Weather News - Winter solstice

Ben Domensino, 21 June 2017

Today will be the shortest day of the year in Australia.

The winter solstice occurs in Australia today, marking the moment the southern hemisphere reaches its furthest tilt away from the sun. At this moment in time, the sun appears at its northernmost point relative to Earth's equator, causing this to be the shortest day of the year in the southern hemisphere.

From tomorrow, the sun will appear to progressively move higher in our sky, causing each day to become slightly longer than the last until the summer solstice in late December.

While the solstices are used in some countries to mark the beginning or end of winter and summer, the Australian seasons are defined by calendar months. Winter commences at the start of June, while summer begins on December 1st. This is in line with the definitions used in the United Kingdom and has the additional benefit of making is easy to compare statistics at seasonal and monthly scales.

The latest sunrise of the year actually occurs a few days after the solstice. So, while this is the year's shortest day, mornings actually will continue to get darker in the near future.

- Weatherzone

© Weatherzone 2017

Weather News - ENSO: Neutral does not mean normal

Ben Domensino, 21 June 2017

Despite signs that the Pacific Ocean had been warming towards an El Nino event in recent months, this has failed to eventuate and is now unlikely to take place during 2017.

A build up of warmer than usual water near the west coast of South America earlier this year hinted at the early stages of a developing El Nino. However, this warm water failed to spread towards the west and has cooled in the last few weeks. As a result, the potential for an El Nino event this year has past, according to the latest ENSO outlook issued by the Bureau of Meteorology yesterday.

With the Pacific Ocean expected to remain in a neutral phase during the rest of 2017, what does this mean for Australia's weather in the months ahead?

It's important to note that neutral conditions in the Pacific Ocean does not mean average weather in Australia. In the absence of El Nino and La Nina, other climate drivers will come into play during the next few months.

Let's take a look at some of them.

The Indian Ocean may influence rainfall in parts of Australia this winter and spring. Half of the international climate models surveyed by the Bureau of Meteorology indicate that a positive Indian Ocean Dipole (IOD) will develop this winter and last into early spring. A positive IOD can cause below average rainfall in southern and central Australia during this time of year. Even a weakly positive IOD can affect rainfall patterns in these parts of Australia.

The Southern Annular Mode (SAM) influences how far north cold fronts progress during winter. A negative SAM causes westerly winds and cold fronts to progress further north, which can lead to more winter rainfall and snow in southern Australia. Conversely, a positive SAM causes cold fronts to be displaced towards the south and can reduce winter rain/snow in southern states. Given the recent warm and dry weather in southern Australia, it's no surprise that the SAM has been predominantly positive in recent weeks. Unfortunately, the SAM cannot be reliably predicted more than a week or so ahead, so it is not useful as a tool for long-term forecasting.

Sea surface temperature anomalies immediately surrounding Australia can also influence rainfall distribution, as these waters provide moisture when rain-bearing systems pass across the country.

Australia's weather in all seasons is also being influenced by a background rise in global air and ocean temperatures.

Even though El Nino is unlikely this year, the remaining climate influences preserve the likelihood of below average rainfall and above average maximum temperatures across much of western, central and southern Australia during the rest of winter.

- Weatherzone

© Weatherzone 2017

Weather News - Western soaking targets Perth

Ben Domensino, 22 June 2017

Perth has just received its heaviest June rain in more than 30 years amid a wet end to the week in Western Australia.

A low pressure system located just to the south of Perth this morning is driving showers and blustery winds over the western districts of the state.

The heaviest falls during the last 24 hours have occurred in and around Perth, with totals exceeding 60mm in parts of the city.

Perth Metro had received 59mm as of 9am today, almost all of which feel during a 12 hour period from 7pm yesterday. This was the city's heaviest rain in four months and the heaviest for June since 1986.

Jandakot's 41mm as of 9am today is their heaviest June rain in six years, while 29mm at Mandurah is their highest total since February.

While the rain has brought some relief from recent dry conditions on the lower west coast, much lighter showers have occurred further north and east.

Geraldton, which only received a third of its seasonal average rainfall during autumn, picked up a paltry 9mm during the last 24 hours. East of Perth, rainfall totals during the last 24 hours dropped below 10mm as you move east of about Toodyay and less than 1mm around Cunderdin, which is 150km inland.

Showers will continue along the west coast today, although falls will ease in Perth as the low moves east. This will allow rain to spread further inland across the South West Land Division and some areas of the Great Southern District may see heavy falls in the next 24 hours. Unfortunately, much of the Central Wheat Belt will see little, if any rain during this time.

A separate band of rain will extend from the Gascoyne down through the Goldfields today and move into the South Interior and Eucla districts by this evening.

Looking ahead, showers will continue in western districts of Western Australia on Friday before clearing during the weekend, as a high pressure ridge becomes established over the state.

- Weatherzone

© Weatherzone 2017