The Sydney Hobart blow of 1998, which claimed half a dozen lives, and the loss of a number of yachts, is a devastating event for those who took part in the race as well as for those of us who have read about the blow (and wonder if we might some day be caught in the same circumstances). We are just now starting to see data on the storm, and hear from those who were close to the event.
The following surface level analysis charts have been kindly sent to us by the Australian Met Service, and they tell an interesting story. This first chart is from the day of the race, December 26 (local time). The forecast was for gale strength winds in the 35- to 40-knot range, nothing out of the ordinary for the a fast-moving frontal system in this part of the world.
There are four interesting features on this chart. The first is the high pressure system over New Zealand in the lower right hand corner of the chart. If you had been watching the faxes for the previous week you would have noticed that as this high pressure system built and moved over New Zealand it remained rather stable. For the past couple of days the center of the high has not moved appreciably.
Next, there’s a depression centered around 25S and160E, northeast of Sydney. This is a long way from the start of the race, but it represents a potential source of energy and should be a caution flag. The third element is the squashed-looking front to the west of Tasmania in the Australian Bight. As the leading edge of this front approaches the southeastern corner of Australia you will have northerly quadrant winds. However, these will quickly shift to the south as the front passes. The shape of surface isobars, so elongated in a north/south direction is another warning. The cause of this shape will be found in the stable high over New Zealand, and the large high pressure system pushing in from the Western edge of Australia.
The front is caught between the two highs and is being compressed. Compression zone winds are always difficult to predict, and can quickly build force while showing little change in barometer, wind direction, or cloud cover.
If you woke up on race morning, all excited about the passage south, and had a look at this chart and the forecast for gale force winds, you would have to admit that the potential for a serious blow existed. You would then think about the currents between Sydney and Tasmania. There’s a wind produced current running in an easterly direction in the Australian bight, a northerly running current along the East Coast of Australia, and frequently a south running stream in Bass Straights itself. Where these converge there are significant differences in sea surface temperature which can increase atmospheric turbulence, surface level winds and gustiness, and of course, wave heights.
Another factor to consider is “ducting,” a phenomenon wherein wave systems become trapped within current strong boundaries. Ducting can significantly increase wave height and steepness.
So what do you decide to do? The prudent approach is to wait a day or two and see what happens with the weather. This is certainly the course which anyone cruising to Tasmania would adopt. But if the race organizers decide the show must go on?
Now let’s look at the surface analysis chart for 12 hours later. The front has moved east as expected and shortly there will be strong southerly quadrant winds to deal with. Two danger signals are readily apparent: the low we looked at on the previous chart to the northeast has deepened to 992mb (from 998mb) and moved to the Southwest. The high over New Zealand is still stable, and the 992mb low is being moved towards Sydney by the counterclockwise circulation around the high (remember, this is the Southern Hemisphere and the weather systems rotate the opposite of the Northern Hemisphere).
There are two serious issues now to consider. First, the subtropical low is a potential source of humidity and warmth to feed into the system in the Bass Straights. This is a big factor in making a rapid increase in winds possible. The other factor is the stable high over New Zealand. The isobars of the low coming through the Bass Straights between Sydney and Tasmania are even more compressed, another ingredient for increasing winds.
If you were not already nervous from the previous chart, this fax would alert you to the fact that it might be better not to start the race at all. On the other hand, the forecast calls for 35 to 40 knots of wind based on this data, and while this is a reasonable projection from what we know, there is always the risk of the forecast being on the low side.
Consider again: the low moving down from the northeast with the potential of significant energy, the compression zone between low and high, the possibility of increased vertical turbulence and gustiness from sea surface temperature, and sea state risks from current against wind and wave ducting within the currents.
The next chart shows the surface conditions 12 hours later, now early morning on the 27th (after the start of the race the previous afternoon). That low we’ve been worried about is continuing to work its way towards us, the New Zealand High is still in the same place, the high from the west is now very close and the isobars in the Bass Straights and up the southeast coast of Australia continue to compress.
Bob McDavitt of the New Zealand Met Service points out that sustained winds near the coast were measured at 50 to 55 knots, which is consistent with mean gust speeds of 70 knots, in the range of what some of the yachts reported. He says that sea heights were measured by doppler radar on the rescue helicopters with heights of 65 feet (20M). This is much higher than you’d find with this fetch, wind strength, and time span in the open ocean, but remember this area has all sorts of confusing currents which can increase wave heights and steepness.
Lets go back to the last surface analysis for a moment and look at the energy potential for this blow. First, the high pressure to the west, with its counterclockwise circulation, is bringing cold dry air up from the higher latitudes in the Southern Ocean. Next, the tropical low we’ve been discussing to the northeast is a source of warmth and humidity, even as the low is dying off. Next, the energy from the that low, along with some additional warmth and humidity from the subtropics is being sent southeastwards by the high over New Zealand (since this is on the western edge the counterclockwise circulation is northeast about this high’s center). These are all components of a strong blow.
The Bass Straights and the stretch of ocean between Sydney and Tasmania are a dangerous stretch of water. The currents, shallow sea bed, fast-moving weather systems, and the type of energy ingredients we’ve seen here come together with some regularity to create a nasty situation. Knowing this, the prudent thing to do is wait until you see a weather window of sufficient duration to make the passage with a reasonable degree of safety.
Of course in defining this weather window you need to look at the official forecasts, and then ask the question “what could go wrong with this?” Are there blocking highs which could cause a squash zone? Is there a tropical system that could cause problems? How about upper level troughs rotating through?. Remember, the Australian Met Service got this almost right. They were close on the time, and close on intensity. Given the huge number of variables in this part of the world, I think they did an excellent job.
A final thought on wind strength and sea state. The forecasting techniques available today are far superior than even a few years ago. Worldwide computer weather models are getting better, and there are amazing satellite resources with which the forecast offices can work. In general, the record for forecasting wind systems on a macro (large-scale) basis is really quite remarkable. However, the creation of weather within atmosphere is a super-complex phenomenon, and while the macro forecasts have become very accurate, what is happening on your own little patch of ocean can vary significantly-especially when there are elements like warm currents or tropical moisture coming into contact with higher latitude air masses.
All forecasts have a degree of fallibility-the key is understanding the risk factors in any given scenarios, and taking appropriate actions.