Southwestern Ontario cities such as London and Sarnia have been buried in record
amounts of snow, stranding hundreds on Highway 402 and forcing a state of
emergency to be declared in the area – yet other places remain relatively unscathed.

Below, Altaf Arain, director of the McMaster Centre for Climate Change and associate
professor of geography and Earth sciences, explains where the extreme weather came
from and how long it might last.

Southwestern Ontario has gotten a lot of snow over the last week – is this normal?

Actually, yes – sort of. This year is a La Nina year, so there are cooler than normal
water temperatures in the eastern tropical Pacific, which shifts the North American jet
stream and pushes warmer air further north. This changes weather patterns
dramatically for eastern North America. Wet winters over the Great Lakes become
wetter during La Nina year. So it's not a normal winter, but it is normal for a La Nina
year.

So if La Nina caused all that snow in London, should we expect it around McMaster?

Not necessarily. We can expect wet weather, which may mean snow, but may mean
rain. We can also “expect the unexpected”, since La Nina tends to produce very
different weather conditions for different areas. Generally speaking, it causes
northwestern Canada to be colder, the south and southwestern United States to be
warmer and drier and the Great Lakes region to be wetter, since winds moving over the
lakes will tend to dump moisture on the other side. La Nina usually lasts for 9-12
months, though, and it will probably only affect our winter and spring seasons.

Can weather forecasters predict La Nina?

They can, but only in the relative short-term. Scientists monitor sea surface
temperatures using satellites and buoys , so we know when they are warming up or
cooling down, and we know that La Nina follows its opposite, El Nino, but we can only
predict either about six months in advance. They happen approximately every three to
five years.

If global temperatures warm, what will happen to phenomena like La Nina?

Some studies have indicated that La Nina and El Nino have become more frequent in
recent decades. Scientists are still studying their relationship to global climate change.
If ocean temperatures warm due to global warming and the intensity of jet streams
increases, North America can expect more heavier rain, heavier snowfall and more
extreme weather events.