1News weather expert Daniel Corbett explains what's been happening with recent weather events - and why we can now look forward to a calmer spell.
In the last few days, the weather has been wild.
It has also been changeable. Every moment you glance down at your weather app it seems like it has changed - and sometimes quite dramatically.
We had only just got over the impact of ex-Tropical Cyclone Vaianu, which brought a tropical deluge and severe winds in some areas, when a new set of storms rolled in, with Wellington taking a battering on Monday morning.
So, what's behind all of this extreme weather?
Watch Dan's explanation of what's been going on on TVNZ+
Pattern shift
In the days after Vaianu our weather underwent a big shift in the pattern.
We went from a humid tropical flow to an unstable and erratic series of heavy thundery downpours that kept pummelling the country.
The shift to the crazy conditions took place just before this past weekend as a surge of colder air came from well down in the Southern Ocean.
This was not just at the earth’s surface but higher up in the atmosphere too. This makes a difference because colder air higher up in the atmosphere above warmer air makes things more unstable.
In other words, it gives more lift in the lower atmosphere which makes the clouds rise more readily to grow storms.
Remember, when cold meets warm that creates a difference or gradient which is one of the main building blocks to create weather and storms.
Surge of cold
In this case the surge of cold was quite intense. It came off a lobe from the recently developed Polar Vortex over Antarctica.
There was also a lot of the residual warmth in the atmosphere from the summer loitering just north of New Zealand.
The gradient between the cold and warm helped create the active front that hit the South Island Last Friday. It brought heavy rain and stronger winds.
What happened after that played a big part in the erratic nature of the weather since then.
As the surge of cold air in the atmosphere moved in the south Tasman Sea it became surrounded by the warmer air elsewhere.
A strong surging jet stream helped carry the front and a cold pool of air further north. The forward momentum and the surrounding warm air encircled the cold pool, which then cut off from the main jet stream flow.
Big spinning lump
This in turn became a big spinning lump of cold unstable air just west of the country. The cold air in the middle made the atmosphere very unstable and erratic, creating heavy showers and thunder.
The cut off low then sat for the next few days spinning lumps of cloud, heavy rain and thundery downpours around its edge in the upper jet's stream flow.
These types of weather pattern are not only very erratic but hard to forecast with just computer models. All the lumps of cloud and rain spin at different speeds based on the pieces of upper energy driving them.
They can also change from hour to hour and day to day.
Many in the Wellington region - Hutt Valley and Porirua especially - will remember the torrential downpours that occurred in the early hours of Saturday.
These were driven by the upper energy in the offshore cut-off low, but what complicated things was a feed of extra juicy air from the sub tropics, from off the Queensland Coast. That fed into these storms, as well as the upper energy combining to make them more intense.
This happened especially given the area of good convergence (winds coming together) ahead of the front.
The computer models at the time were not keeping up with the actual atmosphere. They also still struggled with the timing of the driving features of the heavy thundery downpours from the upper energy.
Erratic weather on the way out
Many years ago, when I was a tornado detection specialist and TV weatherman in Tornado Alley in the States we would rely less on the computer models and more on the observational data to help drive our short-term forecasts.
Computer models these days have improved greatly but still have weaknesses.
The good news through all of this is, after several days of spinning across the Tasman Sea and New Zealand, bringing storms and torrential downpours - the wild and erratic rainmaker is on the way out.
Finally, some more settled weather will arrive as we head in the later parts of this week and the long Anzac weekend.
Time to give the raincoat break - or to dry out at least!
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