A change of weather pattern is coming to the Pacific Northwest with a series of wet weather systems. This will bring a consistent chance of rain through Sunday, with the best chance being Thursday afternoon and evening. The west side of the Cascades stands to get soaked with this pattern. Some estimates have the Coast Range in Oregon having isolated spots getting over 15″ of precipitation (that’s two years’ worth of rain for the Tri-Cities). Because of the trajectory of these storms, our rainfall amounts will be a fraction of that.
Our chance of precipitation starts Wednesday night with a possible wintry mix. A wintry mix can include rain, snow, sleet, and freezing rain depending on the temperature setup. This weather pattern will provide vertical mixing of the atmosphere allowing us to get rid of the inversion, but there is a chance that precipitation will start before the inversion is entirely cleared. If we do get frozen precipitation in the Columbia Basin, accumulations are expected to be light.
Even still, commuters should plan for slick roads and slowdowns as it doesn’t take much ice on the road to cause big problems. This was the case earlier this week when people driving too fast for the conditions caused two car pileups, forcing the closure of the eastbound side of the Umatilla Bridge. Freezing fog was the source of the ice buildup that morning.
Once the bulk of the rain starts on Thursday, it should come in the form of light to moderate rain showers. These should taper off Thursday night. We have repeated low (20%-30%) chances of rain lasting from Friday into the weekend. It appears most areas in the Columbia Basin can expect up to a quarter inch of rain.
For those traveling west, this will be a headache inducer. The stream of precipitation will be like an atmospheric firehose, dumping tons of moisture in and west of the Cascades. Snoqualmie, which is the lowest of the major mountain passes in the Washington Cascades, stands to gain up to a foot and a half of snow by Friday. Those planning to cross the mountains should keep tabs on the WSDOT website for Washington and TripCheck for Oregon.
Snow levels should rise Friday, turning precipitation from snow to rain on the lower slopes and passes in the Cascades. This will come with heightened river flow due to the combination of melting snow and added heavy rainfall. Flooding is most likely on the west side of the mountains, which is where most of the precipitation will fall. Rivers draining the Washington Cascades are expected to rise, but current guidance suggests that rivers will not reach flood stage as a result of this pattern.
I will continue to monitor this situation and post updates. You can find those on Facebook at Tri-Cities Weather and on Twitter where I’m @markaingalls
The featured image for this post is modeled precipitation values from the ECMWF as displayed by Windy.
