It's almost spring. Don't be surprised when the weather isn't good
Wednesday, 29 August 2018
It's a couple of days until spring, when people rejoice the end of the winter months but sometimes forget it's not quite summer yet.
Niwa meteorologist Ben Noll said: 'People just want to jump right to summer. The amount of calls we get about summer at this time of year is incredible. They just turn the spring outlook into the summer outlook, they're all excited'.
Spring starts on Saturday, and winter's seeing itself off this week with wet, rainy weather for much of the country after a few days of settled weather.
Spring was generally a season of 'ups and downs' and 'topsy turvy weather', Noll said.
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'You can have winter-like patterns in spring, you can have summer-like patterns in spring. It's a shoulder season so you kind of get this interesting blend,' Noll said.
'Put all the weather you experience in a year in a blender and that's spring. It's a transition. It can be an exciting time of the year.'
Niwa has said that this spring is expected to be … well, pretty much like spring. That means the usual, typical storms.
Niwa's climate outlook forecasts near normal rainfall totals for the country, with temperatures near average in the North Island, but above average in the South Island.
From August to October soil moisture levels and river flows are forecast to be near normal in the north and east of the North Island. However, in the east of the South Island, Niwa forecast below normal soil moisture levels and river flows.
'There will inevitably be wet and dry days, and hot and cold days, within a season. The exact range in temperature and rainfall within each of the three categories varies with location and season,' it said.
However, it could be the calm before the storm as the probability for El Nino conditions increases as we approach summer in the Southern Hemisphere - Niwa says there is a 78 per cent chance for El Nino conditions from February to April 2019.
The strength and characteristics of this event - if it eventuates - remain uncertain at this point, but indications so far are that it is unlikely to be in the strong category.
Spring 2017 was the second-equal warmest spring on record for New Zealand, Noll said.
Storms caused chaos including daytime deluges, severe thunderstorms, flashing flooding and landslides.
TRICKY TO FORECAST
High and low pressure systems change as the year progresses to summer.
'You have a gradual shortening of the wavelengths of the troughs and ridge patterns … that basically means things can start happening quicker, things can move on the weather map a little bit faster and that can lead to some exciting weather patterns.'
It made spring a challenging season to forecast weather for. 'It's very challenging to predict convective rainfall - that's rainfall that deals with thunderstorms and more nebulous weather features you can get during the summertime.'
On the other hand, winter was characterised by strong high and low pressure systems.
'That's easier for the weather models to grasp onto. In spring, you get a mix of those two things so gradually, the predictability has a decrease - it's not by anything increase, but it has a subtle decrease.'