The polar vortex: Cold weather gripping the Midwest and Northern Plains of US

Madison, Wisconsin, Jan 30: Owing to the polar vortex a desperately cold weather is now gripping the Midwest and Northern Plains of the United States, as well as interior Canada.

According to NASA, a large area of low pressure and extremely cold air usually swirls over the Arctic, with strong counter-clockwise winds that trap the cold around the Pole. But disturbances in the jet stream and the intrusion of warmer mid-latitude air masses can disturb this polar vortex and make it unstable, sending Arctic air south into middle latitudes.

That has been the case in late January 2019. Forecasters are predicting that air temperatures in parts of the continental United States will drop to their lowest levels since at least 1994, with the potential to break all-time record lows for January 30 and 31. With clear skies, steady winds, and snow cover on the ground, at least 90 million Americans could experience temperatures at or below zero degrees Fahrenheit (-18° Celsius), according to the U.S. National Weather Service (NWS).

The map above shows air temperatures at 2 meters (around 6.5 feet above the ground) at 09:00 Universal Time (4 a.m. Eastern Standard Time) on January 29, 2019, as represented by the Goddard Earth Observing System Model, Version 5. GEOS-5 is a global atmospheric model that uses mathematical equations run through a supercomputer to represent physical processes.

The figure above is not a traditional forecast, but a reanalysis of model input fixed in time-a representation of atmospheric conditions near dawn on January 29, 2019. Measurements of temperature, moisture, wind speeds and directions, and other conditions are compiled from NASA satellites and other sources, and then added to the model to closely simulate observed reality. Note how some portions of the Arctic are close to the freezing point-significantly warmer than usual for the dark of mid-winter-while masses of cooler air plunge toward the interior of North America.

Meteorologists at The Washington Post pointed out that temperatures on January 31, 2019, in the Midwestern U.S. will be likely colder than those on the North Slope of Alaska.

What is polar vortex?

US weather service defines the polar vortex is a large area of low pressure and cold air surrounding both of the Earth's poles. It ALWAYS exists near the poles, but weakens in summer and strengthens in winter.

The term "vortex" refers to the counter-clockwise flow of air that helps keep the colder air near the Poles. Many times during winter in the northern hemisphere, the polar vortex will expand, sending cold air southward with the jet stream (see graphic above).

This occurs fairly regularly during wintertime and is often associated with large outbreaks of Arctic air in the United States. The one that occurred January 2014 is similar to many other cold outbreaks that have occurred in the past, including several notable colder outbreaks in 1977, 1982, 1985 and 1989.