Mid-February to See Coldest Weather of the Season in Southeast Michigan
by Thomas Kovacs
Up to now the 2020-2021 Winter season defined as December, January, and February has been much warmer than average defying the outlook for the winter season published in mid-October 2020. However, that is all about to change. Back in January the polar vortex began to weaken and break apart. The polar vortex is a circulation of Arctic air that circumnavigates the poles during the polar night centered in the stratosphere, but its effects can extend into the troposphere. For the North Pole, the polar night is from the autumnal equinox (September 19, 2020) to the vernal equinox (March 20, 2021). Most years the polar vortex begins to break apart near the end of the polar night during an event centered in the stratosphere called sudden stratosphere warming. This year the polar stratosphere warmed by over 30 C (54 F) in 5 days (Dec. 31 - Jan. 5) causing the polar vortex to begin to break apart. Once the Arctic air is no longer confined to the Arctic regions a highly amplified wavy polar jet stream can cause parts of the polar vortex to move equatorward. This process usually takes weeks. Now, part of that air is expected to move across the northern Plains and Great Lakes states causing much colder than normal weather starting February 5 and extending to at least February 16 for southeast Michigan.
The following figures show the Climate Prediction Center's forecast for February 8-12 (first figure) and February 10-16 (second figure). The figures show the distribution of probabilities relative to the average for the period 1981-2010. The color code is on the bottom of each diagram and represents the chance of above or below average values for the entire 90-day period. Equal chances mean that there is a 33.3% chance of above, below, or average values for the period whereas only values above or below 33.3% chance are colored. With average highs in the low 30s and average lows in the low 20s during this period, we can expect temperatures significantly below these averages especially between Feb. 8-12, 2021.