An area of low pressure in the tropics won't likely become a tropical cyclone -- but it is a tropical rainmaker, says a WeatherWatch spokesperson.

'The area of low pressure had the chances of becoming a cyclone however for a storm to form all the low pressure energy needs to be centred in one location. This low is stretched out with the energy spread too far, which limits how strong the winds get.”

The weather service says that high pressure in the New Zealand area is also limiting the chances of a storm growing.

'But as we said on Friday, this is more about the rainfall risks for NZ than it is about a storm coming our way.”

The tropical low is expected to bring heavy summer rain to a number of regions, but high pressure may just hold on to the North Island.

'With much of the North Island further east than the South Island, the tropical rainmaker may miss out bringing rain to very dry North Island regions,” says a WeatherWatch spokesperson.

'The line is very fine between drier than normal and wetter than normal as this tropical system drops southwards.”

Humidity and overnight temperatures are expected to rise.

'Even if the North Island misses out on heavy rain from the tropical low, it's still likely to get more humid,” says a WeatherWatch spokesperson.

'Overnight lows are going to jump nationwide.”

The service says there are two good extremes to highlight.

'Auckland has a week coming up of overnight lows in the 20s. Coupled with higher humidity the Humidex (or feels like) temperature will be around the mid-20s at midnight this week.

'Southland had a light frost on Friday morning as the overnight lows dropped closer to freezing. This week, the warmer northerly airflow will push the overnight low up to +17C by Tuesday night, which, by the way, is warmer than Thursday's daytime high once the tropical system has moved through. 'Also worth noting the daytime highs will be around 30 degrees to kick off the start of February.”