The summer outlook has been issued by the Weather Bureau – and it looks not too wet or dry, and not too hot or cold. The forecast is 50/50.
Let’s start by noting that our wet weather, in terms of what we see day to day, is controlled by how much moisture is able to interact with low pressure. One way to see what the season ahead may be like is to see how Australia’s tropical oceans are doing. If our waters are warmer than usual, then that would provide extra moisture. If we can get a higher amount of moisture to feed down to Victoria then that should produce more rain.
That is one part of the equation – the other is whether it can interact with low pressure – and that is not as easy to tell. So, our seasonal forecasts are based on what is happening off Queensland and northern WA. Whether we are in a positive or negative Indian Ocean Dipole (IOD) and an El Nino or La Nina.
The Pacific Ocean has been in a neutral state, and is forecast to stay that way into the middle of next year. This means that it is not warmer or cooler than normal, off the Queensland coast. There were signs in October that it would head back to El Nino next Autumn, but that is no longer the case.
The Indian Ocean is exactly the same: neutral. It is not warmer or cooler than normal, off the northern WA coast.
So, both of these are not giving us much.
“Than normal” refers to a broadscale balance between Australia, South America and Africa. So, there are no broadscale shifts across the Oceans. It is important to note though, that the waters around Australia are currently warmer than the long term average.
The odds of a wetter than average summer across Victoria are roughly 50/50. In every 10 years like this, half would be expected to be wetter, half drier. The far east is only a 40 to 45 % chance (drier), whereas areas near the southwest coast are closer to 60 % (wetter). This means, we have no strong forcing – but – if you can get a channel of tropical moisture lined up, and low pressure moving through at that time, then yes, we will see significant rain. If they don’t line up for a while, it will become very dry over Victoria – especially considering the dry Spring across the north of the state.
In terms of temperature, it is the same guidance – both maximum and minimum temperatures have 50/50 odds. So, there is no strong forcing with temperature either. It will all come down to how the high and low pressure systems move through. If we have a high sit to our east for a period of time, then we will have a heatwave. If lows move through on regular intervals then that will bring cool spells.
So, there is no strong forcing from our oceans, and the official guidance is 50/50. Watch the weather maps, and keep up with my forecasts, and I will let you know when heatwaves, cool spells, and significant rain are lining up to affect Victoria.
There is no “chance” or “possible” anymore – this is developing and will affect Victoria tomorrow.
Tropical moisture left over from Cyclone Alessia is combining with a trough, and huge upper level energy, and this will sweep a band of rain and thunderstorms across Victoria.
The north and east of Victoria will receive the heaviest falls – with more than 20 mm likely. Many areas have the risk of recording more than 50 mm of rain in less than 24 hours.
Here is the weighted average of all the models this morning:
Here is the latest ACCESS model:
And, of course check www.yr.no (EC model) so you can zoom into your town. I was asked about Serpentine earlier, so here is what it says for them this morning:
The weather this week will involve lots of dramatic change.
We will see dry heat ahead of a weak cool change, before widespread rain with heavy falls. It will then sharply turn much colder, with wintry showers and alpine snow.
We have heat and sunshine today. By 9:30am it was already over 30C in the northwest. Melbourne is heading for a top of 36C. A weak milder change will slowly drift across western Victoria this afternoon and tonight. Luckily the winds are not strong, or the fire danger would be much worse. Instead it is a Total Fire Ban in the Mallee:
The weak change washes out overnight, so we are warm to hot again tomorrow.
Rain is on the way – all weather models have it, so confidence is quite high. It will pass through on Tuesday night and Wednesday, cooling things down. Here is the latest outlook from BOM: http://www.bom.gov.au/jsp/watl/rainfall/pme.jsp
You can also get a good feel for how much you will see and when using the EC model: http://www.yr.no/place/Australia/Victoria/.
It will then turn much, much colder for Thursday. We should have widespread showers, wintry with thunder and icy small pellets of hail. Snow is likely in alpine areas. The temperature change will be huge – Monday compared with Thursday:
It will then all ease on Friday as the next high comes in.
To get rain we need both moisture and instability – and there is a system brewing next week. If you are harvesting, I would keep a close eye on what this weather system does.
It has a feed of tropical moisture, running into low pressure, and there may be widespread rain across Victoria on Wednesday and Thursday next week.
Now, the models were hinting that yesterday’s rain system would be huge, 8 to 10 days ago – and it turned into most areas receiving less than 5mm. But this one still has big falls, and we are getting into the five day out period where confidence increases.
The image above is the weighted average of all the weather models, from this morning’s run. It updates each morning at BOM/WATL.
You can keep an eye on developments for your area at www.yr.no. Select English (its my favourite weather model, the EC, on a Norwegian website) and enter your town. It will update each day at 6am and 6pm.
Here is what it is currently saying for Horsham:
I thought the windscreen wipers weren’t going to last…