Forecast: December 20th, 2022

Last updated 9:45 PM, Tuesday, December 20th, 2022 

By Anthony Edwards

Happy Tuesday!

Hopefully everyone enjoyed the snow day in Seattle! The UW Seattle and Bothell campuses suspended operations today for those staff members that are still working over winter break. Bothell is closed through Friday. UW will open at 10 a.m. Wednesday.

The expected snowstorm delivered a sharp north to south gradient of snowfall, with more than a foot accumulating at spots near the Canadian border and zero snowfall down in Olympia. In the middle, Seattle saw snow to start and end the event, with a few hour period of rainfall Tuesday morning.

Seattle ended up with a few inches of snow, but the official observation site, Sea-Tac, will go down with even less due to being on the south side of the city. That helps balance out earlier this month with the airport received quite a bit of snow, while downtown was mostly left out.

Some showers linger around this evening and overnight. Spotty accumulations of an inch or two are possible.

Attention now turns to extreme cold (at least for Western Washington standards) for the next few days.

Temperatures won’t rise above freezing until Friday, with highs Wednesday and Thursday likely in the mid-20s. Seattle hasn’t had a high temperature below 25 degrees this decade. The last such instance was in 1999. Thursday’s current forecast high is 24ºF.

Overnight lows will be even more extreme. Temperatures Tuesday night will be in the mid-to-upper 20s in the city and colder in outlying areas. Wednesday also has a shot to be our coldest night this decade. A low below 14 degrees would do this, but current thinking is that we’ll stay “warmer” in the mid-to-upper teens. Thursday night may also touch the upper teens. Areas with more snow cover and away from the water may hit single-digits Wednesday and Thursday nights.

Protect plants, pipes, and people from cold weather! If it’s too cold for you, it’s too cold for your pet!

Thursday afternoon into Friday may be our most hazardous weather day in the past decade in Seattle.

A widespread freezing rain event is in store for Western Washington. This is a dangerous precipitation type that is rare for Seattle. We wrote about what to expect for freezing rain in a post earlier today.

Precipitation begins Thursday evening, starting at the southern Washington coast and moving toward the metro area throughout the night. This will mainly start as snowfall, aside from the immediate coast, where rain or freezing rain is possible from the get-go.

Heading into Friday morning, warm air spreads over the deep cold airmass in place. This means liquid precipitation will freeze immediately upon hitting a frozen surface.

Any accumulation of freezing rain is impactful, turning roadways to ice, but as it accumulates it can begin to topple branches, power lines, and structures, especially those already weighed down by snowfall.

The timing of the surface warming above freezing is tricky to forecast, but the warm-up will be slow, and many places such as the southern and central Puget Sound may be stuck in freezing rain all the way through the day Friday. This could lead to accumulations from a quarter to a half-inch.

Flight delays are a near certainty at Sea-Tac Airport and roadways will be a disaster. Do anything in your power to stay home Friday.

Christmas Eve and Christmas Day will be above freezing for everyone in Western Washington, well into the 40s. Urban flooding is possible with heavy rainfall on top of all the frozen precipitation. Take it slow if you’re driving around for holiday festivities.

Happy holidays!

Reach forecaster Anthony Edwards at theuwdawgcast@uw.edu or on social media at Twitter: @TheUWDawgcast and Instagram: @UWDawgcast