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Will we have a good snow season this year?

There were two decent rounds of alpine snow last month and much of Australia's southeast shivered through their coldest temperatures in at least six months last week.

Early-season cold snaps and snow flurries always bring one question to the fore: Will the snow season be good this year?

The first thing to point out when discussing Australia's snow season is that it is fickle. The position of Australia's alps in the mid-latitudes and the relatively low height of our skiable mountain peaks means that even during winter, snow can quickly turn to rain. In Australia, the quality of a season can be determined by one or two big snow events.

Now onto the season ahead.

The two ingredients needed for snow are: precipitation and freezing temperature. Both need to be present at the same time for natural snow to occur.

While we can't determine the timing or snow amounts of individual events this far out, we can use seasonal temperature and rainfall forecasts to get an idea of how the upcoming snow season might unfold.

Firstly, let's consider the weather patterns that will be influencing Australia's southeast during the middle of 2017.

The Pacific Ocean is currently in a neutral phase, meaning neither El Nino or La Nina are in force. However, we could be moving towards El Nino in the coming months and five of eight international climate models exceed El Nino thresholds by September.

The Indian Ocean is also in a neutral phase at present, although the majority of forecast models anticipate that a positive Indian Ocean Dipole (IOD) event will occur this winter.

Both El Nino and a positive IOD typically cause below average rainfall across southeastern Australia during the middle of the year, while the latter can also cause warmer-than-usual conditions in the Alps. In fact, these two climate drivers can work together and exacerbate their effects.

According to the Bureau of Meteorology, Australia's maximum snow depth during El Nino years is around 35 centimetres lower than the long-term average, while the length of time with a snow depth above one metre decreases by a fortnight.

Unfortunately for snow-lovers, the current outlook means that there is an increased likelihood of below average snow depth and duration during the upcoming season.

There is a silver lining to this ominous-sounding situation though. The fickle nature of Australia's snow season can work in our favour. Only one or two strong cold fronts can overcome the background influence of the Pacific and Indian Ocean climate drivers. So, good snowfall is still possible under the regime of an El Nino/positive IOD double-header.

The strength of cold fronts in southern Australia is influenced by an index called the Southern Annular Mode (SAM). When SAM is negative, cold fronts extend further north and can cause more snow than rain.

The average peak snow depth during a negative SAM year is a whopping 240cm, according to the Bureau of Meteorology. This is 80cm higher than positive SAM years (when cold fronts are pushed to the south). Unlike El Nino and the IOD though, SAM cannot be reliably forecast beyond a couple of weeks and therefore can't be used to predict seasonal snow quality.

On average, Australia's alpine snow depth exceeds one metre between the middle of July and October, with a peak depth of close to two metres usually occurring in late-August or early September. However, global warming has decreased the season's peak depth closer to 1.5 metres in recent years.

While the odds are against a bumper snow season in Australia this year, the alps can be full of surprises. We will most likely have a slow start to the season and the majority of our snow should from a handful of strong frontal systems.

If you are planning on a trip to the slopes, keep an eye on the two-week snow forecast during the months ahead at: http://www.weatherzone.com.au/snow

- Weatherzone

© Weatherzone 2017

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