Record Warmth Defines Winter 2025-26 Across the Western U.S.

Meteorological winter 2025-26, encompassing December through February, concluded as the warmest on record for a vast stretch of the western United States, according to preliminary data. A large area from Southern California to the High Plains and Northern Rockies experienced unprecedented warmth, based on 131 years of climate data. Although the contiguous United States as a whole recorded its second-warmest winter, cooler temperatures in the eastern half of the country prevented it from being the warmest on record.

Warmest Winter on Record for Many Cities

Several cities are reporting winter 2025-26 as their warmest on record, with data spanning over a century of observations. These include Salt Lake City, Utah (152 years of data), Tucson, Arizona (130 years of data) and Rapid City, South Dakota (114 years of data). Phoenix, Arizona, surpassed its previous record by nearly 3 degrees.

Beyond Temperature: The Accumulated Winter Season Severity Index

The impact of the mild winter extends beyond just temperature readings. The Midwestern Regional Climate Center’s Accumulated Winter Season Severity Index (AWSSI) – which considers both temperature and snowfall – indicates record mild winter seasons across much of the West. The AWSSI measures the cumulative impact of winter weather, and can extend beyond the traditional meteorological winter timeframe, continuing as long as winter-like conditions persist. Weather.com reports that many locations are experiencing record mild seasons.

What Caused the Warm Winter?

The dominant factor behind the unusually warm winter in the West was a persistent ridge of high pressure. This pattern allowed warmer air to prevail and steered storm systems northward, resulting in limited snowfall in the mountains. This trend is linked to the negative phase of the Arctic Oscillation (AO), a global atmospheric circulation pattern. A negative AO weakens the polar vortex, allowing colder air to spill into the eastern U.S. While trapping warmer air over the West.

Broader Winter Outlook

NOAA’s Climate Prediction Center (CPC) indicated that La Niña conditions were favored to continue into the Northern Hemisphere winter, with a likely transition to ENSO-neutral conditions in January-March 2026. National Weather Service. The Old Farmer’s Almanac predicted a mostly mild winter with pockets of wild weather, forecasting near-normal to slightly milder temperatures across most of the country, with colder-than-average conditions expected in the Appalachians, Southeast, Florida, and Ohio Valley. The Old Farmer’s Almanac

Key Takeaways

• Winter 2025-26 was the warmest on record for much of the western United States.
• Cities like Salt Lake City, Tucson, and Phoenix experienced record-breaking warmth.
• A persistent ridge of high pressure and a negative Arctic Oscillation contributed to the mild conditions.
• The lack of snowfall, coupled with warmer temperatures, has implications for snowpack and water resources.