Saturday 12 May 2018

Tasmania hit by flash flooding.

Parts of Hobart and southeastern Tasmania have been hit by flash flooding after record-breaking rainfall in the area this week. Hobart received 129.2 mm of rain in 24 hours on Thursday 10-Friday 11 May 2018, the highest rainfall within 24 hours ever recorded in May in the city, and only the fifth time that rainfall in excess of 100 mm has fallen within 24 hours in the area at any time of year, with over 40 mm falling in one hour, the highest rainfall ever recorded in Hobart, while 236.2 mm of rainfall was recorded over the same period on Mount Wellington, the highest rainfall in 24 hours ever recorded in Tasmania.

 Floodwaters in Hobart this week. Patrick Gee/The Mercuary.

There are no reports of any casualties in these floods, but a number of people had to be rescued from vehicles and local emergency services are warning against unnecessary travel. Cars have been swept away in parts of Hobart, with many homes and businesses effected by flooding, as well as hospitals and the University of Tasmania. High winds have also caused damage to many homes, as well as knocking down trees and power lines; about 12 000 homes are thought to be without power across southeaster Tasmania.

Debris on the streets of Hobart, Tasmania, following flooding this week. Edith Bevin/ABC.

The flooding has been linked to a low pressure system over the Pacific Ocean to the northeast of Tasmania. Low pressure systems over oceans are caused by warming of the air over the ocean, which causes the air over the affected area to rise, while fresh air is drawn in from elsewhere and in turn be warmed and rise. At the same time the heat causes high levels of evaporation from the ocean, so that the rising air is also waterlogged. Eventually this air rises high enough in the atmosphere that it matches the pressure of the air around it, at which point it stops rising and drifts with the prevalent wind. Eventually it passes into cooler areas, where it starts to lose its water as precipitation (rain), as cooler air cannot hold as much evaporated water as warmer air. As land has different thermal properties to water, this often means that the cooler areas encountered by the warm, waterlogged air are over land, leading to high rainfall in coastal areas. This system has also been linked to snowfall in New South Wales this week.

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