Wednesday, April 24, 2024

When the forecast itself is a bit foggy

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Forecasters know precisely what is happening and why people are going to start complaining, but the local forecast you look at may not give any hints, explains WeatherWatch.
Despite some hugely saturated and muddy regions frequently making the news, the NIWA soil moisture image shows about half of New Zealand is either close to normal, or even leaning drier than normal, right now.
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By Philip Duncan, WeatherWatch

Sometimes forecasters seem like apologists, often offering an argument or defence when the weather doesn’t pan out as we expect. But knowing why a forecast went wrong is a critical part of understanding the weather itself. 

Recently I’ve been saying to people, “It’s predictable – when a weak and large low pressure zone stalls near New Zealand we get complaints about rain forecasts being inaccurate.” 

We as forecasters know precisely what is happening and why people are going to start complaining, but the local forecast you look at may not give any hints. This is why understanding the big picture of the weather is so critical – because how your weather is made is just as important as how it is forecast. 

The reason I’m bringing this up is because big, large, lazy low pressure zones are a nightmare to forecast on a very local scale – big picture is easy – and in 2023 we’ve had no shortage of large, slow-moving low pressure zones. 

When a classic cold front moves in a forecaster has an easy job. We work out the speed of the front, the timing of it arriving at your place, rainfall, wind direction change and temperature drop. But with a large stalled low it’s like a tyre stuck in the mud, going around and around. 

The rain then fractures into pieces, not a perfect line of rain moving in one direction, but spinning in broken pieces – like a spinning firework that is just about finished.  Our mountains and ranges alter these downpours (one side is wet, the other side dry – we see this in every single region across NZ).

So next time you see rain or showers in your 10-day forecast, dig a little deeper to get context. I can’t fly to London in one hour from Auckland and I can’t get 100% perfection in forecasts – the science and technology aren’t there for either (yet!). 

But you can bring your forecasts to life and get more accuracy simply by understanding the big picture driving the weather pattern each and every day.

There’s been a lot of news of all the rain lately, and it’s sadly too muddy for some. But let’s not make the mistake of other outlets and assume everyone is in the “too wet” basket. Yes, rainfall in 2023 has been exceptional and record breaking for some, but for others it’s panned out to quite a normal mid-June set-up (at least soil-moisture-wise).

NIWA’s recent soil moisture deficit maps shows the three driest areas right now are all in places where we’ve been getting complaints about wet weather recently: Eastern Northland, East Cape and Canterbury. A big chunk of the western North Island is also basically normal for this time of year, along with the West Coast and Southland. That’s maybe 50% of NZ around normal, or even dryer than normal. 

Upcoming highlights:

• A westerly change to dry out the eastern North Island

• Drier than average in a large part of the South Island

• Not as cold as you’d expect for winter

• Still a messy neutral weather pattern

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