Meteorological winter begins Dec. 1 as Western WA braces for colder weather

Meteorological winter starts on Dec. 1. Yet, winter doesn’t officially begin until the winter solstice, which is on Dec. 21, just after 7 a.m. Why is there a difference when winter gets underway?

The winter solstice, the shortest day of the year in the Northern Hemisphere, is based on the astronomical seasons people have used to mark time for thousands of years. Astronomical seasons are based on the position of Earth as the planet rotates around the sun. Earth’s 23.5-degree tilt and the sun’s alignment over the equator determine both the winter and summer solstices, along with the spring and fall equinoxes.

Many think of winter as the coldest time of the year and summer as the warmest, with spring and fall being the transition seasons. That is the basis for meteorological seasons. Meteorologists and climatologists around the globe divide the seasons into groups of three months based on the annual temperature cycle and the calendar.

The coldest months of the year are December, January, and February, hence determining meteorological winter in the Northern Hemisphere. The calendar also makes it much easier to calculate seasonal statistics from monthly weather data versus astronomical seasons — very useful for commerce, agriculture, and a variety of other needs.

Right on time, the coolest weather of the season thus far will unfold this first week of December. High temperatures will struggle to rise beyond the 40s with lows in the 30s. Frost or freezing fog is quite possible, particularly in the south sound region through Wednesday. On bridges and overpasses, slippery, icy spots are more likely. Have that ice scraper handy for those frosty windshields and windows.

Before the end of the week, a much more active mild weather pattern is anticipated. The first in a parade of Pacific weather systems is slated to bring rain to Western Washington late Thursday into Friday. If longer-range weather charts are on track, additional weather systems are forecast to follow well into next week.

Mountain snow

Snow enthusiasts anxiously await the arrival of a lot of snow in the mountains and the opening of the ski areas. Last year, many resorts had opened by Thanksgiving weekend. Not the case this year.

The more active weather pattern developing late this week and into next week promises to deliver snow in the mountains. This weekend, snow levels are expected to be around 3,500 feet, meaning healthy amounts of snow for many of the ski areas. For motorists on mountain passes, the shift to a cooler, snowier weather pattern means being prepared for winter driving conditions.

November rain and temperatures

November just wrapped up, and it is usually the wettest month of the year. For much of the month, that reputation held firm. Then, higher pressure aloft built over the Pacific Northwest, resulting in a dry finish to the month along with some sunshine.

Despite what seemed to be a wet month of November, the Seattle-Tacoma International Airport (SEA) finished close to a half inch of rain below average, with nearly five and three-quarters of an inch. November was the tenth month of the year with below-average rainfall. March was the only month that was wetter than average by about an inch.

Along the coast, both Hoquiam and Forks also finished the month drier than normal. In contrast, Olympia was about a half inch wetter than average, while Bellingham was over an inch above normal.

For the year, Western Washington remains much drier than normal. SEA is close to eight inches below average; Olympia has a rain deficit of over 11 inches; Hoquiam, about 10 inches; and usually soggy Forks is now more than 28 inches drier than usual.

According to the U.S. Drought Monitor, much of Western Washington remains in moderate to severe drought conditions.

November was also warmer than usual. SEA was more than two degrees above average, while Olympia was three degrees warmer than normal for the month.

Weather outlook for December

Long-range weather guidance indicates a wet period next week, with snow in the mountains. Toward the middle of the month, it is possible that a much cooler weather pattern could develop. This change bears watching at this point. With a La Niña pattern in place heading into this winter, it is not uncommon to have at least one lowland snow event.

For those who desire a White Christmas, the odds of that kind of day for Western Washington are slim — about a 5% chance. The last significant White Christmas was in 2008, when many parts of Western Washington had more than six inches of snow on the ground for a festive day.

Meteorological winter is underway. With La Niña in play, at least cooler weather should unfold as winter approaches. Stay tuned!

Ted Buehner is the KIRO Newsradio meteorologist. Follow him on X and Bluesky. Read more of his stories here.