Why warmer weather sticks around for “colder” months
Even though we are well into December and almost entering the new year, it hasn't quite felt like the winter season. Yesterday, Christmas Day, the afternoon high temperature was measured to be 70 degrees at Columbia Regional Airport. That was 4 degrees shy of the record of 74 degrees for the Christmas Day high temperature in Columbia, measured back in 1889. Climatologically, the average temperature for Columbia on December 25th is only 41 degrees, meaning it was almost 30 degrees warmer than what is expected for this time of the year.
This kind of weather was observed because a large high-pressure system has been present over the United States over the past week. This high-pressure system has been driven by the subtropical jet stream, one of the two fast-flowing "rivers" of air in the upper levels of the atmosphere that direct much of the weather at the Earth's surface. The subtropical jet stream sinks south and grows closer to the equator during the winter seasons, keeping much of the warm, moist air to the south as well.
The polar jet stream, which is the second fast-flowing "river" of air, also sinks to the south and grows closer to the Central United States. This often pulls dry, cold air from the north to the south, leading to the colder weather observed during the winter seasons.
In the setup that has led to the abnormally warm weather this past week, the subtropical jet stream has resided in much of the Northern United States. This pulled all the warm air from the south into many areas that climatologically would be experiencing colder weather during this time. This may bring up the question: Even with this setup, days are much shorter in the winter seasons, so wouldn't the Earth's surface have less time to gain heat and therefore be colder?
This can be answered by bringing up an example of a cooking pan. The pan gains heat from the stove under it and stores the heat throughout the pan itself. Even after you take it off the stove and away from the heat, the heat stored inside the pan remains, and it takes a while for the heat to fully exit. Comparing this to weather, it is a similar concept to the sun heating the Earth. Even though our shortest days are in December and less heating from the sun is received, the coldest weather of the winter season often doesn't arrive until January due to the time it takes to lose that stored heat.
Looking at the forecast, it seems as though this trend of the coldest weather holding off until January is being reinforced. After a noticeable cold snap on Sunday, temperatures will be close to the single digits Monday morning, and only warm into the mid-20s by the afternoon hours. Afternoon temperatures will then get into the upper 30s and low 40s by the end of the year, which will lead to temperatures kicking off the new year staying right around average.
