A 50-degree day in December is an unseasonable event driven by multiple atmospheric forces. Understanding this unusual warmth requires looking beyond the single day to immediate weather patterns and long-term climate trends. These factors—short-term air flow, multi-year oceanic cycles, and a warming baseline—work together to influence the likelihood and intensity of a mild winter spell.
How Short-Term Weather Systems Create Warm Spells
The most immediate cause of a December warm spell is the temporary configuration of the upper-level wind known as the jet stream. Normally, this fast-moving river of air separates cold polar air to the north from warmer mid-latitude air to the south. For warmth to extend into northern regions, the jet stream must develop a large northward bulge, or ridge, over the affected area.
This pronounced ridge effectively steers the cold Arctic air well to the west or east of the region, preventing its southward intrusion. When the jet stream loops far north, it allows warm air to be pulled from the subtropical south. This horizontal transport of heat energy by wind is called advection, which is the direct mechanism for the rapid warming felt at the surface.
A high-pressure system, often called an atmospheric block, is frequently associated with this northward jet stream ridge. The high-pressure dome is relatively stationary and acts like a roadblock in the atmosphere, diverting the normal west-to-east flow of weather systems. This blocking pattern traps the warm air mass, allowing it to linger for several days or even a week. Without the constant push of fresh, cold air from Canada, the warmth builds up, leading to the 50-degree reading.
The Influence of Natural Climate Cycles
Large-scale, naturally occurring oceanic and atmospheric oscillations set the stage for how winter will behave overall. The El Niño-Southern Oscillation (ENSO) in the Pacific Ocean is one of the most significant of these multi-year cycles. During an El Niño phase, warmer-than-average sea surface temperatures influence the atmospheric circulation across North America.
El Niño conditions typically push the polar jet stream farther north than usual, especially across the northern United States. This shift tends to favor warmer and drier winter conditions in the Midwest and northern tier of the country. The region is less exposed to cold air masses originating in the Arctic, making warm spells more likely and intense, and reducing the frequency of severe cold outbreaks.
Another important influence is the North Atlantic Oscillation (NAO), based on the pressure difference between the Icelandic Low and the Azores High. When the NAO is in its positive phase, it directs the jet stream to track northeast toward Europe. This positive phase results in higher atmospheric pressure and reduced storminess across eastern North America, limiting the frequency of cold-air intrusions from the Arctic. This promotes milder conditions across the eastern half of the continent.
Long-Term Climate Change and Rising Baselines
The most profound, underlying factor contributing to unusual warmth is the long-term trend of rising global temperatures. Though short-term weather events are driven by atmospheric dynamics, they occur against a significantly warmer baseline than they did decades ago. Global average surface temperatures have increased by approximately 2 degrees Fahrenheit since the pre-industrial era, with the rate of warming accelerating notably since the mid-1970s.
This baseline shift means that when the jet stream creates a pattern favoring warm air, the resulting temperature is higher than it would have been in the 20th century. For instance, a weather pattern that might have produced a 40-degree December day 50 years ago now results in 50 degrees because the starting point is warmer.
Data confirms this rising baseline through the increased frequency of warm weather records during winter. In many locations across the United States, the number of very warm winter days is increasing, and the coldest day of the year is now occurring at a significantly higher temperature than in previous decades. This long-term thermodynamic change amplifies the effects of the natural atmospheric patterns, making unseasonable warmth a more common and intense experience in the modern December.