The polar vortex is an area of cold low pressure that typically circulates and strengthens around the Arctic region in the wintertime and then weakens in the summer. During winter, pieces of the polar vortex can break off and be sent southward with the jet stream to either the Europe/Asia side of the North Pole or to North America. Meteorologists try to track the positioning of the polar vortex as a way of predicting Arctic air outbreaks that potentially could drop southward into the mid-latitudes. Computer forecast models can show the position of the polar vortex through their forecasts of heights of different atmospheric pressure levels. Last night’s European computer model 10-day forecast of the 500 millibar height field (~18,000 feet up in the atmosphere) clearly shows another rather typical example of the polar vortex being distributed to mid-latitudes – in this case to both sides of the pole (see forecast map). This particular upper-level setup is forecasted to occur in 10 days and it will continue the bitter cold weather pattern that begins early next week in the central and eastern US. One major Arctic air outbreak arrives in the Mid-Atlantic region this coming Tuesday and, given the expectation for the polar vortex to be distributed southward by later this month, another major Arctic air outbreak can be anticipated here in the mid-latitudes as we close out the month of January. This upcoming deep freeze will rival the outbreak earlier this month with sub-zero readings in many areas across the central and eastern US and it will last for a considerably longer period of time.