Record-Breaking Warmth Across the Western US: A Deep Dive
The winter of 2025-2026 has officially gone down in history as the warmest on record for much of the western United States. But this isn't just about breaking temperature records; it's about understanding the complex factors at play and their far-reaching impacts.
The Warmest Winter Ever?
According to preliminary data, a vast expanse of the western half of the country, stretching from Southern California to the High Plains and Northern Rockies, experienced its warmest winter on record. This is based on an impressive 131 years of climate data, highlighting the unprecedented nature of this winter's warmth.
Record-Breaking Cities
Several cities are now boasting their warmest winters ever, even those with extensive historical climate data. Salt Lake City, with 152 years of records, Tucson with 130 years, and Rapid City, South Dakota, with 114 years, all set new benchmarks. Phoenix, Arizona, shattered its previous record by a staggering 3 degrees, a remarkable feat in just three months.
More Than Just Temperatures
While temperatures are a key indicator, the story doesn't end there. The Midwestern Regional Climate Center's Accumulated Winter Season Severity Index (AWSSI) takes into account both temperature and snowfall. Interestingly, many western locations haven't just experienced warm temperatures; they've also endured a lack of snow, which can have long-term consequences due to reduced snowpack.
What Caused This Warmth?
The answer lies in the atmospheric dynamics. A persistent ridge of high pressure dominated the western half of the country, leading to consistently warmer weather. This high-pressure system also influenced the jet stream, causing storms to be steered farther north, resulting in limited snowfall in the mountains.
This phenomenon is linked to the negative phase of the Arctic Oscillation (AO), a global atmospheric pattern. When the AO is negative, the polar vortex weakens, allowing colder air to spill into the eastern US while trapping warmer air in the west. This natural cycle played a crucial role in the unusually warm winter across the West.
The Eastern Contrast
Interestingly, the eastern US, with its cooler temperatures, prevented the entire country from achieving a record-breaking warm winter. This contrast highlights the delicate balance of weather systems across the nation.
