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Education: The most accurate ways to read a rain forecast & get fewer nasty surprises

REPOST from our Education Pages — It’s always interesting when we get a complaint about a rain forecast, just how often they haven’t looked at the ‘big picture’ or read the ‘fine print’. We have the most accurate forecast data in NZ, but that doesn’t mean it’s perfect.

While globally rain forecasts are increasingly becoming more accurate and more localised, the terrain across NZ’s regions is so varied that the rain totals do often differ quite a bit from farm to farm, orchard to orchard, suburb to suburb, island to island.

What some may not realise is that there is ‘fine print’ in the rain forecasts – and when you look at it, the forecast comes to life in a far more accurate way.

If you follow all these steps, you’ll have FAR FEWER FALSE ALARMS!

  • Step 1: Look at the Weather icons. You can see when any RAIN SYMBOL shows up. First indicator of wet stuff (and means the chance of wet weather is above 30%).
  • Step 2: Check the Text Forecast. Here you can see what sort of ‘precipitation’ (wet stuff that falls from the sky) is coming: Ie, Rain, showers, hail, graupel or snow, etc.

This is where most people stop. They’ve seen the wet weather icon, or “Rain developing” in the forecast. Surely that’s enough?

Not even close!

  • Step 3: What is the % (chance of) rain? If it’s below 60% that doesn’t bode well for being an overly wet day (unless it’s a random thunderstorm).
  • Step 4: What is the rainfall total for that day? If less than 20mm, that’s usually a “low” amount of rain or showers. 20 to 40mm would be more moderate to high and 40mm+ is high rainfall. Sometimes, under 10mm may not even occur as you’re really in the weeds of accurately tracking borderline areas of rain or showers across a hilly island nation like ours, but it’s still helpful to know.
  • Step 5: Now you know how much rain is likely coming – how is it spread? In one hour? Or many hours across the day? Daytime or nighttime? At and our new App you can visually see the rain in graph form, hour by hour, for 10 days out. It’s a great way to get a ‘snap shot’ of when the rain starts, peaks and ends. Keep it simple.
  • Step 6: It’s all very well knowing what is forecast in your area – but what about the area next to you? It’s important to see the RAIN MAPS to get your head around where the peak rain will be across NZ and to see if you’re in the ‘thick of it’ or just on the fringes (being on the fringes means a higher chance of missing out on the forecast rain).
  • Step 7: Watch our weekday weather videos, to get full context. Here our head forecaster Philip Duncan explains the type of rain that may be showing up, helping make sense of what may be forecast and why some forecasts may change. We get far fewer complaints about our forecast rain accuracy from people who follow our daily videos as it gives context to your 10 day forecast.
  • Step 8: Look for instability. That’s basically being underneath, or near, any low pressure zone as this means rain forecasts can “break” – along with the direction of airflow (which can change a lot when the centre of a low is over you or near by) – and when the airflow moves/shifts over mountains and ranges it and can then break the rain forecasts (lifting totals for some, dropping totals for others). Quite a few moving parts! So this is when using the rain radar is critical. A thunderstorm is a great example of instability and unexpected rainfall – sometimes a thunderstorm can add 50mm+ on top of what was forecast. In the January 27, 2023, flood in Auckland the Severe Thunderstorm added 200mm+ above what was forecast. Thunderstorms and afternoon instability are still the most difficult rain events to accurately forecast in both maximum totals and precise location… and, of course, sometimes they don’t occur – leaving you with a dry day when you’re expecting more. NZ’s lack of open weather data also limits the accuracy of ALL forecasters in the country. Forecast accuracy is improving year on year, but tracking live on the rain radar on the day will help. Which leads to Step 9…
  • Step 9: On the day itself, nothing beats monitoring the actual Rain Radar. Coupled with our hourly forecast graphs they can give you a good understanding of risks, start and end times and peak. Also, to monitor if the forecast wet weather is on time, late or early – or falling apart. We are now pro-actively working with MetService to hopefully bring more real time rain alerts in the future, filling a much needed gap in NZ’s weather industry. Watch this space! **A second Government review on the lack of open weather data and double up of commercial Niwa/MetService tax funded weather services is expected in the first quarter of 2024.

All of these resources laid out in the steps above can be freely found at, our new App, or here at – they will help you better plan ahead. Yes, forecasts do certainly get it wrong from time to time, but you can significantly increase the accuracy of your local forecasts if you follow the steps above – you should get fewer nasty surprises.

If rain matters than much to you then the couple extra minutes to take the few steps above will really make a big difference.

Here you can visually see how the rain peaks and which days are dry at a glance.
Now break down that day with rain in it and really drill down deeper. In this example, the chance of rain is only moderate at 50% and the rainfall total is just 5.2mm. In other words – even if the forecast says “Rain” it’s not likely to be an overly wet day. You could possibly even still dry the laundry in this set up!
The big picture – Compare your rainfall with regions nearby and work out if you’re in the thick of it all or just on the fringes.
Seeing this will make your local rain forecast make much more sense.


Josh on 5/05/2024 9:00pm

i just found this article. i remember a couple flashes and rumbles at 10pm on january 27th last year. that was an extreme and deadly flooding event for auckland.

Obai Tashkandi on 11/02/2021 6:28am

Hi WW, what an awesome article! I love it and I learnt from it! Thank you so much! Although I have a question, when can the rainy days be locked in? Of course, 10 days are way too far, but when are you sure that it will rain, how far?

WW Forecast Team on 11/02/2021 6:18pm

Hey Obai, thanks for the thanks! Highest confidence is within 48 hours, but sometimes even a long range rain event can be pretty much locked in (more so in winter when things move with a bit more speed). If the chance of rain is over 60% more than 5 days out that’s a fairly high chance of rain (but rarely locked in that far out).
– WW

Ajay on 5/01/2023 11:40pm

Allow me to thank you for thanking and thanks from me too 🙂

Derek Butcher on 11/02/2021 12:00am

An excellent article and I learned a lot from it. My general weather knowledge has increased a lot since I started visiting this site a few years ago.

Thanks to WW.

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