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From extreme heat to severe storms, Australia has been through the weather wringer in recent weeks.
In
the past few days it has been hail; the same weather pattern triggering
storms in Melbourne, Canberra, Sydney and the Gold Coast, according to
Dr Joshua Soderholm, research scientist at the Bureau of Meteorology
(BOM). He said the broad surface trough and strong upper-level low system that moved through eastern Australia was a "quite remarkable, multi-day event" (which is meteorologist for lots of exclamation points).
What was the set-up?
Synoptically (that means on a national scale), Dr Soderholm said it was a fairly unusual set-up.The trough and low reached all the way from Victoria to Queensland and moved slowly across eastern Australia from Friday through until Tuesday.
The upper-level low brought instability to the atmosphere, cooling the mid and upper-level air temperatures.
"Then the surface trough was drawing in moisture from the Coral Sea right down into Victoria," he said.
This hot moist air, combined with the instability aloft, made the perfect conditions for thunderstorms and hail.
Dr Soderholm said the trough that moved through Canberra was particularly vigorous.
"Even early in the morning we had rain and some weak thunderstorms through the Wagga Wagga, Gundagai region."
There were multiple storm systems along the trough, and by the late morning some started to intensify.
"The Canberra cell in particular, it actually had developed along the trough line just east of Yass. This was only on the order of one hour before it impacted Canberra," Dr Soderholm said.
"So, we saw initially a shower and then a weak thunderstorm, then it was rapidly intensifying into this severe thunderstorm which eventually impacted Canberra."
The conditions were ripe for severe hailstorms and supercell thunderstorms — and they popped up all over the place.
"It was just unfortunate that one of them had to impact Canberra," Dr Soderholm said.
What makes a storm super?
A supercell thunderstorm is basically a thunderstorm on steroids.
Lots of cold air aloft creates instability in the atmosphere as well as strong winds.
This creates a special environment where storms can get more organised than they normally do, by generating a spinning updraft.
"This allows them to become more intense and persist for longer, which allows them in turn to generate larger hail," Dr Soderholm said.
"Every hailstone starts as a little ice crystal; we call it an embryo."
The embryo moves from the outer edges of the thunderstorm into the centre, where the strong updrafts and moisture being pulled from the surface allow it to grow.
"Supercells are special because they have very wide, strong and persistent updrafts, so you can imagine the hail growth conditions are ideal for hailstorms.
"This little ice crystal will grow from a few millimetres up to a few centimetres, five to six centimetres, in the order of 20 minutes or so within the supercell storm."
How much warning do we get?
Not long at all. Those hail embryos have a very short incubation."It's only in the order of tens of minutes that we can go from a marginal storm to a very high-end storm which is producing giant hailstones," Dr Soderholm said.
He said the BOM watched these thunderstorm cells and issued its warnings up to an hour ahead.
On days when storms are looking likely, there will be a watch out over a large area around 24 hours in advance.
"But if we have a thunderstorm which rapidly intensifies and starts producing giant hailstones only 20 minutes before it impacts the city, then that's when you'll get the cell-based warnings."
In the case of the Canberra hail on Monday, Dr Soderholm said there were cell-based warnings out for several cells along that line in the hours before it approached the capital.
"That specific cell, they attached the 'very dangerous' tag to it approximately 20 minutes before it hit Canberra, and that was just as it really got going and started to produce those giant hailstones."
Is this normal for Canberra?
South-east Queensland and northern New South Wales are Australia's giant hail hot-spots, but southern areas do get pelted from time to time.Dr Soderholm said south-east Queensland and northern NSW had one to two severe hailstorms per region every year; in Canberra it is more like once every five years (approximately).
But he did say that Monday's storm in Canberra was significantly more than just a run-of-the-mill storm.
"It was producing giant hail, so we would call that significantly severe."
It is difficult to say how often these significantly severe storms occur in any given region because they are so rare and records are relatively short.
But there may be more opportunities for observation in future as climate change is expected to make thunderstorms more common.
Dr Soderholm, however, said what it would do to hail was less certain.
"The general consensus is that the environments are going to become more unstable, so we're going to be seeing more deep convective thunderstorms developing across, particularly, through south-eastern Australia," he said.
"However, the link to hail is less certain because if you think about a warm atmosphere, hail is also going to be melting as well in a warm atmosphere to a greater extent than what it would now."
What now?
Believe it or not, we are back to talking about fire weather. What a difference a few days makes.The trough which brought all the rain and hail has finally moved off and a new front is making its way across the country.
The front is dragging down warm air from the interior and bringing blustery conditions as the front moves east, upping the fire danger.
The recent rain has at least helped to dampen things down a little, but the fires are far from out.
Meanwhile, Brisbane and much of the east coast is forecast to stew under muggy heatwave conditions for the rest of the week.
Beautiful one day, pummelled the following, and sweaty the next.
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