Wednesday, April 24, 2024

Rain and the angle it comes in from

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Unlike many other nations, when NZ starts to have drought we need to worry about two things, says Phil Duncan.
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By Philip Duncan, WeatherWatch

In New Zealand a lot of our weather is all about the “angle” it comes in from. Our location on Earth means we’re highly exposed to lows and cold fronts from the Southern Ocean, Tasman Sea and sometimes the tropics. 

Despite being in such a prime location, we can get very dry. Did you know that Hobart in Tasmania is Australia’s second driest capital (626mm per annum)?

Despite Tasmania being far more exposed to Southern Ocean storms, the city of Hobart (along with much of eastern Tasmania) is much like Canterbury. Mountains to the west, the sea to the east – and that big westerly flow keeps eastern areas dry. 

The reason I’m bringing this up is because NZ’s mountains and ranges play the biggest role in our weather. Unlike flatter islands, like Great Britain, where the rain bands move mostly right across from west to east, our ranges block and enhance rainfall, block and enhance cloud and often boost our winds. 

This also explains why central parts of NZ are so dry – and northern NZ. The Southern Alps are the main reason places like Marlborough and Wairarapa are so dry, because in an El Niño period (and especially in autumn and spring) our nation is dominated often by west to southwest airflows.

This angle blocks rain into central NZ due to the mountains caught up in this airflow. No different to a shed on your farm creating shade and shelter. 

For northern NZ it’s less about mountains (although they still play a role) and more about the fact Australia is often in the way. The angle of attack for wet weather often curves around the continent in a way that swipes Fiordland but not Northland. Especially in an El Niño period.

So when NZ starts to have drought we need to look for two things, whereas many other nations only have to worry about one thing – rain. We also have to work out if the mountains and ranges are going to “break” the forecast. 

When rain is forecast, we then have to look at the angle of attack. For example, Nelson and Marlborough could do with a decent rain event with northerly quarter winds – to really feed it into these regions. 

Now that autumn is here properly, we do tend to get more variety in our weather – different angles of rain. But the next week or so ahead looks dominated by high pressure, just some wet weather in the mix. 

El Niño continues to fade going into winter, but may give autumn more of a southwesterly lean to our weather. So we may not see an immediate fix to the dry regions – but hopefully as chaotic weather starts to form in the shorter, darker, months ahead we’ll get more variety in the angle of attack of rain bands.

7am Sunday March 24 through to 7am Sunday March 31 shows the bulk of the wet weather in Fiordland and Westland.

Upcoming highlights

• Not much positive news for those who need rain this week, sorry

• Windy westerlies dominate the start of this week

• High pressure dominates the second half of the week and weekend

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