It’s only Halloween, but winter is on the way. Two storm systems moving out of the Gulf of Alaska are on track to bring the first substantial snow of the 2017-18 winter season to the Sierra Nevada, starting Friday, and widespread rainfall across the Bay Area over the weekend. Forecasters said Monday that gusty winds and 1 to 2 feet of snow are likely Saturday and Sunday along California’s main mountain passes, including Donner Pass near Lake Tahoe, Tioga Pass at Yosemite, Ebbetts Pass and Carson Pass, with perhaps a foot along the shoreline of Lake Tahoe this weekend.