For the east slopes I would use NWS Spokane, not Seattle. You can dial it in by the map. I would also recommend reading the "forecast discussion" they are notes on the forecast that are not published and have the forecaster thoughts on several models and what has happened historically with similar circumstances.
Also as a general rule, dry air cools @ 5.5 degree F per 1000 feet. In Wa we don't have a lot of dry air hitting the Mnt a good rule is 3.5 degrees per 1000 feet. For example Leavenworth is forecasted to be at 48 degrees today, elevation 1850, and you want to pack in the general area to 5500 subtract 12-15 degrees from that. There might be warmer air a loft, because the E Slopes can get a fair amount of cold air damming at the valley floors in late fall but that will get you a general idea.
