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Polar vortex could set new records for cold, bring snow in Ontario

Thursday, May 7th 2020, 9:09 pm - The higher sun angle in May helps moderate the frigid temperatures associated with the polar vortex, but we could still see record-breaking cold in southern Ontario.

Forecast models are now certain that the coldest temperature anomaly on Earth will sit right over parts of eastern North America this Friday and Saturday.

You probably have some questions, I know I did.

Q: How is this the polar vortex? Temperatures aren't even that cold at the surface.

A: The polar vortex is the culprit, but there's a big difference between this event occurring in mid-spring versus the heart of winter.

The angle that sunlight strikes the Earth at during May is exponentially more powerful than the sun angle in January. The higher sun angle gives many more hours of daylight and prevents temperatures from plunging below the -20 °C and -30°C mark at the surface.

The polar vortex is a feature way, way up from the surface. Specifically, we're looking at a region of the atmosphere called the tropopause. When this cold lobe of air crosses southern Ontario Friday evening, it'll be roughly 8 km above the surface, featuring temperatures of -45°C!

See the grey shade below? That air aloft originated in the Arctic Ocean.


Q: I have seen snow on the ground many times in May, is this that unusual?

A: Ontario is a massive province and naturally the further north you go, the higher the probability is that you'll see accumulating snow in May. It's uncommon in southern Ontario, but northern regions of the province can expect to see a few centimetres of snowfall every May.


Q: If it's this cold out, is there still a risk of a sunburn?

A: As parts of southern Ontario struggle through one of their top ten coldest May days on record Friday and Saturday, the UV index will still likely be moderate or high depending on the amount of sunshine.

Sun elevation in the sky, the amount of ozone and pollution in the atmosphere and cloud cover are the determinant factors of UV intensity, not surface temperature.



Q: A few cold days in a row in May, so what? This can't be that uncommon.

A: Several factors are making this May a memorable one. Firstly, the number of days forecast below the freezing mark is incredibly unusual.

Using Toronto as a proxy, it's rare to see consecutive frosts during May in the GTA. Toronto recently recorded its first May sub-zero temperature in 15 years on May 5th, and we’ll like see several more throughout the rest of this month.

A similar anomaly exists for daytime highs. Even one or two daytime highs below 10°C is noteworthy. Depending on how warm it gets on Sunday, Toronto might just string together four or five of them.


We're carefully watching the daytime highs on Friday across Ontario. It appears Sault Ste. Marie and Sudbury might record their coldest daytime highs on record during May – and that's a pretty big deal.

OntarioRecordHighs (1)

A front moves in from northwestern Ontario, dropping the temperatures drastically for Friday and Saturday. It will feel more March-like weather. Areas in northern Ontario may see up to 10 cm of snow.

The lakes are relatively warm compared to the airmass, so that means that lake-effect snow is still possible this time of year. Several bands of snow will flow off of Georgian Bay and Lake Huron by late Friday evening. Locally 5-10 cm of snowfall is possible in regions that see persistent snow throughout Saturday.

Most of southern Ontario including the GTA is likely to see snow or graupel showers on Saturday, but accumulations for most will stay below trace amounts.

Temperatures will only hit 4°C or 5°C in much of southern Ontario for Saturday. Overnight lows Friday could dip to -4°C, making it sufficient for snow, but warm surfaces will make it difficult to stick.

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