Canadians on the West Coast and in parts of Atlantic Canada may be in for a cooler than normal autumn, Environment Canada said Thursday as it unveiled its outlook for the next three months.

For most of the rest of the country, temperatures may be above normal for September through November, according to the outlook.

Precipitation levels for the period may also be below normal for the Prairies and Atlantic Canada but about normal everywhere else.

As for the historical accuracy of the outlook, CBC meteorologist Johanna Wagstaffe said there is some confidence in the temperature outlook, particularly for the coasts, northern Ontario and Quebec, but there is less confidence in the precipitation outlook.

Wagstaffe says the transitional seasons — spring and autumn — are all about dramatic changes in the weather.

A temperature outlook of "above normal" could mean that temperature swings from very cold to very warm days might average out above normal, not that most days will be warmer.

Also Thursday, United Nations weather forecasters predicted a higher chance of a La Nina developing through the rest of this year.

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The World Meteorological Organization says there is now a 50 per cent probability of La Nina conditions developing, up from a 25 per cent chance from an earlier forecast.

If La Nina conditions do develop, they are expected to be weak to moderate, and the WMO said there is almost no chance of an occurrence of El Nino.

La Nina occurs when the ocean is cooler than normal, and results in plentiful rainfall in affected countries, which form a belt that runs east-west and spans from just north of the equator down to Australia and the South Pacific. It is also associated with dry conditions for parts of eastern Africa, southwestern Asia and the southern United States.

A moderate to strong La Niña dominated the tropical Pacific Ocean from September 2010 to February 2011 before it dissipated by May 2011, UN forecasters said.