Rain showers diminish Tuesday afternoon and evening, though a few isolated showers are possible during the day Wednesday. After that, skies are forecast to clear over the Columbia Basin allowing for temperatures to get below freezing in most lowland areas. This has prompted the National Weather Service to issue a freeze warning for everywhere in blue on the map above, including the Tri-Cities, Yakima, Walla Walla, and Hermiston.
Most areas will see lows in the 28° to 33° range, but clear skies and fairly calm winds will foster an environment for strong temperature inversions to develop. Well sheltered locations may see temperatures drop as low as 25° by sunrise Thursday. Sensitive vegetation should be protected from frost and now is a good time to ensure outdoor plumbing is ready for freezing temperatures.
Also shown on the map at the top of this article is a winter weather advisory (purple) for the Blue and Wallowa Mountains where 4 to 10 inches of snow may be seen above 3000 feet between 05:00 Tuesday and 05:00 Wednesday. The green area in the far northwest of the map is a flash flood watch for possible flooding, landslides, and debris flows from recent burn scars in that part of the Cascades.
On top of favorable surface conditions for cold temperatures, a strong upper level trough is forecast to pass over the Pacific Northwest Wednesday into Thursday. This will bring with it subarctic air aloft to enhance cooling overnight temperatures at the surface. On the map above, the colder subarctic air associated with the trough is seen in the pink and purple shading to the north of the jet stream (marked in yellow).
The trough continues eastward and the jet stream over the Pacific Northwest becomes more westerly on Friday to bring in somewhat warmer temperatures, increased clouds, and a renewed chance for rain.
The westerly jet stream will tap into a robust plume of moisture extending toward the Pacific Northwest from the tropical West Pacific to create an atmospheric river event. Rainfall totals west of the Cascades are likely to run up to a few inches in the lowlands with several inches close to the windward side of mountains. The Columbia Basin is likely to observe some rain out of this event, but not more than an inch or so.
The weather pattern is likely to remain active into next week, and active weather favors above freezing temperatures this time of year. Plants that are sensitive to freezing temperatures may have their growing season extended some beyond Thursday morning’s freeze if they are properly protected. Of course, I am a meteorologist and not a botanist so mileage may vary depending on the plant, level of protection, etc.
