There is good agreement between the leading computer models that a deep low pressure system will form over the Tasman Sea this Boxing Day. This has been a relatively common occurrence for the Christmas-Boxing Day period over the past 10 years, providing a strong influence on surf conditions and weather through Southeastern Australia. The most notorious Tasman Low was the system that created havoc for the Sydney to Hobart race of 1998, with extreme winds and possibly the largest recorded wave of all time.

1998 This year is infamous in both sailing and weather forecasting history. A small scale low formed over Eastern Bass Strait late on Boxing Day and produced extreme weather that was underplayed by the leading computer models. A paper writtern by Peter Joubert (a retired Professor of Fluid Dynamics from Melbourne University), covered some of the extreme aspects of this low, including the measurement of a 42.7m wave by rescue helicopters and also mean wind-speeds in excess of 68kts (125km/h+). The maximum wind gust recorded at Wilsons Promontory was an incredible 92kts (170km/h). The wave height reading of 42.5m (120ft) is the highest on record and was taken from a rescue helicopter using a radio altimeter. This severe weather event took place during the Sydney to Hobart Yacht race, where 6 people tragically lost their lives. 

The above chart (produced by Peter Joubert) displays the small but intense low in the vicinity of Wilsons Promontory. It also shows the difference between the observed data (analysis) and the forecast data (numerical models).

1999 This year was in vast contrast to 1998, with the charts displaying a typical summer pattern. The chart below shows a heat trough over inland NSW and a broad high over the Southern Tasman Sea. Another high is positioned to the south of the Bight, which is typical of the summer months. This pattern is not especially exciting for Australian surf prospects, although areas such as the East Coast of Victoria and the Southwest Coast of WA would have had clean conditions thanks to offshore winds. 

2000 While there was no Tasman Low as such this year, a large long-wave trough to the south of Tasmania spawned a very large and deep Southern Ocean Low. This system generated a large Boxing Day swell for both Victoria and Tasmania. Winds held from a NW'ly direction through Southern Victoria, producing great conditions at many locations.   

2001 A small low that passed to the south of Tasmania on Christmas Day produced hot NW winds through Sydney and extreme fire conditions. At the same time the low was dragging cold polar air across Tasmania, resulting in snow over higher parts. This system produced a decent swell increase for Victoria and Tasmania, but not a great deal for NSW. This is because the low weakened as it moved towards the Tasman Sea on Boxing Day.

2002 It was another cool Christmas through Tasmania and Victoria in 2002, but not quite as extreme as the previous year. A deepening low and cold front swept up into the Tasman Sea on Christmas Day, leaving the southern states under a cool SW'ly airflow. This low was well positioned to produce a fairly large S'ly swell for the NSW coast as it moved into the Tasman Sea. 

2003 A low pressure system passing to the south of Tasmania on Christmas Day held reasonable swell prospects for Victoria but weakened as it moved east into the Tasman Sea, limiting the amount of size seen along the NSW coast over the following few days. Once again it was a cool Christmas through Victoria and Tasmania, but a W'ly airflow produced hot weather along the NSW coast. 

2004 In yet another cool weather setup for southern Tasmania, a deep low passed to the south of the state on Boxing Day. A strong S/SW fetch between this system and a large high over the Bight sent a large swell towards Victoria and also Southern and Western Tasmania. It should be noted that the high is ridging along way south behind the low, which is not only setting up the broad S/SW fetch, but also helping to drag cold polar air northwards towards Tasmania.   

2005 An impressive Southern Ocean low passed to the south of Tasmania on Christmas Day. This low had a very low central pressure of 950hPa, which helped to create very strong SW winds along its rear flank. This low was responsible for a very large and powerful swell through both Tasmania and southern Victoria in the days following Christmas. The high slipped southwards on Boxing Day, allowing winds to tend N/NW'ly along the Victorian Coast. As in 2000, this created excellent conditions at many locations. The NSW coast missed out to a large degree due to the weakening of the low and also its subsequent southeastwards movement.  

2006 This year is shaping up amazingly well for surfers along the East Coast of Australia. All models are in agreement that a low will start to deepen to the east of Tasmania on Christmas Day, setting up a a broad and strong S'ly fetch through the Tasman Sea. As was seen in 2004 (although a little further to the west,) a strong high to the south of the Bight will contribute to the size and strength of the fetch. It will also help to drag very cold air up across Tasmania and southern Victoria on Christmas Day. Snow over the higher parts of Tasmania looks likely at this point and the Alpine region of Victoria may also see snow. The swell from this system looks like peaking mid week along the East Coast, but check the regular forecast updates for more precise details.  

Overall it can be seen that the Christmas period and the month of December more generally is still quite a transitional period from spring to summer. Volatile weather is still quite common as cold polar air masses clash with warmer air to the north, leading to the formation of mid latitude low pressure systems. More settled weather tends to be more common into February and March as heat becomes more evenly distributed through the Southern Hemisphere. This year the timing of the Tasman low looks excellent for East Coast surfers as we move into the holiday period, so make the most of it!