When riding Blewett in the summer I avoid Friday (especially afternoons) going north to Leavenworth/Wenatchee and Sunday coming south. The reason is there's just too much traffic from the Seattle area which inlcludes a lot of RV's and with the narrower shoulders the last few miles to the top can make for a little more stressful ride. If you do have to ride those days then definitely take the Old Blewett road.

I've done the ride on a loaded touring bike (going north from Ellensburg) on a Monday in summer and traffic was much, much better.

At 4000 ft it's not that bad on top when the temps are in the 90's in the valleys. On the Cle Elum side you have the Swauk and Mineral Springs campgrounds where you can refill with water.