Updated — The first band of rain is moving down the upper North Island and the “melting” effect moves south with it!  Often with weather forecasting it can be difficult to accurately describe something technical, in a way that makes it clear to understand for everyone.

Hopefully our odd description of “melting rain” might help you understand (if you need it) why some forecasts have really struggled to work out what is happening this weekend rainwise.

As many in northern New Zealand know, we have a low moving in at the moment – but for many it’s felt as if it’s ‘going to rain’ any time soon, but it hasn’t arrived.  The various forecasters throughout this week have all shifted the arrival time of the rain as well.

So here we are at 4:30am on Saturday and the first of the rain arrives in Northland. It’s patchy as predicted.

But, as we mentioned this week, worth paying note of those watches and possible warnings this weekend. This event isn’t a widespread rain event, it’s another of those hit and miss ones where a heavy rain watch/warning may be issued and the rain is torrential for some but next to nothing for others.  Worth watching/warning about definitely, due to the sub-tropical nature. But hopefully nothing too major from this low.

Overall we expect the rain clouds to fade out as the low drops south and runs into a high – the low itself will also weaken over the weekend.

The rain maps show heavy rain moving towards Northland and the upper North Island this weekend.  The winds have picked up and the clouds have arrived already – but it’s been like this for days now.

That’s because the high is holding the low up, like a broken down bus on a one way street and the low is the car waiting behind it to move. As the rain clouds move south – away from the low and down the upper North Island – the air pressure will rise quickly due to the high still remaining over the eastern North Island.  The high pressure will then help turn those heavy rain clouds into isolated showers then drizzle patches then dry, the further south you go.

This action means the rain (which is torrential in places in the subtropics) falls apart as it slowly moves south, thins out, then dries out completely.  The result?  If you look at the rain radar – or our rain forecast maps now – you’ll notice how the big band of rain thins out quickly in the south – you can see it’s wanting to move south, but the high is not letting it…and won’t let it move until the high A) weakens or B) moves away – which it’s predicted to slowly do this weekend and into Monday while at the same time the low will weaken.

It’s for this reason we’ve been using showers more than rain in many of our forecasts – as it’s a very fine line to work out where the rain stops and where the dry begins, but the high is so far dominating – doing a good job at keeping those rain clouds north of NZ for now.  The further the high moves away, the further south the heavier rain can go before it starts to “melt” and fall apart.

Despite the fact drizzle patches may fall instead of rain for some areas, there is the risk of torrential downpours for Northland. It’s hit and miss, but keep up to date with any possible weather watches and warnings over the next few days.

– Image / File, Napier last month, WW

– WeatherWatch.co.nz