From Pierce County Emergency Management:
A strong cold front will sweep across western Washington tonight. In addition to creating gusty winds, this front will usher in a colder air mass. Snow levels will be in the 2000 to 2500 foot range during the day Saturday.
This means that the heavier showers or any thunderstorms that develop will likely contain a large amount of small hail, possibly completely covering the ground to a depth of an inch or two. The snow level will lower into the 1500 to 2000 foot range Saturday night which will only increase the threat of hail with any showers. Also if the showers are heavy enough wet snow might be pulled briefly to near sea level.
Improving conditions are likely Sunday under weak high pressure.
For more information to: www.weather.gov/seattle