Alexandra

Weather Brief 24 January: Turning Dry Again for the North Island

After heavy rain brought some relief to parched areas of the North Island during the weekend, going forward it appears that high pressure could cause another period with little if any precipitation for the same areas.

A front moving north will deliver heavy rain to the West Coast from tonight into early Wednesday morning, but the westerly wind flow will leave only isolated showers for regions such as Otago and Canterbury. In the North Island, scattered showers will reach southern and western areas on Wednesday, but rainfall in these places will generally be light. Isolated showers may also impact Auckland, Hawke’s Bay and Gisborne later in the day, but rainfall amounts in these places will be minimal. Also, with winds behind the front shifting to the southwest temperatures will once again be cooler than average.

Showers Will Move North Across the Country on Wednesday


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Thursday will feature high pressure delivering pleasant conditions to the North Island along with the northern and eastern South Island. However, a front draping across southern New Zealand will bring periods of rain to the West Coast and Southland. In fact it will likely be a fairly wet day for Invercargill, while scattered showers will move as far north as Dunedin.

Friday could see a few light showers in the South Island during the morning before a large area of high pressure in the Tasman Sea causes clearing by the afternoon. The dry weather should also be found throughout the day in the North Island. This is good news for those planning to go out on Friday evening, as dry albeit cool weather is expected across the country.

High Pressure Will Be the Dominant Pattern for the North Island in the Near Future


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Looking into the longer-term forecast, there are indications that high pressure will be a dominant feature around the North Island heading into early February. While this will mean generally tranquil weather that will be compatible with outdoor plans, it also means that rainfall will likely be sparse in the coming weeks. Unfortunately this is bad news for dry areas of the North Island, including Auckland, Northland and the eastern coast. It is likely that soil moisture levels will continue to fall in the near future, which is problematic for farmers and also increases the risk of brush fires.

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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