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Weather Brief 27 April: A Wet Weekend for Many, Then Turning Cool

The recent dry weather that New Zealand has enjoyed is about to break with the arrival of a storm this weekend. Behind this storm temperatures will turn cooler as a southwesterly flow develops.

After the incredibly heavy rain endured by much of New Zealand in the first half of April thanks to the Tropical Torrent and Cyclone Cook, the second half of the month has been generally dry as high pressure took control of the region’s weather. Unfortunately wet conditions are set to return this weekend as a storm will move across the North Island. In northern New Zealand, the best bet for dry weather will be Saturday morning, but by afternoon and evening conditions will be fairly soggy. Sunday will remain showery as well, so any outdoor plans you have may be best rescheduled for another time. The weekend rain could give the North Island a good soaking, with amounts of 20-40 mm likely in many locations.

Percentage of Dry Hours Expected This Weekend


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Much of the South Island will avoid the wet weather on Saturday, with eastern areas even enjoying periods of sunshine. However, showers will become more likely in the South Island on Sunday, so you may want to keep an umbrella handy.

Southerly Winds Will Bring Cooler Temperatures Next Week


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Once the weekend storm passes winds will shift to the south, ushering in noticeably cooler air to start next week. High temperatures on Monday will be several degrees below average across New Zealand, with the overnights being chilly as well. You’ll certainly want to bring a coat with you if you’ll be out in the evenings or during your morning commute. While temperatures may rebound a bit in the middle of next week, a reinforcing shot of cold air will arrive behind a front on Thursday.

Two Day Rainfall Map


This map shows the extent and intensity of rain that’s expected to affect New Zealand during the next two days.

Click the map to view the animated forecast.

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NIWAWeather keys


The colour gradient you see behind your town or city forecast depicts the amount of cloud cover expected during daylight hours - ranging from clear blue to dark grey (overcast). If rain or snow is expected, raindrop or snowflake symbols will also appear in the background. The further down the screen the symbols appear, the heavier we expect the rain or snow to be. Read more...

About NIWA


NIWA is the National Institute of Water and Atmospheric Research. We’re New Zealand’s leading supplier of atmospheric, freshwater, environmental and marine science services.

We collect, store and manage data on New Zealand’s weather, climate, marine and freshwater resources and other important environmental parameters. We deliver that data via a range of tools and services that help Kiwis manage their interactions with the environment and its resources more productively and sustainably.

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Meet FitzRoy


He’s the 18-tonne brain behind NIWAWeather’s forecasts. His real name is IBM p575 POWER6, but he prefers FitzRoy – or just ‘supercomputer’ if he’s in one of his moods.

He’s a fussy individual. He occupies a specially constructed, climate-controlled (appropriately!) room at NIWA’s site in Wellington. But we don’t mind, because he works tirelessly, 24/7, with the computational effort of about 7000 laptops. And his outputs are vitally important to all New Zealanders.

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Weather Q&A


What causes heavy rain, versus light rain?

The amount of water vapour present in a cloud, and the strength of the rising air ('updraft') in that cloud, determines the intensity of rain it can produce. The depth, or thickness, of cloud determines how much water is available.

The heaviest rain is usually associated with very deep clouds such as thunderstorm cells, which may extend from about one kilometre above the Earth’s surface to over 10 kilometres high. These clouds also contain very strong updrafts, which means large drops of rain can be held up in the cloud until they grow so big that the rising air can no longer support them.

Prolonged heavy rain can also fall from very thick, slow-moving stratiform clouds known as nimbostratus.

In New Zealand, the heaviest rain usually falls on the western side of the mountain ranges, where moisture-laden winds are forced upwards by the mountains, creating very deep clouds.


How does rain come out of clouds?

For rain to fall, the water vapour that makes up clouds needs to combine and grow into droplets heavy enough to counteract rising air (‘updrafts’) in the cloud. But the process relies on more than water alone. Tiny specks of dust and other microscopic substances, known as ‘condensation nuclei’ are required. Condensation nuclei are everywhere in the atmosphere. Water vapour wraps itself around them, which begins the process of raindrop growth. As the drop gets bigger, it collides with more and more droplets and growth accelerates until the drop is heavy enough to fall.



How is rain measured?

Forecasters use a plastic or metal open-topped cylinder called a rain gauge to capture rain. Rain gauges have standard dimensions so that measurements can be compared from place to place. The amount of rain collected is measured over a certain timeframe – usually hourly or accumulated over 24 hours. Measurements can be made manually or electronically. In New Zealand we measure rain in millimetres. Ten millimetres of rain in Wellington means that if all the rain that fell within the city was spread out over hard flat ground (so it couldn’t drain away), it would form a puddle 10 millimetres deep.


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