Bad weather on horizon

Heavy rain and gales are expected to hit northern regions later this week, ending a short reprieve from the worst of winter's weather.

A large but "fairly docile" low moving in from the northern Tasman Sea could bring heavy rain, WeatherWatch forecaster Philip Duncan said.

"Between the incoming low from the north west and the higher air pressure over New Zealand we can expected gales to develop in some northern regions, especially coastal areas," he said.

"The wind is created between the high air pressure and the incoming low but the main feature does appear to be slow moving rain bands, which could cause flooding."

Northland, Coromandel Peninsula, Bay of Plenty and East Cape were expected to bear the brunt of the heavy rain with some isolated thunderstorms possible on Thursday night and Friday morning.\

The low was forecast to reach Gisborne and Hawke's Bay by the weekend.

Mr Duncan said the gales were not expected to cause damage.

MetService has issued a severe weather watch for the upper North Island, saying there was a moderate risk of heavy rain and severe gales.

Easterly gales could spread as far south as Kapiti and the northern Marlborough, it said.

A Winter Storm | MetService Blog

Since Wednesday 6 July, stormy westerly conditions have affected New Zealand. In this blog, we’ll look at why.

Below is the mean sea level analysis – the weather map – for 6am Sunday 10 July. In between big highs over the mid South Pacific and south of western Australia is a really large trough; it’s the area shaded light blue. The weather map has looked like this, more or less, since Wednesday 6 July: that is, the big features on it aren’t moving much.

There’s good reasons why these big features aren’t moving much. They reflect the so-called “long waves” in the troposphere (the troposphere is that part of the Earth’s atmosphere in which the weather occurs), which are stationary at the moment. Below is an image made from a Fourier analysis of the wave pattern in the Southern Hemisphere at midnight Saturday 09 July. There’s a trough (blue) in this wave pattern more or less in the same place as the one shaded light blue on the weather map above. And either side of this trough, there are ridges (pink) in about the same place as the big highs on the weather map above. For more about this, see my blog post on Wave Three .

On its way to New Zealand, this air travelled over a long stretch of ocean. It will have been colder than the surface of the sea it passed over, and will therefore have taken up heat from the sea surface. As heat transfers from the sea surface to the air immediately above it, “blobs” of air become warmer than their surroundings and rise upwards. This process is known as convection (see Chris Webster’s blog post about predictability and popcorn ). If convection continues for long enough, showers and/or thunderstorms are the result. In the satellite picture below, more or less all of the clouds over the Tasman Sea, New Zealand and the seas to the south of the country are “blobs” of air which is rising (or has recently risen) convectively.