Thunder rarely happens when it’s snowing, but it did during this week’s so-called polar blast in a phenomenon known as thundersnow.
It was unusual to get thunderstorms and snow at the same time, but on Wednesday about a dozen lightning strikes were recorded in parts of Southland, Otago and Canterbury, where it had been snowing, MetService duty forecaster Heath Gullery said.
Thundersnow is uncommon for the South Island, and “extremely rare” for the North Island, where snow rarely falls to sea level.
The US National Oceanic and Atmospheric Administration (NOAA) describes thundersnow as a rare weather phenomenon, in which thunder and lightning were accompanied by snow, rather than rain.
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”The ingredients that create thundersnow are similar to the dynamics of a typical thunderstorm — moisture, instability, and a lifting mechanism.”
Niwa Weather meteorologist Ben Noll said the weather processes of the past few days had been driven by the difference between the temperature of the ocean, and the temperature of the atmosphere some way off the surface.
”It’s all kind of tied together. All these processes are being driven by the fact we had an extremely cold air mass emanating from Antarctica.
”For New Zealand, we’ve had warmer seas than average for sometime now.
”The difference between the air mass above and seas below is what’s driving things like the ocean effect, as well as the thundersnow,” Noll said.
The ocean effect involved the cold air moving over relatively warmer waters, resulting in cellular-type snow showers.
Thundersnow events were similar to thunderstorms, except the cloud tops were not usually as high.
The prospect of thundersnow usually came with a high accumulation of snow. “We did see that in parts of Southland, in Dunedin in the hills, and to a lesser degree in the eastern hills of Christchurch.”
Snow, hail and graupel – a kind of grainy snow – had fallen during the thundersnow events.
Thundersnow was not foreign to New Zealand. It would happen during winter in the Southern Alps, but no one would be around to hear it, Noll said.
“I think the time of the year makes this event more unusual.”
NOAA said air rarely became unstable when it was snowing because the air temperature in winter snowstorms was consistently cold, at both high and lower points in the atmosphere.
Air was considered unstable if it continued to rise on its own after getting a nudge from a cold front or warm front.
“In some winter storms, shallow layers of warm air are lifted and continue to rise on their own – increasing snowfall and causing enough electric charge separation for lightning to occur,” NOAA said.
“Thundersnow can be found where there is relatively strong instability and abundant moisture above the surface, such as above a warm front.”
The UK Met Office said lightning appeared brighter during a thundersnow event, because the light reflected off the snowflakes.
But the sound of the thunder was dampened by the snow within the thunderstorm. While thunder from a typical thunderstorm might be heard many kilometres away, the thunder during a thundersnow event would only be heard within about 5km.
Niwa’s Noll said the event of the past couple of days had ticked just one of the two boxes needed for heavy snowfall.
”This time we had a southerly airflow. There was some moisture, but air parcels coming from the ice sheet are colder and drier.
”If we had, say, the same air mass from the ice sheet, but interacting with moisture from the sub-tropics, that would be a recipe for a lot of snow,” Noll said.
MetService meteorologist Amy Rossiter said no more thundersnow events had been recorded overnight, or on Thursday morning.
There was some lightning over Wellington about 11pm to midnight, and there were reports of some snow flurries at the same time, but that wasn’t considered thundersnow.
There had also been a considerable amount of thunderstorm activity well offshore, to the east.
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