Discussion: Arctic outbreaks during winter are generally caused by stratospheric warming events in the polar region. When the stratosphere warms, the weather atmosphere (called the troposphere) which lay beneath the stratosphere contracts and cools. The colder and dense air piles colder (and colder) at ground level beneath or downwind the stratospheric warming episode and an arctic air mass is born. This set of circumstances was initiated Dec. 21-25 (Fig. 2).
Fig. 2: In the stratosphere an area of warming across northern Canada occurred Dec. 21-25, 2017 and was the catalyst for the following U.S. arctic outbreak.
The arctic air mass formed Dec. 24-28 across Canada (Fig. 3) which was shock cold given the near record warmth for the first 2/3 of December for western Canada to the North-Central U.S. The arctic air struck Chicago Christmas Day slowly expanding south and east to the southern Plains and Ohio Valley Dec. 27th and later that same day into New England and the northern Mid-Atlantic.
Fig. 3-5: Sequence of formation and migration of an arctic air mass in North America during late December and early January.
The arctic air intensified and widened across both the Great Plains and East U.S. Dec. 29-Jan. 2 (Fig. 4). Bitter cold shifted into the North-Central U.S. early in this period while lake-effect snows combined with a stalled low pressure trough widened Midwest and Northeast snow cover which acted as an inviting cold carpet for the North-Central chill to advance eastward on New Year’s Day. Many daily record cold temperatures were set on New Year’s Day from the Central to East U.S. thanks to a 1060 MB high pressure center over the northwest Great Plains.
During Jan. 3-7 the core of the cold shifted to the East U.S. (Fig. 5). Interestingly, the intensity of the arctic air eased slightly but a snow storm affecting most of the East Coast meant the cold air that followed would be enhanced and was the most extreme of the entire sequence for the East and South. In fact, the peak cold for the east was accompanied by high wind Jan. 3-5 causing hostile wind chill well below zero into the Mid-Atlantic States. The arctic high pressure finally abated with quick warming to follow Jan. 8.
Locations: The Midwest and East U.S. produce a high energy demand zone. Let’s take a look at a few key cities and compare their cold peak to the coldest on record for each location. We’ll examine Chicago, Cleveland, Pittsburgh, Boston, New York City, Baltimore, Richmond, and Atlanta. Of the 8 locations the coldest peak of the arctic outbreak set a daily record for half the stations and all stations were within the top 3 coldest days for the given date.
Chicago: The coldest day (1/-9) of the arctic outbreak was Jan. 1 and ranked the coldest in history for that day which is impressive given the 1871-2018 history.
Cleveland: The coldest day (10/-2) of the arctic outbreak in Cleveland was Jan. 5 which ranked 3rd coldest in the 1938-2018 period of record.
Pittsburgh: The coldest day (9/-1) of the arctic outbreak in Pittsburgh was Jan. 5 and ranked 3rd coldest in the 1948-2018 period of record.
Boston: New Year’s Day featured the coldest day (13/0) of the arctic outbreak and was the coldest Jan. 1 on record in the 1872-2018 climatology.
New York City: The coldest day (15/8) of the arctic outbreak was Jan. 6 and ranked as the coldest of the 1939-2018 period of record for that day.
Baltimore: The coldest day (22/1) of the arctic outbreak was Jan. 7 ranking 2nd all-time for that date given the 1939-2018 history.
Richmond: The coldest day (23/-3) of the arctic outbreak was Jan. 7 which ranked as the coldest day on record for that day given the 1930-2018 climatology.
Atlanta: New Year’s Day was the coldest (29/18) of the arctic outbreak and ranked 2nd all-time for the date given the 1940-2018 period of record.