Every winter the headlines warn that “the polar vortex is coming.” It sounds like a storm with a name. It isn’t.
The polar vortex is a huge, permanent swirl of cold air high above the Arctic. It’s there every winter. Most of the time it’s strong and tight, and it keeps the coldest air bottled up in the far north.
Sometimes it weakens or gets pushed out of shape. When that happens, lobes of Arctic air can slide south, and one of the easiest paths is straight down the east side of the Rockies into Alberta. That’s when we get those stretches of −25 °C (−13°F) to −35 °C (−31°F).
The jet stream’s part
The jet stream is a band of fast wind about 9 to 12 km up that flows from west to east. It marks the boundary between cold air to the north and milder air to the south, and it steers storms along it.
- When the jet stream runs fairly straight across Canada, our weather tends to be changeable but not extreme.
- When it buckles into big waves, a dip south over the Prairies lets Arctic air pour in, and a ridge to the north brings unusual warmth.
Why we feel it so sharply here
Lethbridge sits right where Pacific air coming over the mountains meets Arctic air coming down from the north. When the pattern shifts, we can go from a chinook to an Arctic blast in a day. The change is often more noticeable than the cold itself.
What to do
When forecasters start talking about a pattern change and the jet stream dipping south, use the extra days to get ready: block heater cords, vehicle kit, a check on pipes and pets. Our post on the cold snap checklist later this month has the full list.