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Tropical cyclone Debbie has made landfall in Queensland as a category 4 cyclone with winds of more than 150 kilometres per hour.

The cyclone crossed the coast near Airlie Beach on Tuesday afternoon. Reports of wind gusts in excess of 200km per hour and rainfall of more than 200mm of rain have been made in some areas along the central Queensland coast.

The Bureau of Meteorology forecasted an average to above-average number of Australian cyclones in its October severe weather outlook. Australia receives 11 cyclones on average each year, with about four of those in Queensland. Debbie is the fifth cyclone of the season for Australia as a whole and the most intense of the season so far.

Anomalously high moisture, warm ocean temperatures, and low environmental pressures seem to have created the conditions that allowed TC Debbie to form and grow in intensity.

Perfect storm

Tropical cyclones are low pressure systems that form over warm tropical oceans. The warmth and moisture of the oceans are what gives a cyclone its energy. The low pressure, which meteorologists measure in “hectopascals”, draws in the surrounding warm, moist air, which then rises into deep thunderstorm clouds. As the air is pulled into the centre of low pressure, Earth’s rotation causes it to spin cyclonically and it continues to intensify.

TC Debbie formed at the eastern end of an active monsoon trough extending from the Indian Ocean across the top of Australia and into the Coral Sea. The monsoon trough is a region of low air pressure and thunderstorms that forms over northern Australia in the summer months, bringing with it the wet season. On March 22, a large region of active thunderstorms began to organise into a weather disturbance off the eastern tip of Papua New Guinea.

Over the following two days the thunderstorms organised about a circulation centre as sea level pressures began to drop and moist air converged into the area. By late on March 24 a tropical depression, a forerunner of a cyclone, had formed and begun to drift south, making a long S-shaped track.

Tropical Cyclone Debbie was named on March 25. It then came under the influence of the subtropical ridge, a zone of stable high pressure that gives much of Australia fine weather during the summer. This drove Debbie west-southwest towards the Queensland coast while it gradually intensified further.

Because of the relatively high amounts of moisture in the atmosphere, and relatively warm ocean waters, Debbie intensified to category 4 by 10 pm on March 27, with the strongest wind gusts reaching 225-280km per hour. On Tuesday afternoon Debbie was a strong category 4 cyclone with a central pressure of 943 hectopascals and surface sustained winds of 185 kilometres per hour. The Bureau of Meteorology downgraded TC Debbie to a category 3 at 4:00 pm EST.

To put Debbie in context, there has been only one cyclone since 1980 to have made landfall in Queensland with a lower central pressure. That was Yasi in 2011.

Of the 46 cyclones to have made landfall in Queensland since 1980, only three others arrived at the coast with pressures of less than 960 hectopascals: Dominic in 1982, Winifred in 1986, and Ingrid in 2005.

Predicting cyclones

Tropical cyclone forecasters use a variety of tools to forecast the storm’s track, intensity, storm surge, and rainfall. Because it is difficult to obtain observations of wind at ocean’s surface under a cyclone, meteorologists have developed tools based on satellite imagery to estimate a storm’s intensity, location, and where the strongest and most destructive winds are found.

Several models are also used to aid in making forecasts – from the complex numerical weather prediction models, to statistical models. Models start by using observations of the atmosphere, and then use these data to make a forecast.

Depending on their level of complexity the models can predict the future track, intensity, rainfall, wave height, and/or storm surge. The forecasters access all of this information to then make their forecast.

Cyclone forecasts have improved considerably over time. In particular, track forecasts have improved so that the 48-hour forecast is now more accurate than the 24-hour ones were back in the early 1990s. Track forecasting has become so reliable that the US National Hurricane Centre now produces 120-hour track forecasts.

Intensity forecasts have improved more slowly, but as models have become more refined and satellite technology has improved, the ability of forecasters to accurately estimate and predict intensity is also getting gradually better.

The prediction of rainfall, the extent of the damaging wind field, and storm surge forecasts are also slowly improving. Now that they are receiving more attention, we can expect considerable improvements in these over the next decade.

Liz Ritchie-Tyo receives funding from the U.S. National Science Foundation.

Hazelwood's closure does not mean imminent blackouts for Victoria. Takver/Flickr, CC BY-SA

Victoria’s Hazelwood power station will be shut down this week after nearly 50 years of supplying electricity.

The imminent closure has led to concerns about blackouts, raised most recently by Deputy Prime Minister Barnaby Joyce, and rising electricity prices.

So what does the evidence suggest?

Blackouts ahead?

Last week The Age reported that Victoria is facing “72 days of possible power supply shortfalls over the next two years”. While that sounds bad, it does not mean the state is facing imminent blackouts.

