BOM gives a positive rain outlook, but first there’s a wintry blast on the way


Australia

It is raining over large parts of the south-east, there is finally a good rainfall outlook after years of drought, and there is a flood of water heading down the Darling River.

So good news generally, but batten down the hatches if you are in the south-east this weekend.

Key points:

  • Cold, wet conditions are expected to continue over the south-east, bringing welcome follow-up rain for inland areas
  • The Darling River is flowing thanks to heavy rain earlier in the year and is expected to reach the Murray by the end of the month
  • BOM’s three-month outlook suggests most of the country has an above 80 per cent chance of above-average rainfall

Rain is still falling after days of widespread falls and the Bureau of Meteorology (BOM) has issued several severe weather warnings.

Blair Trewin, senior climatologist for the BOM, said the days of widespread rain were the result of a trough that initially formed over eastern Australia and developed into a weak low over Victoria.

The trough dragged moist air from the Indian Ocean, which is abnormally warm and therefore providing ample moisture at the moment.

External Link:

BOM severe weather update.

“It brought rain initially over outback South Australia, and over the last day or so quite widely through south-east Australia, particularly inland New South Wales, much of Victoria and eastern Tasmania,” Dr Trewin said.

The low itself is expected to move to the south-east, leading to a south-westerly-to-southerly air stream over south-east Australia with several fronts on the way.

“The south-southerlies and south-westerlies will bring continued showers and quite cold weather, particularly to Victoria and Tasmania over the weekend,” Dr Trewin said.

There shouldn’t be any problems with overcrowding at south-eastern beaches this weekend, with temperatures well below normal.

External Link:

Packsaddle

“For example, the forecast for Melbourne this weekend is 16C for Saturday and 16C for Sunday; that’s at the bottom end of the range you would expect at this time of year,” he said.

“With that, we will probably see some snow on the higher parts of the mountains in Tasmania, Victoria and southern New South Wales.”

Snow at this time of year is unusual but not unheard of.

Wet start to the year for some

The rain is a welcome follow-up to what has been a better start to the year than we have been used to of late.

Eastern New South Wales was particularly wet, with the first quarter in the top 10 wettest years.

According to Dr Trewin, most of the rest of the country has been average to above average without being too extreme.

External Link:

Monthly rainfall decile

“Really, the only significant areas which have been below average have been parts of the northern tropics, particularly the Northern Territory, Top End and parts of Cape York Peninsula.

“Darwin, for example, looks on track to have its second successive wet season with rainfall well below average.”

The cropping regions of South Australia have also been relatively dry to date, but that is the norm for this time of year and still better than this time last year.

Outback South Australia is tracking above average this year to date, thanks to a few systems getting through, including the one at the moment.

Oodnadatta got more rain in two hours on Thursday morning than it did for the whole of last year.

The best flows for years
External Link:

Menindee Lakes

The Darling River is flowing, and Tony Webber from WaterNSW is excited.

“It’s a wonderful good-news story that’s been a long time coming,” he said.

“What we hear from communities is that there’s just a degree of jubilation, not just at Menindee, but all those communities upstream that have had their town water supply replenished, their town weir is full again.

“The recreational opportunities and the cultural significance of the river has been replenished with this flow.”

It is relief for communities which have experienced arguably the longest and most severe of the drought conditions in the state, according to Mr Webber.

External Link:

Tolarno Station flow

The upper Darling is flowing thanks to repeated heavy rain events in the Namoi and Gwydir valleys this year, and more flows coming from southern Queensland, especially down the Calgoa.

“So that has generated the best flows for a long time in some years and resulted in something like 200 gigalitres arriving in Menindee Lakes already, with anything up to another 190 gigalitres still to come.”

The flow was initially held in Menindee, but since Thursday last week water has been released to flow down the Lower Darling.


Infographic:
Inflows in the Upper Darling are making their way towards Broken Hill.
(Supplied: Murray-Darling Basin Authority)

The first release was at high volume in an attempt to mitigate any adverse impacts of the lower-quality water typically seen in a first flush.

“So once we feel as though we’re confident that lower-quality water has been pushed through, we’ll reduce those flows back to something of the order of 300 megalitres a day, about a tenth of what they currently are.

“But even on conservative estimates, our modelling indicates that’s 12 to 18 months of constant flow in the Lower Darling for the first time in years.”

But there will still be some water left in the lakes.

Mr Webber said even if there was not another flow event, even with the releases into the Lower Darling, it was likely the lakes would go from almost functionally dry to well beyond 20-per-cent full in a matter of weeks.

He said it was reasonable to say the water would meet the Murray at Wentworth towards the end of April, if not before.

The drought isn’t over

This is all certainly good news, but it doesn’t mean we are out of the woods with the drought.

“There has been widespread, welcome rain across most of the drought-hit areas and follow-up rains are forecast in the next 48 to 72 hours, but the corresponding benefit to the large supply dams hasn’t been quite as good,” Mr Webber said.

External Link:

Karoola Station Flow

With the exception of Keepit Dam on the Namoi River, which has shown significant increases in the past few months, but the rest of the inland is not looking so good.

“Inflows into the Macquarie had been well below the previous worst case ever experienced,” Mr Webber said.

“We are hoping that rainfall in the coming days, the next 48 hours, will bring some inflows for those central west dams, so Burrendong Dam on the Macquarie, Wyangala Dam on the Lachlan, but they remain quite low.

“If there’s going to be any prospect in those hardest-hit areas of irrigators getting better access to water allocations, we certainly need a lot more rainfall in those catchment areas that generate significant inflows into those dams.”

Wonderfully blue outlook

Hope of those significant flows are looking like significantly less of a pipedream given the BOM’s latest outlook, which is blue. Lots of it. More than there has been in years.


Infographic:
This outlook doesn’t guarantee rain but it is good to see the odds tipping towards wetter conditions.
(Supplied: BOM)

Dr Trewin said there was a high chance of above-average rainfall over the majority of the country.

“Getting up to 70 to 80 per cent chance of above-average rainfall for the three months for a lot of the Murray-Darling Basin and the agricultural areas of southern Victoria, South Australia and also south-west Western Australia.”

It’s thanks to that warm Indian Ocean again, which is pretty favourable for north-west cloud bands, which are a big influence on rainfall at this time of year, according to Dr Trewin.

External Link:

Rain in Parkes

For the real weather buffs, it is too early in the year to be seeing an influence from a potential negative IOD, so at the moment we will just have to be happy with the above-average Indian Ocean temperatures and keep our fingers crossed for favourable drivers in the months ahead.

“The only parts of the country where the outlook is leaning dry are parts of the east coast and some parts of the northern tropics, which are just starting to run into their dry season anyway,” Dr Trewin said.

The outlook doesn’t guarantee good rain, but there is definitely reason to be hopeful.

Source: https://www.abc.net.au/news