The first half of October brought mostly sunny skies and warm temperatures to the Pacific Northwest as a persistent ridge of high pressure sat overhead, forcing the storm track into Southeast Alaska. The Tri-Cities have recorded ten days with highs at or above 80° so far this month, tying the record set in 1945.
Temperatures have already moderated somewhat so that another 80° high is not likely this week, though mostly sunny skies remain in place. This pattern comes to an end this weekend as a strong upper level trough digs south out of Alaska, bringing the storm track with it.
The show starts Thursday when a weak shortwave crosses the region to generate a westerly breeze through the Cascade mountain gaps. Gusts to 30 mph are possible along Interstate 84, but lighter winds are likely elsewhere.
Friday, a modestly strong surface low pressure system is forecast to make landfall along the Washington coast bringing the first soaking rain of the season overnight and into Saturday. Some models, such as the ECMWF, suggest rain starting in the Tri-Cities area as early as 17:00.
Rain chances continue through the weekend, but the chance of precipitation is much lower. By the time we roll into Monday, around half an inch to an inch of rain is possible for the Columbia Basin (most of it falling Friday night/Saturday morning). Breezy to windy conditions are also possible at times.
The low and associated cold front will also usher in much cooler temperatures. By Sunday, highs around 60° are expected. Snow levels in the mountains are forecast to drop to around 3000 to 4000 feet for the first significant snowfall of the season.
Travel impacts should be minimal on Snoqualmie Pass, but heavy snow is possible on higher pases (such as White Pass) on Saturday with a chance of rain and snow showers for the foreseeable future. Up to a foot of snow is possible this weekend on the upper slopes of higher elevation ski areas in the Cascades.
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