A wet July, but a warmer outlook for August

RADAR VIEW: This is the 24 hour gauge-corrected radar view from July 9. The pink and red shading indicates the highest rain accumulations, showing more than 125mm in 24 hours.

If there were a meteorological equivalent of the 'seven summits', New Zealand would certainly be on the list. As any farmer knows, we get a lot of weather.

This is for two reasons: firstly, we're located in the middle of the roaring forties and weather systems can come in from the southern ocean or the subtropics; and second, because New Zealand is a land of hills and mountains.

Wind flows are the critical factor on whether the weather comes into your place, or not, because of the effect of these hills. This is why forecasters focus on circulation (wind flows) when they look longer-range (on a monthly or seasonal time frame).

Circulation is a handy tool to help identify New Zealand temperatures longer-term. It is not quite as straight forward as 'northerlies are warmer, southerlies are cooler' -- but there is an element of truth in that! Wind flows also guide rainfall forecasts for many regions of New Zealand.

Just ask a farmer on the Canterbury Plains what the weather will be like at their place in a strong northwesterly, and they'll respond "warm and dry". Rather the reverse answer to the bloke in Hokitika. Even the El Nino/La Nina response here is driven to a large degree by wind flow versus our mountains.

So let us focus on circulation here. It serves as a useful way to describe winter 2014 (so far), and we'll use it to estimate the coming conditions.

June and early July saw a large number of extreme events for those regions exposed to the north.

The atmosphere was stuck in what forecasters call a "blocked" regime. Highs were often slow-moving east of the country, with lower pressures over the Tasman Sea. This meant persistent northerly winds and a relatively warm start to winter.

The lows and fronts that were trying to move in from the Tasman Sea couldn't progress very far, with the blocking high fending off the incoming rain band.

These stalled rain bands proved to be a real problem, with relatively warm, moist air fed down from the subtropics, producing heavy rain and extremely large rainfall accumulations.

The week of extreme rain and severe northeast gales in Northland in July is a good example of what blocking can do. Already saturated soils copped nearly 500mm of rain in the worst-affected areas.

This is a total more often seen on the West Coast, rather than in Northland. More details about the Northland floods can be found on the Metservice website.

The second half of July saw a shift to prolonged, cold southwesterlies. Temperatures dropped markedly across New Zealand, with several snowfall events.

The North Island's east coast bore the brunt, with a battering from showers/sleet/snow (depending on the location).

Looking ahead

As we start August, northwesterlies return over the country, bringing stormy conditions over southern New Zealand.

Above normal rainfall is forecast for the West Coast and Southland. Relatively warm conditions are signalled for the South Island, especially eastern areas.

The second week of August shows highs sitting over the Tasman.

Drier conditions are indicated for northern parts of both Islands; possibly wetter for Southland.

For the second half of August, more typical westerly winds are expected, with 'near average' climate (for the time of year) looking the most likely outcome.

Of course, this doesn't rule out extreme weather events -- it's often an unsettled month -- so it will pay to keep up to date with the latest forecasts and warnings at www.metservice.com.

Looking further ahead, there remains a solid chance (70 per cent) of El Nino development during spring. The global oceans are already primed (with the El Nino signal of warmer than normal waters in the equatorial Pacific Ocean already present). However, the atmosphere has yet to respond. The latest climate models suggest that if El Nino does develop, it is increasingly unlikely to be a strong event.