Chilly November in North America

In the winter in the Northern Hemisphere, forecasters keep a close eye on something called the Arctic Oscillation Index—a measure of atmospheric pressure and wind patterns over the mid-to-high latitudes. In November 2018, the Arctic Oscillation Index was unusually negative. “This tends to push storm tracks farther south, which allows for the dominant flow over land to be from the north, which draws cold air from the poles,” explained Joseph Munchak, a meteorologist based at NASA’s Goddard Space Flight Center.

As blasts of cold air from the Arctic spread across the central and eastern United States in November 2018, storm systems brought unusually early and expansive snow cover to North America.

However, one of the realities of the atmosphere and weather is that when a cold snap hits one area, another nearby area often bakes. In this case, while people in eastern Canada and the eastern United States scrambled to dig out winter clothes in early November, Alaskans basked in abnormally warm and snow-free weather.

The map above shows land surface temperature anomalies in North America for November 1–30, 2018. Based on data from the Moderate Resolution Imaging Spectroradiometer (MODIS) on NASA’s Terra satellite, the map depicts temperatures for November compared to the 2000–2012 average for the month. Areas with warmer than average temperatures are shown in red; near-normal temperatures are white; and areas that were cooler are blue.

Note that the map depicts land surface temperatures, not air temperatures. Land surface temperatures reflect how hot the surface of the Earth would feel to the touch in a particular location. They can sometimes be significantly hotter or cooler than air temperatures. In this case, the extensive snow cover in the continental United States and a snow-drought in Alaska likely contributed to the temperature patterns that MODIS observed, noted Munchak. (To learn more about LSTs and air temperatures, read: Where is the Hottest Place on Earth?)A long meander (or Rossby wave) in the jet stream was the common thread that connected the warm weather in Alaska and west of the Rockies with the chill to the east. As the crest of a Rossby wave—a ridge—pushed unusually far toward Alaska, it brought warm tropical air with it. In response, the other side of the wave—a trough—slid into the eastern United States, dragging along pulses of dense, cold Arctic air. Neighboring ridges to the east kept Greenland and Scandinavia unusually warm, as well.

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