Whanganui is likely to have mild weather for the remainder of the school holidays, a forecaster says.

WeatherWatch head analyst Philip Duncan said it was likely to be cooler than normal for the time of year.

"It should be a bit cooler with south westerlies blowing through which then become more westerly," he said. "I don't mean it's going to be cold but it's not going to be the hot, humid weather you normally get.

"There could be a few showers around January 13 and some showers again around January 17."

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MetService is expecting rain in the region this week, with temperatures in the low 20s. Overall, January is likely to be cooler than usual nationwide with a mix of dry and wet, according to MetService.

Meanwhile, the National Institute of Water and Atmospheric Research (Niwa) has predicted near or below average temperatures and rainfall for Whanganui in its outlook for December 2015 to February 2016.

It said the El Nino effect, with its westerly winds, would continue and weaken in Whanganui through the summer.

This season's El Nino effect is a strong one, linked to higher than usual ocean temperatures. Those temperatures are now reducing, but are still 2C higher than usual in the central and eastern Pacific.

The last three El Ninos happened in the 1972-73, 1982-83 and 1997-98 seasons, and all made for extremely dry seasons in parts of New Zealand. Strong westerly winds are typical of them, with drier than normal conditions in the north and east.

This spring's westerly winds are likely to last through into autumn, Niwa says. By the July-September quarter, climate should be back to either a normal pattern or a La Nina pattern - with calmer days and more wind and weather from the north.