The sudden and severe wintry spell in both the US and Canada may be the only one of this season as La Nina takes over.

With holes in the Arctic Sea ice weakening, the jet stream – a pool of freezing air that formed near the North Pole – has descended through Arctic Canada and into the US Great Plains.

The temperature dropped suddenly and, at worst, in Manitoba, Saskatchewan, and Alberta, thermometers registered below -40 [the figure where the scales of Fahrenheit and Celsius meet].

Fort McMurray, a cold place in winter, but whose normal day/night range is -10C to -20C, reached the depths of -41.1C on December 16.

Edmonton, Alberta, dropped to -33C and Winnipeg recorded a maximum temperature of -24C. In this part of Canada, the Arctic air sat for about 10 days but has now been blown away. The cold hit the US states of Montana, Colorado, and the Great Plains and Midwest last weekend, but was a blessedly brief visitor.

At their lowest, temperatures were down to -30C in Minnesota, -22C in Kansas and in even -16C in Oklahoma.

Dallas had a sub-zero day with a max of -1C on Sunday followed by a -9C night. You won’t be surprised to hear that the average range in December is 14.5C by day and 5C by night.

This sudden cold spell was a result of Arctic leakage but another influence on the weather pattern has now become established – La Nina. This is a reflection of El Nino and as such has different effects on the world’s weather.

The definition of La Nina once again refers to the surface temperatures in the Pacific Ocean. In opposition to the prevailing eastern Pacific warmth of an El Nino, we now have below-average surface temperatures in the eastern equatorial Pacific.

The prevailing weather pattern in these La Nina conditions, for North America, tends to be colder than normal in the west and warmer than normal in the east.

Precipitation is also increased, especially in the west, in California, Washington and British Columbia.

Equally, more frequent low-pressure systems are likely to form in the lower Plains states and run up through the Great Lakes.

This will tend to bring enhanced rain and mild weather to the southern and eastern states of the US and eastern Canada.