2017 SNOW SEASON OUTLOOK
ISSUED: MONDAY MAY 29 2017
My summary of what could influence this years snow season, discussing the main drivers of our weather systems, and how much or little of these variables we may see this year.
Every weather system that pushes across the alps and brings us snow has three main variables: moisture, energy and temperature. Let's see how these look this year.
Moisture is the part we can look at month's ahead.
The Pacific Ocean
After lots of talk of an impending El Nino, the most reasonable forecast for winter is a continuation of Neutral conditions. This would mean that, on a large scale, precipitation is neither pushed away from Australia, or towards. Instead, local moisture sources are the main player.
In an El Nino, warm water out there (east of the date line, at the equator) would be offset by cool water off Queensland - but we still have a large area of warm water near Australia.
We also have the continuation of very warm water in the Tasman Sea from Tasmania to New South Wales.
The Indian Ocean
Last year brought Australia's second wettest winter and spring on record, largely thanks to a Negative Indian Ocean Dipole (IOD).
This year will either be Neutral or a Positive IOD.
The waters from Broome to southern India are generally cooler than average, and the water off Kenya are warmer than average. If this persists, and the atmosphere follows suit, then precipitation will be pushed away from Australia.
This means less feeds of moist air coming down from the northwest.
But, a thin line of warm water persists off Indonesia, and that cool water off Broome isn't supported at depth, so I think the reasonable forecast is for a Neutral IOD.
Its also worth noting the cold temperatures to Australia's southwest - and the large, persistent, warm area off Madagascar - this could help fuel our weather systems as they cross the Indian Ocean.
Energy is how these weather systems behave when they get here.
This is the part that is very difficult to predict month's ahead. A negative Southern Annular Mode (SAM) helps us out, but we don't have good guidance on how this will behave in the future. Instead, we will have to see how each weather system behaves.
Look for the main high pressure system to be centred well back to the west (west of Western Australia is ideal). If its too close to the alps then the weather system loses energy.
Look for not one front but several lining up (with that high out of the way).
Look for a cut-off low pressure centre, the pièce de résistance. Cut-off - meaning separated from the westerly airflow to our south. This lets the system slow down, and deliver more as it moves through. And, if we can get a low to cross Bass Strait or travel just south of Gippsland/near the New South Wales south coast - then we are laughing (if the temperature element is working too :)).
Temperature is usually what will have us most anxious. Think about how much it can vary in Melbourne in just one day, yet the difference between snow and rain is just one degree.
We consider mainly variables in the atmosphere to determine if a particular system will be rain or snow, or what time that changeover will be.
Seasonal outlooks predict the likelihood of above or below average temperatures, but this is an average overall, and will be offset by any individual weather system.
The coldest air arrives in the alps when the high is well back to the west and a cold pool comes up from the Southern Ocean. But, a system that looks 'tropical' can still deliver (and deliver a lot), if it has the right feed of colder air at a higher altitude.
The seasonal forecast is for overall, average to above average temperatures.
We won't have as much moisture coming down from the northwest this year - but the tap won't be completely turned off. We should still see a nice feed from time to time - and I'd keep an eye on what that warmth off Indonesia does.
The Tasman Sea warmth could be a good source, in the right set up.
And I have a good feeling about the warmth off Madagascar.
Then its just a question of how each weather system will move through - and that's the big question, as it is each year ;)