This was based on a report from the Australian Energy Market Operator (AEMO), which is in charge of making sure that Australia’s energy markets work.

Every week, AEMO produces something called the Medium Term Projected Assessment of System Adequacy. This report assesses the expected supply and demand of electricity for the next two years.

In a recent report, AEMO did indeed forecast a “reserve shortfall” for 72 days in Victoria in the coming two years. AEMO has actually been forecasting many days of reserve shortfall, since early November last year when Engie announced the closure of Hazelwood.

AEMO has also been forecasting an even greater number of days of reserve shortfalls in South Australia for well over a year.

The shortfall forecast is based on a combination of factors. This includes the amount of local energy supply, the import and export of electricity from other states, the maximum daily demand for electricity, and the “reserve requirement”. The reserve requirement is essentially “spare” capacity that can be used to maintain a reliable supply if something goes wrong.

If there is not enough supply to meet this requirement, there is a reserve shortfall.

Forecasting maximum demand is incredibly challenging and uncertain. AEMO does it by using probabilities. This gives us a measure of the probability of a particular demand forecast being exceeded in a year.

For example, a 10% chance would be expected to be exceeded one year in ten. A 50% chance would be expected to be exceeded one year in two.

To illustrate the point, AEMO forecast that demand over the past summer in Victoria had a 10% chance of exceeding 9,900 megawatts. In reality, the maximum demand was only 8,747MW. That’s not to say the forecast was wrong, but rather that it was not an exceptional (one year in ten) summer.

In the recent outlook, AEMO has found 72 days on which a reserve shortfall might occur. The likelihood of this happening on any one of those days is low. For a reserve shortfall to actually occur 72 times over two years is incredibly unlikely.

However, AEMO still plans for this possibility. Indeed, this is largely the point of producing these forecasts: signalling potential capacity shortfalls so the market and operator can respond.

What will happen when Hazelwood closes?

Another way of illustrating the role of Hazelwood and the effect of its closure on the broader Victorian energy system is shown below.

In this figure, I’ve plotted the 10% and 50% thresholds for exceeding maximum demand in the coming summer, and also the “load duration curve” for previous years. This curve shows that the periods of greatest demand are also the least common (the left side of the graph). The vast majority of demand is much lower, and the “base load” is about 4,000MW.

† Interconnection capacity (from other states) at times of peak demand is much less than the total theoretically possible. ‡ Firm wind is about 7.5% of total rated capacity in Victoria. Author

I’ve also included “firm capacity” (the minimum power we know we can get) with and without Hazelwood, to the right.

As can be seen, there is more than enough capacity in Victoria to meet the base load. There is even enough local firm capacity to meet the peak load and reserve requirements for the one-in-two-year maximum demand event. For the one-in-ten-year event, power needs to be imported from other states to ensure secure supply at the peaks.

AEMO reaffirmed security of supply in a media statement last week. As noted, Victoria and other states have available power generation resources that are not switched on or are operating at less than full capacity. This electricity can be made available to replace the power that Hazelwood supplies.

What about prices?

The question of what replaces Hazelwood brings us to prices. Many, including AEMO, expect to see increased generation from currently underused power plants. These include New South Wales’ black coal power plants. Last year NSW’s black coal was used at 56% of its total capacity. Bumping up these stations’ output would also reduce NSW’s reliance on Victorian exports.

Reducing the capacity of brown coal will mean logically that Victoria relies on more expensive forms of generation such as black coal or gas. This is particularly so if the availability of cheap imports is limited, and more expensive local generation such as gas is needed.

Black coal power stations generate electricity comparatively cheaply. Even so electricity prices are already so high that an increase in black coal generation may not have a dramatic impact on prices. With NSW prices averaging A$137 per megawatt hour this year, it is clear that the cost of coal is not determining electricity prices.

The Victorian wholesale market will also become a more concentrated market. As a result, there may be more opportunities for market power to be exercised. Perhaps the recently announced ACCC inquiry into power prices will put generators on their best behaviour.

Any price rise may be short-lived. The Australian Energy Market Commission, which sets the rules for the energy market, has reported that more renewable energy supply is expected to reduce wholesale electricity prices.

Hazelwood’s closure should not compromise the security of the Victorian electricity system over the next few years. This is not to say that there definitely won’t be a blackout. A one-in-50-year storm, a plant failure, a flooded mine pit, an interconnector outage – any of these events could strain the system beyond what is manageable.

At this stage, what ultimately happens to prices is anyone’s guess. Whatever the case, it is clear that Victoria has plenty of supply to meet the state’s base load. New capacity might be required to meet the maximum demand – and that new capacity could take the form of energy storage.

Dylan McConnell has received funding from the AEMC's Consumer Advocacy Panel and Energy Consumers Australia.

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