The Northwest Ridge route is a great snow climb up Mount Islip. Access depends on yearly snowfall amounts and the SR-2 being open. I went in May 2005 and was lucky. That day we found deep snow 6-9 feet deep all the way to the covered summit. As a non-snow route I have not personally hiked it, but I think it might be a bit brushy at some points. At some point I would like to do this as a dry hike and update this. If anyone has down this as a dry route please inform me.
Take the Angeles Crest Highway SR-2, drive 40 miles on Angeles Crest Highway to Islip Saddle just a little bit past the tunnels. Park here.
Hike up the beginning of the PCT trail for the first few swithbacks until it gains the ridge slope. Take this ridge slope and go straight up following the ridgeline all the way to the summit. The ridgeline at times crosses steep slopes to the west so take care.
Round trip 2.75 miles, elevation gain 1600 feet.
For snow route you will need a ice axe, snow boots, crampons, and warm snow clothing.
Mount Islip is on the Hundred Peaks List
For current road conditions go to Angeles National Forest.