Rain in December

Observation by Denail Henderson:

It has been raining in Alaska for a couple days even though it is early December. Usually by now it never rains and is constantly snowing, but it has been unusually warm. I noticed a couple days before it started raining that when I opened the door it wasn't as cold as it normally is. Most of the time I try to close it quickly after letting my dogs out, but recently its been cool and refreshing instead of bitterly freezing. When it started raining the next day I knew I wasn't just painstakingly tired or crazy and it was actually warmer than it should be.

LEO Says:

After two December's with frequent winter snow storms and shoveling roofs and driveways in 2022 and 2023, it appears we are in a warmer December to wrap up 2024. Southern 'chinooks' (chinook winds) can roll in during winter time in December increasing the chances of rain. Chinook winds which often are the harbingers of warm weather are not as common as in some other regions, such as the eastern slopes of the Rocky Mountains and Canada. However, they do occur occasionally and in Anchorage and are typically more frequent in December when warm, moist air from the Pacific Ocean moves through the region. The frequency of chinook winds in Anchorage can vary from year to year, but they are generally not a regular occurrence. When they do happen, they can lead to significant temperature changes, sometimes causing snow to melt rapidly. If you are interested in specific data or patterns, local meteorological records or the National Weather Service in Anchorage would provide more detailed information.

Consult by Rick Thoman:

The 0.21 inches of rain that fell December 4-5 at Anchorage Airport is the most rain in December since 2018, when 0.37 inches of rain fell December 2-3. The rain was caused by a warm surge of air from the eastern Pacific, pushed north by the combination of strong high pressure aloft over British Columbia and a storm system that tracked northward through the Gulf of Alaska to southern Cook Inlet before dissipating.

Comment from Gemini:

The attached observations highlight the variability of December weather in south-central Alaska, particularly contrasting the recent rain event with the heavy snowfall of Decembers 2022 and 2023. In 2022, Anchorage experienced its wettest December on record, although snowfall accumulation was second highest to 1955, with heavy, wet snow causing traffic disruptions (Anchorage digs out..., 2022). This wet trend continued into 2023, with near-double snowfall compared to average by mid-February (Anchorage has seen..., 2023). The change from heavy snow to rain underscores the influence of warmer temperatures and Pacific air masses, as explained by Rick Thoman in the main observation. The 2021 Fairbanks observation (Impacts of Fairbanks..., 2021) further illustrates the disruptive effects of mid-winter rain, with rain percolating through snowpack and creating icy conditions. The 2016 Kenai Peninsula observation (Freezing Rain..., 2016), while focused on freezing rain, underscores the vulnerability of the region to changing weather patterns and the potential for hazardous conditions with fluctuating temperatures. The shift from historically snowy Decembers to rain in 2024 aligns with broader climate trends in Alaska, including shorter snow seasons and concentrated snowfall periods (December was Anchorage's..., 2023). The Alaska Climate Change Impacts Assessment Report (https://www.snap.uaf.edu/sites/default/files/Ch14_Coastal.pdf) documents these changes, noting increasing temperatures and altered precipitation patterns across the state.