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Tasmania just had its sixth warmest spring on record, and the outlook is for a hot summer

By the Bureau of Meteorology's Dr Andrew Marshall, Sunday December 6, 2020 - 09:21 EDT
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Rainfall for the summer is likely to be above average in the northeast, and average to below average in the southwest. - ABC

Spring 2020 brought a mixed bag of weather to Tasmania, as is typical for our state at this time of year.

It was the sixth warmest spring on record, with days and nights much warmer than average for most areas.

Average overnight temperatures were the warmest on record across parts of the north-east.

It was a dry start and end to spring, with rainfall close to average for the east, below average for the west and very much below average for parts of the south.

September: drier and warmer than average

September was drier than average from the Central Highlands to the southeast coast, and in parts of north-western Tasmania where rainfall totals were below average for the fourth month in a row.

Averaged across the state, rainfall was 21 per cent below the long-term average for September.

Hobart (Ellerslie Rd) recorded 35 mm, which was below the long-term average of 52.7 mm, while Launceston (Ti Tree Bend) recorded 51.8 mm, below its long-term average of 64.7 mm.

Days and nights were warmer than average for September, and in fact it was the warmest September for daytime temperatures since 2016.

Despite these overall warm conditions, there was a mix of colder and warmer days and nights across the state. Standouts include:
Hottest day: 23.4C at Bushy Park (Bushy Park Estates) on the 7th
Coldest night: -7.5C at Liawenee on the 1st

The Bureau declared an on September 29, meaning Tasmania would likely see some above average rainfall in parts of the north and east over the coming months.

October: wettest and coolest since 2016

After a dry start, October brought above average rainfall to parts of the East Coast and Southeast districts, mostly due to slow-moving rainbands early in the month.

Rainfall totals for Hobart (122mm) and Launceston (107.4mm) were well above their long-term averages for October, and it was Tasmania's wettest October since 2016.

Both daytime and night-time temperatures were close to average during October for much of Tasmania.

However, some cooler days in the east made it the coolest October since 2016.

Standouts include:
Wettest day: 140.0mm at Gray (Dalmayne Rd) on the 9th
Coldest day: 0.0C at kunanyi (Mount Wellington Pinnacle) on the 5th

November: driest since 2007, warmest since 2017

The see-sawing conditions of spring 2020 continued into November with below average rainfall and above average temperatures recorded across the state.

Rainfall was 52 per cent below average, making it the driest November since 2007, and the ninth driest November on record for the state.

It was Tasmania's fourth warmest November on record for daytime temperature and the fifth warmest on record for night-time temperature.

It was also the warmest November since 2017, with records set at several locations.

Standouts include:
Hottest day: 34.8C at Scotts Peak Dam on the 11th
Warmest on average overall: 17.0C at Hobart (Ellerslie Road)

Despite La Niña being active in the tropical Pacific, November ended up being much warmer and drier than average due to temporary changes in some of the other drivers of Australian climate, both in the tropics to our north and over Antarctica to our south.

These affected the pressure pattens over Australia, making rainfall less favourable across most of the country.

Summer 2020-21

Rainfall for the summer is likely to be above average in the northeast, and average to below average in the southwest, with near average rainfall elsewhere.

Temperatures are likely to be above average across the state, partly due to fewer cool westerly winds and more humid easterlies, but we will also be influenced by above average ocean temperatures around Tasmania.

Our models suggest the waters around parts of Tasmania could be 2 to 3 degrees above average later in summer.

While the expected increase in rainfall for northern Tasmania is consistent with the typical impact of La Niña, the summer outlook is also being influenced by another mode of variability known as the Southern Annular Mode, or SAM.

The SAM refers to the north-south movement of the strong winds that flow from west to east around the Southern Hemisphere, including over Tasmania.

The SAM is currently in its positive phase, which means these westerly winds have moved south of their usual position.

A positive phase of the SAM is expected to persist through into January.

For Tasmania, this is why we are likely to see an increase in summer rainfall over the east of the state and a decrease in summer rainfall over the southwest.

Under the influences of La Niña and positive SAM, there is an increased risk of flooding in areas with wetter soils and higher rivers, particularly in north-eastern Tasmania.

Summer heatwaves are expected to be less intense, but they may last longer and be more humid.

There is also a risk of grass fires over the coming months due to more vegetation growth in spring this year, compared to last year.

Being aware of these potential hazards will help in our preparedness for a safe and enjoyable summer season 2020-21.

Dr Andrew Marshall is a senior research scientist at the Bureau of Meteorology


© ABC 2020

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