GARDNERVILLE, Nev.—A Northern California wildfire crossed into Nevada, prompting new evacuations, but better weather has been helping crews battling the nation’s largest blaze in southern Oregon. The Tamarack Fire south of Lake Tahoe had burned more than 68 square miles of timber and head-high chaparral in national forest land. It erupted on July 4 and was one of nearly two dozen blazes sparked by lightning strikes. More than 1,200 firefighters were battling the Alpine County blaze, which has destroyed at least 10 buildings, forced evacuations in several communities, and had closed parts of U.S. 395 in Nevada and California. Fire officials expected active or extreme fire behavior on Thursday, which could see 14-mph winds and temperatures approaching 90 degrees. A request for voluntary evacuations was also issued for portions of Douglas County, Nevada. An evacuation center was set up at a community center in Gardnerville, Nevada. Evacuee Morgana-Le-Fae Veatch said …