This one goes back to the "soundings" or atmospheric temperature and dewpoint at elevation. To find this, you can go to NWS and just search for Quillayute sounding. A graph will pop up with two plots, a plot of temperature and a plot of dewpoint. When the two come together at certain elevations that means that there is cloud cover (the temperature is near the dewpoint) and with orographic effects, likely precipitation. I think one would find in these conditions, Pine Express, that clouds will go to 30000 feet or so. Post cold frontal passage and orographic effects are more likely to have tops halfway up the Muir Snowfield. The sounding for 4pm at Quillayute (UIL) shows solid clouds to 570 mb (maybe 16000' and multiple cloud layers extending to 500mb probably to 18000 ft.
weather.unisys.com/upper_air/skew/skew_KUIL.html
I believe there was a previous discussion on soundings and I think it was said that Salem, Ore. also has soundings. During a Pineapple Express, the upper level winds would usually be from the SSW, or on a line from Salem to Mt. Rainier. Hence, the Salem sounding would translate in an hour or two to the conditions on Rainier. Soundings are updated at 4 am and 4 pm in our area. There is also a sounding at Spokane. Is there one at Boeing Field too?
Anyway search for the soundings tomorrow AM and the freezing levels will be obvious. Freezing level on this sounding about 8500'. One can also look at 850mb and 700mb temperatures in various model runs. Try "Unisys 700mb models" for starters.
