From Enumclaw [1], the winding route of Highway 410 cuts through the northeast corner of the park until it hooks east at Crystal Mountain [2], threading through Chinook Pass [3] over to Naches and the Yakima Valley [4] on the eastern side of the mountains.
Highway 410 southeast of Crystal is a fair-weather route, closed in winter and often dusted with snow as late as June.
Links:
[1] http://www.moon.com/destinations/washington/south-cascades/cle-elum-area/enumclaw
[2] http://www.moon.com/destinations/washington/south-cascades/chinook-pass-and-highway-410/crystal-mountain
[3] http://www.moon.com/destinations/washington/south-cascades/chinook-pass-and-highway-410/chinook-pass
[4] http://www.moon.com/destinations/washington/columbia-gorge-and-wine-country/yakima-valley