Tuesday is likely to be the warmest day of this weather pattern for the Columbia Basin with the Tri-Cities reaching into the upper-70s (26°C). Temperatures in some areas west of the Cascades peaked on Monday. Portland recorded a high of 77°F (25°C), a new record for March 18, while Seattle peaked a few days ago. The warmth shifting east of the Cascades is the result of a strong upper level ridge over Western North America shifting east to favor the Columbia Basin a bit better.
Despite cooling to near or below 70°F (21°C) along I-5, most people in Western Washington and Western Oregon should be able to experience one last day of mostly sunny skies. Onshore flow increases some over the course of Tuesday for increased clouds. Stratus is already present Tuesday morning along a lot of the coast. This moves slowly inland and the I-5 corridor between about Vancouver and Olympia will likely be mostly cloudy by sunset. Portland and Seattle may notice increased clouds in the afternoon but most cloud cover moves in overnight.
The onshore push will be notable through and downwind of mountain gaps like the Columbia Gorge. Westerly wind gusts to 30 mph are possible from Bonneville Dam to Boardman. The Tri-Cities, Yakima, and Hermiston may notice increased wind (gusts to 20 mph) during the afternoon and evening. This eases overnight into early Wednesday except higher elevation spots like the Horse Heaven Hills and in/near the mountain gaps. Wind picks up again in the Columbia Basin and Yakima Valley after sunrise Wednesday.
The ridge breaks down midweek allowing wet weather to return. The first system works through Wednesday and Thursday but is fairly weak with only light precipitation in and west of the Cascades. A moderately strong Pacific low approaches the Oregon Coast Friday into Saturday. Region-wide rain and mountain snow are forecast for the weekend. Highs will be much cooler by this point – mid-50s (13°C) for Portland and the Tri-Cities, low-50s (11°C) in Seattle, and mid-40s (7°C) in Spokane.
On the heels of this low is a shortwave trough to keep widespread rain and mountain snow in the picture into Sunday. Snow levels dip below 3000 feet (900 m) near and north of Mount Hood in Oregon over the weekend. Spokane could even see an inch (2 cm) of wet snow out of this pattern. Their National Weather Service office gives the chance of one inch of snow over the course of Friday through Monday at 40% for Spokane and 75% for Sandpoint. Note that that’s for over the course of the entire weekend and, especially for Spokane, the precipitation may oscillate between rain and snow with snow becoming more possible later in the weekend.
Travel may be difficult at times over Cascade passes through the weekend. As of end of day March 17, Pacific Northwest snowpack was at 87% of median for that date. This week’s snow will add to the snowpack but the region has likely peaked for the year; earlier than the median peak date of April 4. Weather models continue to favor the cool and moist weather lasting into the beginning of April. While snowpack is likely in the melting phase for the season, the rate of melting is likely to be slow at least for the next two to three weeks.
A few ensemble models do have an additional heat wave around the end of March but these represent only a small fraction of modeled solutions. The majority of ensemble members show 850 mb temperatures closer to average with repeated chances of precipitation. Early Spring is the coldest time of year for 850 mb temperatures over the Pacific Northwest, thus near average temperatures foster an environment for showers and thunderstorms in the 12-24 hours after a cold front passes. This is especially true during the afternoon on days with some clearing due to daytime heating at the surface being stronger. Conditions like this lift moisture up to create clouds more efficently.
