The National Weather Service has issued a winter weather advisory for the Columbia Basin and Yakima Valley from 10:00 Monday to 10:00 Tuesday. The Cascade Mountains also have an alert posted, but Walla Walla was left out of the advisory area.
Light precipitation is forecast to spread from west to east across Eastern Washington and Eastern Oregon Monday with showers lasting overnight. This will result from a low pressure system approaching Alaska which will drag a warm front followed by a cold front across the Pacific Northwest.
This is a messy forecast with temperatures forecast to be close to freezing and multiple possible scenarios being reasonable. The alert from the National Weather Service specifically lists 1-2 inches of snow and a tenth of an inch of freezing rain in the lowlands.
It looks like the most likely situation around the Tri-Cities and Hermiston is for precipitation to start as freezing rain before transitioning to rain as temperatures warm behind the warm front. Some pockets of freezing rain may linger, and snow is possible in higher elevation neighborhoods around Kennewick and Richland.
Cold air damming, which captures cold air against the leeward side of a mountain range, may cause precipitation to start as snow but then switch to freezing rain in the Yakima Valley. Freezing rain is likely to last longer around Yakima than around the Tri-Cities before transitioning to rain.
Ice accumulations will be light in the Yakima Valley but it doesn’t take much to cause travel problems. Pockets of snow are possible here, too, but most of it will be toward the north in places like Wenatchee and Ellensburg.
Locations east of the Tri-Cities are on the windward side of the Blue Mountains and won’t experience cold air damming. This includes Walla Walla and Pendleton which are likely to just get rain.
What is described above is what I see as being the most likely scenario. There is about a 30% chance of the Tri-Cities seeing just rain out of this event and a 20% chance of precipitation falling as snow before the transition to rain.
People traveling during the advisory period should be prepared for winter driving conditions. If snow does fall in the Tri-Cities and/or Yakima, it will likely be light and messy and not something looking like a scene from a Hallmark Christmas movie.
Light precipitation may start mid-morning but the bulk of it is likely to fall Monday night into early Tuesday. Regardless of what form the precipitation takes the expectation is for it to be the equivalent of up to a quarter inch of rain. For folks that see this as all snow, this comes out to up to two inches.
