La Niña's back. Here's how it will affect the weather

For the second straight year, the world heads into a new La Niña weather event.

This would tend to cool western Canada, dry out parts of an already parched and fiery American West and boost a busy Atlantic hurricane season.

Just five months after the end of a La Niña that started in September 2020, the U.S. National Oceanic and Atmospheric Administration announced a new cooling of the Pacific is underway.

La Niña's natural cooling of parts of the Pacific is the flip side of a warmer El Niño pattern and sets in motion changes to the world's weather for months and sometimes years. But the changes vary from place to place and aren't certainties, just tendencies.

La Niñas tend to cause more agricultural and drought damage to the United States than El Niños and neutral conditions, according to a 1999 study. That study found that La Niñas in general cause $2.2 billion to $6.5 billion in damage to U.S. agriculture.

How strong will it be and how long will it last?

There's a 57 per cent chance this will be a moderate La Niña and only 15 per cent that it will be strong, said Mike Halpert, deputy director of NOAA's Climate Prediction Center.

He said it is unlikely to be as strong as last year's because the second year of back-to-back La Niñas usually doesn't quite measure up to the first.

This La Niña is expected to stretch through spring, Halpert said.

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What does this mean for the weather?

In Canada, La Niña tends to be linked to winter weather that includes above average precipitation in British Columbia, colder-than-normal temperatures in the Prairies and more rain and snow than average in Ontario and Quebec, according to Environment Canada.

For the entire southern third of the U.S. and especially the Southwest, La Niña often means drier and warmer weather. The West has been experiencing a two decade-plus megadrought that's worsened the last couple of years.

But for the Northwest — Washington, Oregon and possibly parts of Idaho and Montana — Halpert said La Niña means a good chance of rain and drought relief.

"Good for them, probably not so good for central, southern California."

The Ohio Valley and Northern Plains could be wetter and cooler. La Niña winters also tend to shift snow storms more northerly in winter while places like the mid-Atlantic often don't get blockbuster snowstorms.

In general, expect it to be cooler in western Canada, southern Alaska, Japan, the Korean peninsula, western Africa and southeastern Brazil.

Much of both southeast Asia and northern Australia are wetter in La Niña — and that's already apparent in Indonesia, Halpert said. Central Africa and southeast China tend to be drier.

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What about hurricane season?

During last year's La Niña, the Atlantic set a record with 30 named storms. This year, without La Niña, the season has still been busier than normal with 20 named storms and only one name left unused on the primary storm name list: Wanda.

The last couple weeks have been quiet, but Halpert said he expects things to pick up again. "Just because it's quiet now, it doesn't mean we won't still see more storms as we get later into October and even into November."

La Niñas tend to make Atlantic seasons more active because one key ingredient in formation of storms is winds near the top of them. An El Niño triggers more crosswinds that decapitate storms, while a La Niña has fewer crosswinds, allowing storms to develop and grow.