An upper-level trough of low pressure is still present over the Pacific Northwest, which will result in stormy weather over the next several days. Periods of precipitation are expected to continue through early next week, with rain in the valley and snow in the Cascades to begin with.
It is projected that the temperatures will drop beginning Saturday night, allowing low-lying areas to be covered with snow by Sunday morning. The amount of snow that will fall is still up in the air, but it will almost certainly snow.
Much colder air will be with us for most of next week, with temperatures in the valleys and along the coast dropping into the teens and twenties at night and reaching temperatures around the freezing level during the day.
There are many convective cumuli connected with this storm, and there was even a lightning strike offshore this morning inside the precipitation zone heading towards the coast.
The risk for thunderstorms today and tonight will remain, mainly over the coast/coast range. However, this afternoon, we can't rule out a tiny probability of thunder in the interior valleys.
The general pattern over the weekend will feature the mean upper trough position over the Pacific Northwest with re-enforcing shortwaves that will increase precipitation at times while also reducing snow levels in certain locations.
This implies a lot of snow for the mountains, with the Cascades perhaps receiving 2 to 4 feet of snow or more by the end of the day Sunday. Because of favorable orographic flow, the Cascades south of Mt. Hood are anticipated to get the highest-end values in the region.
With 6-10 inches of snow above 1500-2000 ft expected today and overnight, with several inches expected in the upper passes around 1000 to 1500 ft along with the coast range.
Snow levels are expected to decrease further overnight, Saturday, and Sunday, resulting in a more serious effect on roadways traversing the coast range and leading to the shore.
As a result of the continuous forecast precipitation, the winter storm warnings for the Cascades and foothills and the coast range have been extended through the weekend.
Snow levels will continue to decline, reaching approximately 1000 feet by Saturday afternoon in the lowlands and along the coast. Sunday will be a transitional day for the highlands and coast.
During the day, some snow showers may reach the valley floor due to the convective air mass, which will enable more vital rains to force the snow levels down to the lowest altitudes in the mountains. However, it will be very hit or miss.
The snow levels should rise to the valley floor by Saturday night and into Sunday, with additional snow possible. However, since this is a showery trend, the amount of snow accumulated might vary significantly.
According to the National Snowfall Model, there is more than 90 percent likelihood of 1 inch or more snowfall ending Sunday afternoon, 70-90 percent chance of 2 inches or more snowfall, and 40-60 percent chance of 4 inches or more precipitation.