What does "chance of rain" mean on a MetService forecast?
The percentage is a probability, not an amount — here's how to read it, why two apps disagree, and how to actually use it when planning your day.
On a MetService forecast, “chance of rain” is the probability of precipitation — the likelihood that a measurable amount of rain will fall at your location during the stated period. A 40% chance means rain is more likely not to reach you than to reach you. Importantly, the percentage says nothing about how much rain falls, how heavy it is, or how long it lasts.
What exactly is the “chance of rain” percentage?
The figure is a probability of precipitation, tied to a specific place and a specific window — morning, afternoon, evening, or a whole day. If MetService shows a 60% chance for Wellington in the afternoon, the honest way to read it is: if that same weather pattern played out ten times, rain would reach that spot in roughly six of them. It is a statement about likelihood, not intensity, coverage or duration.
“Measurable” matters too. It means enough rain to register in a standard gauge. A brief shower that just dampens the footpath can satisfy the threshold every bit as much as a steady downpour — which is one reason a high percentage can still coincide with a day that feels mostly dry.
How does MetService work out the probability?
MetService forecasters begin with numerical weather prediction (NWP) — computer models that simulate the atmosphere — and layer on local knowledge of New Zealand's terrain and weather patterns. Two ideas feed the number: how confident forecasters are that rain will occur somewhere in the forecast area, and how much of that area is likely to be affected. Blend the two and you get a single percentage.
Ensembles sharpen this. Instead of running a model once, forecasters run it many times with slightly nudged starting conditions. If seven of ten runs produce rain over your district, that supports a figure around 70%. When the runs scatter, confidence — and the percentage — drops.
Does 70% mean 70% of the day, or 70% of the area?
Neither, and this is the most common misconception. It does not mean rain for 70% of the hours, nor rain over 70% of the map. It is the chance that at least one measurable rain event reaches a given point during the period. The table below translates the bands into plain English.
| Chance of rain | What it means in practice |
|---|---|
| 0–20% | Unlikely — plan outdoors, keep half an eye on the sky |
| 30–40% | Possible — worth carrying a jacket |
| 50–60% | Close to a coin toss — have a wet-weather plan ready |
| 70–80% | Likely — expect rain at some stage |
| 90–100% | Almost certain — rain is essentially locked in |
Chance of rain versus how much rain
The percentage and the amount are separate figures, and you should read them together. A 90% chance of 0.5 mm of drizzle is a very different day from a 50% chance of a thundery burst dropping 30 mm. New Zealand's geography makes this vivid: the Southern Alps and other ranges wring moisture out of the westerlies, so the West Coast can be soaked while Canterbury, in the lee, stays dry under the same system. Always pair the chance of rain with the rainfall accumulation — and, when totals climb, with any MetService watches or warnings.
Why do two apps show a different chance for the same day?
Because they lean on different models. The European ECMWF, the American NOAA GFS and the German DWD ICON each simulate the atmosphere in their own way and frequently disagree, especially three or more days out. One might see a front clipping the coast; another pushes it out to sea. That disagreement is exactly what checkweather.io turns into a plain-English forecast-confidence score — when the models line up, confidence is high; when they scatter, today's 50% may swing tomorrow. Because New Zealand's own forecasters keep their models commercial and publish no open data, checkweather compares the three global heavyweights and links through to MetService for the official national word.
Does the percentage change through the day?
Yes, and following it pays off. A single day's forecast is usually split into blocks — morning, afternoon and evening — and each carries its own chance of rain. A day might read 20% in the morning, climbing to 70% by evening as a front approaches; the headline daily figure smooths those blocks into one number and can hide the timing that actually matters to you. If you can shift a walk or a job to the drier block, the hour-by-hour breakdown is where you'll find the window. The percentages are also revised as fresh model runs arrive through the day, so a figure you glanced at over breakfast may have firmed up or eased by lunchtime. Checking the trend across those updates tells you whether the atmosphere is settling on rain or backing away from it — information a single snapshot can never give you.
How to actually use the chance of rain
Match the number to the stakes. Hanging out washing on a 30% day is a fair bet; heading into the backcountry on a tramp, or taking a boat out, you'd want a far lower figure before you relax. Watch the trend across successive forecast updates, too — a chance that climbs run to run signals a system the models are growing more certain about, while a falling figure suggests the threat is drifting away. Read it alongside wind, the “feels like” temperature and any warnings, and the percentage becomes a genuinely useful planning tool rather than a coin toss.