCESS forecast model to seven days

http://www.bom.gov.au/australia/charts/viewer/index.shtml?domain=combinedW&type=sigWaveHgt – Wave Watch 3 model to six days

http://www.bom.gov.au/jsp/marine/wind/index.jsp – This model utilises the ACCESS model for Day 1 and 2 and the ECMWF model for days 3 to 7. Local scale: utilising high-resolution NWP models. The ACCESS C model wasn’t operational at the time of writing this article. Keep checking. This model will have 5km horizontal grids. In the meantime check out the 12km ACCESS A model:

http://www.bom.gov.au/australia/charts/viewer/index.shtml?domain=IDY20202&type=windbarb&level=10m NSW/Vic – forecast winds to three days.

http://www.bom.gov.au/australia/charts/viewer/index.shtml?domain=IDY20201&type=windbarb&level=10m Vic/Tas – forecast winds to three days.

The pull-down menu on the domain button under the Interactive Weather and Wave Forecast Maps heading gives you an idea of the model output available.

By now you should have formulated a conceptual model for what is happening based on the latest analysis and observations. You would have used NWP guidance (forecast product) to predict larger scale controls on local weather.

You must take a critical approach to NWP. You should assess how well it has performed over the past few days. If it has done well, it means that you will have a greater confidence that it will do well into the future. If you can, compare the output of different NWP models. Get to know whether the model(s) have been pushing cold fronts through your area of interest faster/slower than observed (a typical cause of forecast failure). Is it (are they) over/under developing low pressure systems, etc.

Your own forecast

Once comfortable with the above, you can produce your forecast. Prepare your hourly or 3-hourly wind and wave forecast (for example) utilising NWP output massaged by you taking account of the following:

1. Localised topographic effects (wind channelling/funnelling, wind convergence/divergence, etc)

2. Diurnal (24-hour) variations of the wind (eg. wind speed normally drops off close to the coast overnight. Wind speed is normally strongest during the afternoon close to the coast. Further offshore the wind speed can be strongest around 3am).

3. Note that NWP models do not run at fine-enough resolutions to resolve a sea breeze situation by day or a land breeze overnight. They will also have difficulty in the movement or timing of cold fronts, more particularly during the warmer months along the NSW coast. You have to do this.

4. Note short-term trends in the wind and waves from your observations noted in your weather log (for example, for the last six hours or so).

5. Note that there can be more than one swell wave train running at any one time. The Wave Watch 3 model does a pretty good job in assisting you with this area.

6. Be aware that the nature of the wind wave will be driven by the surface wind velocity (direction and speed).

7. Apply any suitable local knowledge to the forecast.

8. Utilise the current coastal waters and high seas forecasts issued by the BoM. This product is an excellent first guess field. It’s a machine and person mix. Why not fine tune this forecast using the above tips?

Remember that NWP guidance and hence your weather forecast or any other forecast can fail, so YOU have to weather watch utilising your regular on-board observations. Be prepared to AMEND your forecast if and when the need arises even if only as a stop gap measure awaiting an updated forecast reflecting the new conditions.

Always be aware of weather warnings. Don’t second-guess them. The BoM strives to issue warnings with at least a 24 hour lead-time.

Ocean Currents

BLUElink is a BoM and a CSIRO project that allows the user to view actual and forecast (to seven days) ocean currents via the following links: http://www.bom.gov.au/oceanography/forecasts/index.shtml

After a couple of clicks, you will obtain a chart that covers the NSW coast. You can then step out to seven days with forecast sea surface temperature and current vectors:
If you click on the Temp or Sea Water Temperature (SST) button, you can obtain the temperature of the sea surface commencing at a given start date and stepping forward by 24 hours with each step out to seven days.

The graduated colour scale indicates the temperature values in degrees Celsius. By clicking on the Temp and Currents button, you can obtain not only the SST but also the direction and speed of ocean currents. The direction of the current tends to flow along contours of SST and the speed is indicated by the length of the arrow or vector. A warm eddy is associated with an anticlockwise flow in the southern hemisphere of the vectors or arrows, whilst a cold water eddy will have clockwise rotating arrows or vectors.

You can readily correlate the patterns of the SST with the flow of the current vectors (the arrows depicting the direction and strength of the current) in this product. An arrow representing the length of a 2.0 m/sec vector is the standard to compare others by.

Any situation where we have waves opposing an ocean current will see a very nasty seaway being generated. An example of this would be a southerly change opposing a south-setting East Australia Current (EAC) along the NSW coast. This situation is common in our warmer months and has seen plenty of retirements from the Hobart race on the first day out. Conditions tend to be worst at or near the Continental Shelf (where the EAC runs at its maximum rate), so placement off the coast is extremely critical in these situations.

For those of you that still like to look at the higher resolution product that has been made available by the CSIRO over the years. Please note that it’s still available at a cost from: http://www.cmar.csiro.au/research/oceans.html

Tidal information for around the Australian coast-line can be accessed via this link: http://www.bom.gov.au/oceanography/tides/index.shtml

The BoM site is very much a one-stop shop but there are also a number of weather and oceanographic sites available (some free, some not).

Word of Caution

Be aware that some of these forecasts are untouched by a meteorologist, so great care needs to be exercised with such product.

Two questions to ask yourself if utilising routeing software: Where do my GRIB winds come from? What is the horizontal resolution of the model? Note that with the model horizontal resolution, the lower the number (ie. the higher the resolution) the more accurate the model should be – a 5km model should be more accurate than a model running at 30km resolution, everything else being equal.

The forecasts and warnings available from the BoM web-site have been produced by a blend of computer and human meteorologist. This is a big plus.

The information provided by the Marine Services link covers everything from the latest forecast, warnings, observations and charts to how to obtain your weather information via HF and VHF radio.

An excellent working knowledge of all this information could just make the difference between winning or losing a race – or even life and death.