False spring has ended and the Pacific Northwest is returning to a cool and wet weather pattern. A quick moving shortwave brought light precipitation in and west of the Cascades Wednesday and Thursday. The next system is larger, has more moisture, and will be slow moving.
Low pressure approaching Oregon will slowly meander near the coast during the day Friday. This positioning will keep most of the precipitation west of the Cascades though high clouds will obscure the sky in the Columbia Basin. It comes onshore Saturday and spins around the region through Sunday.
Over the course of the weekend, rainfall totals along the I-5 corridor are forecast to range from around three quarters of an inch (20 mm) for Portland south through Eugene to closer to half an inch (12 mm) from Longview to Seattle and lower in the Olympic Mountain rain shadow. Higher values are expected on the coast with the southern Oregon Coast looking at more than two inches (50 mm).
Rain doesn’t start in the Columbia Basin until Saturday with showers continuing into Sunday. Rainfall ranges from less than a tenth of an inch (2 mm) for Moses Lake and Ephrata to around that value in the Tri-Cities and about a quarter inch (6 mm) at Yakima. Central Oregon is also looking at around a quarter inch (6 mm). On either side of the Cascades, isolated thunderstorms are possible through the weekend but copious amounts of cloud cover will limit daytime heating. Areas that do see significant sunbreaks could recieve strong thunderstorms.
06z HRRR modeled radar reflectivity at 20:00 PDT Friday. (WeatherBell)
Mesoscale modeling keeps trying to swing a strong rainband from the low across Western Oregon beginning Friday afternoon and moving north to reach British Columbia a few hours after midnight Saturday. It may be strong enough to produce isolated to scattered thunder while it passes through.
Some lightning was observed early Friday west of Eureka, California as part of this band, lending confidence to the idea of observing thunder further north later in the day. There is a chance that an embedded supercell or two form within it as it progresses northward, but that could happen anywhere from the Cascade Crest to offshore in regions south of Tacoma.
Freezing levels are going to be dropping through the course of this event. As of Friday morning, freezing levels throughout the Northwest are around 7000 feet (2100 m). By early Saturday they drop by about 1500 feet (500 m). On Sunday the higher passes will likely get snowed on, but Snoqualmie Pass east of Seattle may only see rain or mixed rain and snow. Spokane cools down Sunday night and could have a little snow mix with rain into early Monday.
This low exits the region at the beginning of next week, heading to Colorado and then up toward the Hudson Bay bringing a possible round of snow to the Northern Plains and Canadian Prairies. The further west and north of the low’s center, the better chance of snow instead of rain. The rain/snow line may fall close to Minneapolis creating a difficult forecast for meteorologists there Monday night into Tuesday.
Active weather continues in the Pacific Northwest after this low pressure system moves out. A cold front from an Alaskan low sweeps through Monday. Later in the week models indicate the presence of some strong lows out in the Pacific coming close to the Pacific Northwest. It is too far out to give reliable specifics on what these will bring but there is a chance one or two of these get close enough to produce stormy activity later in the week.
The featured image is infrared satellite at 03:00 PDT Friday via Windy.https://ingallswx.com/2024/03/22/wet-pacific-northwest-weekend-with-slow-moving-low-pressure-system/#orwx #pdxtst #wawx #Weather
