High pressure, which had brought our brief period of mostly clear skies and dry weather across the Inland Northwest, will begin to shift east during the day today. This will open the door for a series of upper waves to move into the region and increase the chances of lower elevation showers and high mountain snow. Expect mostly cloudy skies in Northcentral Washington today and fairly mild temperatures with highs this afternoon near 60 degrees.
By Wednesday morning, the first weather wave will transport a decent fetch of moisture and precipitation into the northern Cascades in the broad southwest upper flow increasing across the Pacific Northwest. As the cold frontal boundary moves into Central Washington, showers will spread into Eastern Washington by afternoon. The Wenatchee Valley has a 50% chance of rain showers in the morning increasing to a 70% chance by Wednesday night with afternoon high temperatures in the upper 50’s. Measurable precipitation in Central and Eastern Washington will be on the light side possibly accounting for only about 0.01 to 0.04 inches with some mountain areas across the northern Cascades seeing as much as a half inch of precipitation.
Thursday through Saturday, an active autumn weather pattern is on tap as a strong Pacific jet sends a series of weather systems through the Pacific Northwest. Weather models have been in good agreement for several days advertising the upcoming wet and windy pattern and confidence is above normal. What is less certain is exact strength and timing of each shortwave passing through. At this time, it looks like there will be at least three distinct waves to focus on.
The first shortwave traverses the Northwest on Thursday bringing windy conditions out of the west 10-20 mph during the day and then a 40% chance of showers by Thursday night with highs once again near 60.
Thursday`s shortwave will carve out a broad upper-level trough across much of the northern tier of the U.S. and we should see a second shortwave sometime in the Thursday night to Friday morning time-frame. This will be a relatively cooler but not as moist air mass with shower concentration once again focused along the Cascade Crest but also expanding into the Columbia Basin when the shortwave dynamics swing trough. There is a 50% chance of rain Friday with highs in the upper 50’s.
The last disturbance dropping into the region arrives on Saturday and has the potential to be the wettest of this forecast period. This will also be the coolest of the three storm systems and bring a decent shot for snow on the area mountain passes and mix of rain/snow down to 2000 feet for the northern valleys of and possibly northern and eastern reaches of the Columbia Basin. With a 50% chance of rain Saturday expect highs in the lower to mid-50’s. Each weather system will deliver breezy to locally windy conditions, especially across the Columbia Basin and Palouse. Scattered showers for Sunday and Monday with afternoon highs in the mid 50’s.