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Jul 8, 2014

Crikey Clarifier: skiing pretty, or snow problems?

Will a hot autumn result in a long ski season? Do the phases of the moon result in better snow on the slopes? Meteorologist Magdalena Roze explains why you should always have your skis ready, just in case.

Would-be skiers were cursing above-average temperatures at the beginning of June, convinced their expensive equipment and snowsuits would languish in the garage all winter. Now with a healthy snowfall, snow bunnies are hitting the slopes. But is there any way to know how much snow Australia is likely to get during a winter — perhaps before one shells out one’s hard-earned at Kathmandu?

What determines Australia’s snowfall? 

This may come as a bit of a surprise, as there are countless theories that attempt to make a connection to the success or failure of our snow season. But things like the timing of first snowfall or the moon or whether autumn was hot (or not) are really not much help to us, because the main thing that’s going to bring the Australian Alps decent snowfall is cold fronts.

What is needed for decent snowfall?

Cold fronts deliver the two key ingredients for snow: precipitation and cold air. And these two must come together as a package deal.

There can be chilly weather galore, but if there’s nothing falling out of the sky to freeze, you won’t get snow. It gets a bit trickier with precipitation because it’s so dependent on temperature. You want buckets of water coming out of the sky, because if it’s cold enough for it to freeze then you’ll get a heap of snow. But if it’s too warm, the water won’t freeze and you’ll just end up with a lot of rain. In Australia, this can often go either way because the alpine climate here is so marginal.

What is responsible for cold fronts?

There’s a belt of cold westerly winds or low pressure that circles Antarctica, moving north and south at varying times of the year. This is known as the Antarctic Oscillation (AO) or Southern Annular Mode (SAM). When the SAM is in a positive phase, the westerlies contract south closer to Antarctica, which means that we get weaker westerly winds and stronger areas of high pressure over southern Australia. These highs not only bring prolonged periods of dry weather, they also block cold fronts from hitting the mainland. This is bad news for snow because we need these fronts coming our way to bring cold air and precipitation.

On the other hand, a negative SAM phase is when the belt of westerly winds expands towards the equator, which means more and stronger fronts and lows hitting southern Australia. This can result in snow if the precipitation is falling in the right place and it’s cold enough for it to freeze into snow. We need the SAM to be in a negative phase to get more cold fronts to increase our chances of snowfall.

Can you predict which phase the SAM will be in?

It’s virtually impossible to predict what phase the SAM will be in more than a few days to a fortnight in advance, let alone a few weeks or months. Therefore, it’s very difficult to predict a snow season.

Why do we have such good snow now?

Another climate pattern, El Nino, which has a major influence on weather around the globe, has stalled. El Nino typically occurs every two to seven years and results in drier and warmer conditions over southern Australia. Warm and dry are basically the enemies of snow.

On average, the peak snow depth measured at Spencer’s Creek (which is between Perisher Valley and Charlotte’s Pass in NSW) is 35cm lower during El Nino years, and the season is 2.5 weeks shorter. In fact, the four lowest peak snow depths were all measured during El Nino years. Above-average snow seasons have occurred during El Nino years, but these are an exception.

The majority of climate models are still forecasting an El Nino this year, but it’s been pushed back to spring as the atmosphere is taking its time to respond to the changes occurring in the ocean.

There have also been plenty of cold fronts hitting southern Australia. Fronts move from west to east, and the fronts in the past week have been particularly juicy because sea temperatures off Western Australia are warmer than normal so there’s more moisture in the atmosphere. These fronts have been picking up this moisture in the west, carrying it east and dropping it off at the Alps, resulting in over a metre of fresh snow for some resorts. But this could turn around.

You may recall dismal snowfall last July when SAM was in a positive phase. It turned around to a negative phase in August, resulting in more cold fronts and a good late season snow dump.

What’s the snow report look like now?

Luckily, it’s still looking good in the near future. A cold front is crossing the southeast on Wednesday and Thursday with a top up of 20cm to 40cm of snow, followed by another weaker system on Friday with 10cm to 20cm of fresh powder.

*Magdalena Roze is a journalist and meteorologist. You can follow her on Twitter.

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2 thoughts on “Crikey Clarifier: skiing pretty, or snow problems?

  1. Glen

    Nice summary. There are of course more things that affect our snow than AAO and ENSO; see (Looks like I’m going to miss yet again … prediction is difficult, especially about the…)

  2. Alex

    Most edificational, thank you!