What caused hail to blanket Grafton during winter storm?

Australia Broadcast Center

15 hours ago

A hailstorm that blanketed a northern New South Wales city in ice was an unusual occurrence for winter, according to the Bureau of Meteorology (BOM).
Grafton looked more like it had been dusted in snow than hail after the storm hit late on Wednesday and left a swathe of damage.
BOM senior meteorologist Tristan Sumarna said while it was unusual to see the amount of small hail that fell, cold-season thunderstorms did occur.
"Large hailstones are most common during the later spring months when the weather has warmed, but you still get that cold air moving across to the North Coast," he said.
"But cool-season storms producing hail is always going to be a risk in winter."
Mr Sumarna said winter hailstorms were caused by upper cold air from the Antarctic moving over an area like northern NSW that had relatively warmer air.
"When you have cold air above warm air, the potential for thunderstorms increases quite a lot," he said.
Mr Sumarna said this caused updrafts, and the stronger and more constant the updraft, the greater the chance of hail.
"Any moisture that is carried within that air parcel gets lifted, it freezes into ice particles," he said.
"Then it circles like a washing machine … and when it does that, it collides into water that is colder than 0 degrees [Celsius], instantly freezing like a shell.
"The more it does that, then you get hail that falls when the droplet gets too heavy."
Stronger updrafts with a colder, drier air mass will keep the ice particles aloft for longer, creating bigger hail.
The hailstorm left vehicles, including five police cars, pocked with hailstones.
The Grafton library had to be evacuated after multiple holes were reported in the building's roof.
One woman told the ABC that her daughter said "it seemed like it was raining inside" when the storm hit, and library staff worked to save the books from water damage.
Clarence Valley Mayor Ray Smith said the library would be out of action for weeks while the extent of the damage was assessed.
"Because of damage to the roof, the hail and rain got in and it's damaged a significant amount of our book stock … and electrical works," he said.
Cr Smith said there had also been reports of damage to private property.
Grafton resident Stuart Murphy saw the dramatic storm approaching and captured the scene with his drone.
"It felt like a summer storm in winter. It was like a mixing of the seasons," Mr Murphy said.
"It wasn't the largest hail I've ever seen, but it was certainly the greatest volume of hail I've ever seen."
Mr Sumarna said a warming atmosphere would lead to more severe thunderstorms.
The Grafton hail dump came less than a fortnight after snow fell within a couple of hours' drive of the city.
Mr Sumarna said while the Northern Tablelands regularly got snow flurries, the amount of snow that fell in places like Guyra and Armidale was highly unusual.
"It was a once-in-a-generation event," he said.
He said while storms that formed hail needed a cold air mass to move over a warm air mass, to form snow on the Northern Tablelands, it was almost the reverse.
"You need a large conveyor belt of warm tropical air over that drier, colder surface to get snow," Mr Sumarna said.
The cold air mass that caused the storms over NSW, from the Hunter region north to the border, is expected to move east on Friday.