Mid-January has arrived which means a couple of things in the Northeast. College students reluctantly return to the grind of classes, football season is coming to an end and the winter cold has arrived in full force. Yes indeed, the bone chilling cold returned last week from a long hiatus and is back with a vengeance.
Recently, (although not so much this week), Arctic air from Canada caused frigid temperatures in locations such as International Falls, Minnesota (-35?F) while cities such as Boston and New York barely made it out of single digits. Although it may appear that subzero temperatures can seemingly come out of the blue, forecasters have the ability to predict the severity of the winter cold far before winter arrives.
Large-scale climatic teleconnections have an important influence on the weather pattern for a specific region. These teleconnections such as the North Atlantic Oscillation (NAO) and the Pacific North American pattern (PNA) relate large-scale weather patterns across a large distance, consequently having a direct impact on the weather we have experienced this winter.
The PNA is one of the most recognized, influential teleconnection patterns in the Northern Hemisphere. The positive phase of the PNA oscillation tends to be associated with warming over the Pacific and the negative phase tends to be associated with cooling over the Pacific. This warming/cooling is directly correlated to the temperature anomaly in the United States. During a positive PNA there is a ridge in the jet stream over the western U.S. with warm air infiltrating from Mexico resulting in above average temperatures. During this stage there consequently is a trough in the jet stream over the Eastern U.S. with cold air coming down from Canada resulting in cooler than normal temperatures. During a negative PNA, the temperature anomaly is directly opposite to the positive stage with cooler temperatures out west and warmer temperatures in the east. Recently, with the cold air that funneled into the northeastern U.S., the PNA shifted to the positive phase.
Another influential oscillation, the NAO is simply a “blocking” pattern that affects the location and intensity of cold air. The positive phase of the NAO tends to bring above normal temperatures along with relatively wet conditions over the Eastern seaboard. These conditions are associated with a fairly strong upper-level jet stream. During the negative phase, the upper-level jet stream weakens, allowing cold air to filter down along the east coast of the U.S. Additionally; the negative NAO tends to bring conditions drier and cooler than normal. The recent cold blast pushed the NAO into negative territory for only the second time since the beginning of winter.
Distinct changes in temperature and precipitation throughout the winter always correlate to the large-scale meteorological patterns. Understanding how these large-scale teleconnections behave during the winter is extremely important for any meteorologist when making a forecast.