Warmer temperatures, dense fog and more rain on the horizon for much of the Northwest

The Pineapple Express, an atmospheric river known for carrying tropical moisture from the Pacific Ocean to the West Coast, is expected to bring unseasonably warm temperatures and buckets of precipitation across Washington this week.

Meanwhile, patchy, dense fog will continue to plague morning commutes in the Spokane area, on the Palouse and in Central Washington, according to the National Weather Service.

The agency issued a hydrologic outlook Monday warning of steadily increasing rainfall Tuesday night through Wednesday morning in areas near the coast, particularly for the Olympic Peninsula and North Cascades. Areas in the region saw up to 4 inches of rain Sunday.

The precipitation will bring a renewed flood risk for rivers and streams in Western Washington, which already are dealing with heavy weekend rainfall and significant snow melt after temperatures in the Northwest reached record highs over the weekend.

The Nooksack River in Whatcom County reached a crest of 18.52 feet Monday, leading to the closure of area roads and a dog park, as reported by the Bellingham Herald.

Temperatures in Seattle hit a daily high of 61 degrees Sunday, beating the previous record of 57 degrees set in 2018. Olympia also beat the daily record high of 60 degrees set in 1976 when temperatures reached 63 degrees. Lewiston, Idaho, recorded a daily high of 59 degrees, breaking the old record set in 1931 by two degrees.

Spokane International Airport recorded a high of 54 degrees Sunday, and a high of 49 degrees on Monday. The high Wednesday is forecast to hit 52 degrees.

Those highs will steadily decrease throughout the state as the weekend approaches. Daniel Butler, a meteorologist with the agency’s Spokane office, said temperatures are expected to drop in the Inland Northwest into the lower 40s by Friday as the Pineapple Express dwindles in strength.

“It’s not looking to be an arctic plunge or anything, but it will be much cooler compared to how it’s felt,” Butler said.

Patchy, dense fog is expected Tuesday morning, later that night and into Wednesday, before giving way to a 30% chance of light rain as the atmospheric river reaches the Inland Northwest. Accumulation totals are forecast to be about a tenth of an inch Wednesday with a high around 52 degrees.

More than 45% of days since Nov. 1 have featured patchy fog, according to the National Weather Service. Butler said that although snowfall totals this winter are below average for the region, total precipitation is right around normal.

That precipitation has led to foggy morning commutes, Butler said.

“Moist soils and light winds have led to quite a bit of fog formation,” Butler said. “And the light snow we have had, when that melts, it also adds to the fog formation.”

The chance of rain will increase across the Spokane area to 50% Wednesday night and Thursday, as temperatures continue to fall. Butler said there is not much concern of flooding in Central and Eastern Washington, as rivers are far from reaching capacity.

“The rivers are running below normal, so we could definitely take a bit of additional moisture,” Butler said.

The Friday forecast calls for more rain in Spokane, with a possible wintry mix in the Cascades if temperatures drop low enough. The nightly low for Spokane is forecast around 34 degrees.

The wintry mix could arrive in the Inland Northwest on Saturday night, with nightly lows reaching the upper 20s.

“We’ll see a lot of moisture, but it definitely won’t be as bad as the western side of Washington,” Butler said.

Reporter Roberta Simonson contributed to this article.