The third system in a train that started late last week is forecast to reach the Pacific Northwest during the day Monday. High clouds from this are already reaching the Interstate 5 corridor as of 07:00. Rainfall totals through Tuesday reaching half an inch (10 mm) are forecast for Portland, Seattle, and Vancouver with higher amounts on the coast and perhaps a quarter inch (5 mm) in Spokane with under a tenth of an inch (2 mm) in the Tri-Cities and Moses Lake. Higher amounts in thunderstorms.
Snow levels are likely to be around 4000 ft (1200 m) during the day Monday and closer to 3000 ft (900 m) on Tuesday except around and south of the Three Sisters west of Bend, which will be warmer. New snow amounts in excess of a foot (30 cm) can be expected in the higher elevations of the Cascades, Vancouver Island, and British Columbia Coast Ranges. Lower amounts in inland mountains.
This starts with postfrontal showers west of the Cascades thanks to moisture and cool air aloft leftover from Sunday’s regional storm. Steady rain from the incoming system reaches the coast mid-morning and Portland and Seattle early afternoon. The Columbia Basin will, of course, experience the Cascade rain shadow but may observe some light rain or drizzle into the evening.
About six hours of steady light rain is likely west of the Cascades before switching to the postfrontal shower regime. Isolated thunderstorms are possible in the Oregon Coast Range Monday evening but cool temperatures from persistent cloud cover during the day should limit this sort of activity throughout the Northwest.
If Western Washington and Oregon can get some sunbreaks Tuesday morning the chances of afternoon showers producing lightning will be improved as sun hitting the ground will increase daytime heating and 850 mb temperatures drop down to around -5°C. It doesn’t appear that this is very likely, but the possibility exists. The Columbia Basin has a better chance of periodic sunshine but mid-level temperatures will be warmer on that side of the mountains so while isolated thunder is possible, most folks in places like the Tri-Cities, Yakima, and Moses Lake aren’t likely to experience lightning.
Some leftover showers may linger on the west slopes of the Cascades on Wednesday but we start coming under the influence of a ridge of high pressure building along the West Coast of North America. Mostly sunny skies are expected in lowland areas east of the Cascades beginning Wednesday and lasting beyond the weekend. In cities like Portland and Seattle, mostly sunny skies begin Thursday.
Weekend highs will probably reach near to above 70°F (21°C) in Portland and 65°F (18°C) in Seattle. This is pretty early in the season to be getting that warm. The average first 70°F (21°C) high in Portland is April 1 while the average first 65°F (18°C) high in Seattle is March 26. There is some uncertainty with highs in the Columbia Basin this weekend. Currently favoring highs closer to those Seattle will see because of possible northeasterly flow bringing cool air off the Canadian Rockies, but if that pattern does not develop the Tri-Cities could get warmer than Portland.
The featured image is satellite imagery for the Pacific Northwest and adjacant Pacific Ocean at 07:31 PDT Monday.
