Early December brings start of ‘meteorological’ winter

November is usually the wettest month of the year, and that reputation held firm for much of the month. Then higher pressure aloft built over the Pacific Northwest, resulting in a dry finish to the month since the 28th, along with some sunshine.

Sunday, Dec. 1 marked the start of meteorological winter. This period of dry weather is expected to continue through the middle of this week as meteorological winter gets rolling. Expect more sunshine after areas of morning fog and low clouds through Wednesday. High temperatures will remain cool, rising mainly into the 40s with lows in the upper 20s and 30s.

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Morning frost and the likelihood of icy spots on bridges and overpasses will be present. Have that ice scrapper handy for those frosty windshields and windows.

By Thursday, the high pressure aloft is anticipated to give way to an incoming Pacific weather system, bringing an end to the week-long dry spell. Rain is expected to develop and extend into Friday and Saturday. High temperatures should moderate, warming into the mid-40s to lower 50s and lows primarily in the 40s.

In the mountains, warmer air aloft associated with the upper-level high pressure will push freezing levels around 10,000 feet through Wednesday, though afternoon pass temperatures will only rise into the mid-30s to mid-40s, helping preserve the current snowpack.

The dry period to finish November resulted in yet below-average rainfall for the month across much of Western Washington. The Seattle-Tacoma International Airport (Sea-Tac) finished about 1.5 inches drier than normal, the eighth month this year that was below average. Only Hoquiam was wetter than normal by about 1.5 inches of rain.

For the year, Sea-Tac is now just over seven inches of rain behind the annual average. Olympia is just under four inches below average this year. Meanwhile, locations closer to the Canadian border are currently above average for the year. Forks is about 5.5 inches wetter than normal, while Bellingham is just over 6.5 inches above average for the year.

As noted earlier, meteorological winter started on Sunday, Dec. 1. Yet, the calendar says winter begins on the winter solstice, this year on Dec. 21, just after 1 a.m.

Why does the start of winter differ?

The winter solstice, the shortest day of the year in the Northern Hemisphere, is based upon astronomical seasons that people have used to mark time for thousands of years.

Astronomical seasons are based upon 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 and spring and fall equinoxes.

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

The coldest months of the year are December, January and February, which determines the meteorological winter. 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.

In summary, astronomical seasons are based on the position of Earth in relation to the sun, while meteorological seasons are based upon the annual temperature cycle and the calendar. And now you are ready for that trivia question!

Ted Buehner is the KIRO Newsradio meteorologist. You can read more of Ted’s stories here and follow him on X