Cold This April Due to Polar Vortex

You're heard all about the "polar vortex" this week, as forecasts called for record cold in parts of the East last weekend. The good news is that the colder-than-average temperatures are happening in April, and not January.

So what role, if any, is the polar vortex truly playing in the chilly conditions across parts of the Midwest and East this April?

 What Is Polar Vortex?

The polar vortex is an area of low pressure in the upper atmosphere, primarily in the stratosphere, a layer of the atmosphere above which most of our sensible weather occurs (known as the troposphere).

 

 

The polar vortex is strongest in winter, thanks to an increased temperature contrast between the polar regions and the mid-latitudes, including the United States.

How It Works

When the polar vortex is strongest, you're less likely to see cold air plunge deep into North America or Europe. (By strongest, we mean the generally west-to-east flow around the vortex is stronger than average). The easiest way to conceptualize this is in terms of a wall. A stronger polar vortex effectively helps to wall off cold Arctic air from the mid-latitudes. Occasionally, though, the polar vortex is disrupted and weakens. This happens when the stratosphere warms. When the polar vortex is weakened a piece of the polar vortex can surge farther south, bringing Arctic cold into portions of North America and Europe.

How It Impacts Your Temperatures

Below-average temperatures, along with snowy conditions, have been the story so far for much of the Midwest and Northeast. This weekend, another shot of Arctic air will bring potentially record cold temperatures to portions of the East. Colder-than-average temperatures are expected for the Midwest, Northeast and parts of the South beginning late this week and continuing into at least part of next week. One of the causes of this temperature change is the weakening of the stratospheric polar vortex that took place in March of last year. In response to the atmospheric conditions, both the Arctic Oscillation (AO) and the North Atlantic Oscillation (NAO) are expected to trend negative into at least mid-April. In addition, a strong, blocking high pressure aloft near or over Greenland, referred to as a Greenland block, is expected to develop. The negative AO and NAO conditions, along with the Greenland block, will help to keep the cold conditions in the East into at least mid-April. Meanwhile, lower in the atmosphere at jet-stream level, a sharp southward plunge of the jet stream will once again surge southward into the East late this week, accompanied by another round of colder-than-average temperatures.

 

https://weather.com/science/weather-explainers/news/polar-vortex-april-2016-cold-outbreak-east