1News weather expert Daniel Corbett explains why the weather that hit NZ packed such a punch.
The intense storm that battered the capital and the South Island this week has certainly left a trail of destruction and disruption across a wide swathe of the country.
In a simple sense, a very deep low – for the time of year – passed just south of the country with an active front that extended further. There was also a big high parked northeast of the country, so a very large pressure gradient developed between the two features.
Over the distance of around 2800km the pressure changed from 1024hpa at the high centre to around 950hpa at the area of low pressure.
The tight pressure gradient at the surface is depicted on weather maps as the zone with plenty of lines very close to each other. The laws of physics say that a set amount of air must exist between each line, or isobar. If they are closer together then parcels of air must speed up to make room for other parcels.
This explains some of it, but not all of it. Since the atmosphere is all interconnected, what happens higher up can affect things lower down and vice-versa.
The big change that has put our weather into warp-drive over the past week is the extreme contrast of air masses around New Zealand.
Late last week a significant surge of warmth high up in the atmosphere spread down from the Indian Ocean into interior Australia. This helped build the big mountain of warm air (high pressure) over the continent. Temperatures climbed into the 40s.
This mass of very warm air then spread into the Tasman Sea and towards New Zealand earlier this week.
At the same time, the big mass of cold air that resides over Antarctica during the winter, the Polar Vortex, began to destabilise and fade away as longer days and warm air interacted with it.
This weakening caused the big solid mass of cold to become wobbly and send surges of cold north. One big lump spread into the Southern Ocean.
The interaction of cold air mass and very warm air mass created a significant gradient over the Southern Ocean and Tasman Sea. Warmer seas currently, and long-term planetary warming, adding some fuel to the mix too.
The end result has been supercharged weather systems this week – well beyond the typical spring gales.
The long weekend will offer a brief reprieve of dryer weather before the next warm-cold interactions give rise to a wet and windy low for the drive home on Monday.
‘Weather turns somewhat to custard’ – Watch Dan’s long weekend forecast on TVNZ+
Enjoy the sun while it lasts, though the good news is there’s more settled and warmer weather on the cards for the end of October and start of November.
