There was no calm before the storm, it appears.

Toronto was struck by a sudden and brief burst of snow pellets — resembling hail — Monday afternoon, bewildering residents. 

But as noted by CBC Toronto meteorologist Jay Scotland, the pellets were, in fact, not hail but graupel. 

These styrofoam-like pellets, often mistaken for hail, "form when supercooled water droplets freeze on snowflakes," said CBC Toronto meteorologist Colette Kennedy.

How can you distinguish graupel from hail? "Graupel is softer than hail and will likely break apart in your hand if you pick it up," Kennedy said.

"Hail is harder and usually whiter in colour and can have many layers," she said.

A major winter storm is expected beginning midday Tuesday, Environment Canada says.

Toronto could see about two to five centimetres by the afternoon and up to 20 centimetres through the night, the weather agency said.