December Cold is Warming
Wednesday December 14, 2016
The next two days promise to bring the coldest air so far this season.
High temperatures Thursday and Friday are currently forecast to remain in the 20s. Early morning lows are forecast to drop into the single digits from Poughkeepsie northward on Friday. These temperatures will be the lowest December temperatures since 2013. For Ulster County, Friday will also be the earliest date that temperatures fall into the single digits since 2005.
So, why the cold?
This week's cold will be due to an arctic air mass sliding southward from Canada. Arctic air masses such as this one are regions of very cold, dry air that always exist over northern Canada, Siberia, and the Arctic. Global winds circulate around the edge of such air masses, encapsulating them. These winds exist in upper levels of the atmosphere, and the collective general circulation of air around the air mass is know as the Polar Vortex since it usually exists over the North Pole - "vortex" being a circular flow of air (the South Pole has its own version). When the circulation of the Polar Vortex weakens, it becomes less capable of containing the cold air encapsulated near the poles, allowing it to seep southward. This week's circulation has actually split into two lobes, one over Siberia and one over Canada. New York will be on the edge of the Canadian lobe.
Caption: This map of northern North America shows relative temperatures as colors (purples = colder, blues = warmer) and the flow of wind (white lines) in the upper levels of the atmosphere for Thursday morning. Dark purples, representing an arctic air mass, over Canada are encapsulated by a circulation of air, in this case the "Canadian lobe" of the Polar Vortex.
Caption: This map shows the same temperature and wind scales as the previous map, just at the surface of the Earth. The coldest air is still encapsulated by the edge of the Polar Vortex, however, because the Vortex circulation exists in the upper levels of the atmosphere, it is not physically discernible at the surface (other than the cold).
Despite the cold, no daily records are anticipated to be broken in Ulster County where the weather archive goes back to 1896.
This is because December cold is becoming warmer.
Since the late 1800's, when Ulster County weather archives begin, December temperatures have been rising. Overall, the 30-year average December temperature has increased four degrees at Mohonk Lake since 1926. This has been slightly due to a warming of the warmest temperatures, but has been more significantly related to a warming of the coldest temperatures. Over the last 90 years, the average maximum December temperature has risen by about one degree; the average minimum December temperature however has risen by seven degrees!
Caption: The chart above represents the change in the 30-year averaged December temperatures at Mohonk Lake since 1926. The highest temperatures (orange line) in the month have risen by one degree Fahrenheit. The overall average temperature (black line) has risen by four degrees. The coldest temperatures (pink line) have risen by seven degrees.
Caption: The pink line from the previous chart is shown here in black. Also shown in orange are the lowest measured December temperatures each year at Mohonk Lake. The early 20th century saw the lowest temperatures fall below zero every couple of years, however, this has not happened since the 1980s.
The trends in Ulster County December temperatures are reflective of global conditions. Since the early 20th century temperatures have steadily risen worldwide, and the top ten years on record since 1880 have all occurred since 2000 [[ >>Link<< ]]. The chart below generated by Zachary Labe of UC Irvine depicts how Arctic sea ice extent (SIE) has been steadily decreasing since the 1980s [[ >>Link<< ]]. This decrease in sea ice has been mirrored by rising arctic temperatures. Altogether this suggests that the a decreasing quantity of cold air over North America during the month of December has lead to a significantly decreased probability of Ulster County experiencing the intense cold snaps of the early 20th century.
Caption: The average Sea Ice Extent (SIE) has fallen significantly since 1979. Sea ice extents in early December 2016 were the lowest on record, and were typical of values that would have been observed in early November back in the 80's and 90's.