I recently recorded some data from my personal weather station that was very odd for this time of year. During December 2025, normal 8 AM temperatures from my personal weather station are about 38 to 45 degrees Fahrenheit. However, from around December 15 2025 to December 25 2025, the temperatures in the morning were unusually high. The highest temperature I recorded during that week was a whopping 64.7 degrees fahrenheit, while the lowest temperature during that week was 51 degrees, which is still higher than the average. I was extremely confused at how a winter morning could actually be warmer than winter afternoons like December 3 2025, where at 2:30 the temperature was only 63.0 F at 2:00 PM.
I did some research1 and it turns out that the reason for this unseasonable warmth is the La Niña weather pattern. La Niña is a weather pattern phenomenon that strengthens the easterly trade winds, resulting in warm water being pushed westward, where California is. The warm water helps to send warm ocean breezes to the east coast of California. Below is a graph of the temperatures of the month of December. As you can see, there is a large spike at the end of the month roughly between December 15 and December 25.
Since this effect is primarily in the San Franciso Bay area, it alludes to the idea that a microclimate is created due to the unique landscape. The Bay Area is located right between the Pacific Ocean to the west and large mountains to the East. The mountains act like a physical barrier that prevents the warm air from La Niña from moving away. During La Niña conditions, the Pacific gusts shift northward, pushing high pressure cold winds up north while bringing down warmer winds to the coast.
The warm coastal air doesn’t spread out much because it is blocked by the mountains on one side and more warm air over the Pacific ocean on the other side. As a result, inland California suffers colder winter while the Bay Area cruises through a warmer holiday season.
Source:
- This is the article I read: https://www.usatoday.com/story/news/california/2026/02/05/why-is-it-so-hot-socal-la-heat-wave/88527523007/
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