Unusually warm days in December usually come from a mix of short‑term weather patterns (like high pressure and warm winds from the south or downslope off mountains) layered on top of long‑term climate warming that is raising typical temperatures over time.

Not Just “Winter Is Broken”

A 70‑degree December day does not mean the season has failed; it means the local atmosphere is set up to bring in warmer air than normal.

Weather is naturally variable, so even in winter there can be strong swings above or below the average.

The Immediate Weather Setup

Meteorologists often see a few recurring patterns behind December warmth:

  • Strong high pressure parked over a region, bringing sunshine and allowing air to warm for several days.
  • Southerly winds pulling warm, moist air north from the Gulf of Mexico or other warm sources.
  • Downslope or “chinook‑type” winds off mountains that compress and heat the air as it flows downhill.

When these line up, temperatures can run 15–25 degrees above the usual December average.

Role of Climate Change

Climate change does not create one specific 70‑degree day, but it shifts the baseline so warm extremes are easier to reach and break records more often.

Recent Decembers in parts of the U.S. have set or challenged record highs, and long‑term data show more warm records relative to cold ones over recent decades.

Why It Feels So Strange

People tend to remember “what winter is supposed to feel like,” so a sweatshirt‑weather Christmas stands out more than a routine cold snap.

Online forum threads are full of comments linking unusual December warmth to both normal variability (“weather changes”) and human‑driven warming of the planet.

TL;DR:
It can be 70 degrees in December when the jet stream and high‑pressure patterns pull in warm air and sunshine for several days, and a warming climate makes this kind of out‑of‑season heat more likely and more record‑breaking over time.

Information gathered from public forums or data available on the internet and portrayed here.