Subzero temperatures, snow and ice are set to hit much of the eastern United States this week, as weather forecasters warn a blast of cold air is set to sweep across much of the country.
Over 150 million people and over two dozen states are expected to be affected by the winter blast, which could cause travel disruption, power blackouts and dangerously cold temperatures.
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The extreme cold and icy conditions that are coming with Winter Storm Fern are being intensified by an expansion of the polar vortex.
That could bring heavy snow and ice to areas of the US that are not as equipped to deal with it, experts have warned.
Polar vortex explained
The polar vortex is a large region of cold air and low pressure that surrounds both the north and south poles.
It's always there, and isn't something that comes and goes, but generally is weaker in summer and stronger in winter.
However, sometimes in the winter months, the polar vortex can expand, sending extreme cold air further south and causing outbreaks of wintery weather including ice and snow.
The term 'polar vortex' has always been known and used by scientists and meteorologists, but, says one science paper published by Harvard University, the term came into popular use during a particularly cold blast in January 2014.
It exists high up in Earth's atmosphere, but the effects of a polar vortex expansion can be felt on the ground, in regions like the United States, as well as parts of Europe and Asia.
A key danger of such an expansion is that it sends cold air further south, often to regions of the world not used to such freezing temperatures, and therefore potentially not as equipped to manage it.
Some scientists suggest that Arctic warming may be destabilising the jet stream, leading to more persistent and erratic cold outbreaks in recent years.
Where will be affected?
By the weekend of 23–25 January, US States facing cold icy air include North Dakota, South Dakota, Montana, Wyoming, Minnesota, Wisconsin, Iowa, Nebraska and Michigan.
States facing heavy snow and/or ice include Kansas, Oklahoma, Missouri, Illinois, Texas, Arkansas, Tennessee, Kentucky, Virginia, Maryland, West Virginia, Pennsylvania, New Jersey and New York.
Conditions could be particularly treacherous in Louisiana, Mississippi, Alabama and Georgia.
Some regions are facing prolonged ice, snow and subzero temperatures lasting over 24 hours, according to the weather forecasting website AccuWeather.
The website also warns that, even when the winter storm has subsided, icy conditions could remain for sometime, causing further delays or disruptions.
For up-to-date weather forecasts, check the National Weather Service, the Climate Prediction Center and the NWS forecast maps.
Preparing for the polar vortex
It's estimated that up to 2 feet of snow could pile up in some areas of the US, which could mean severe disruptions to transport.
Weather forecasters are suggesting commuters and weekend travellers check for flight, traffic or transport delays before travelling.
Other advice given by bodies such as the National Weather Service includes dressing appropriately if you do need to leave your home, and ensuring you have emergency kits both at home and in your cars.
