Via weather.com and NOAA:
Several inches of snow are possible in parts of the Cascades and northern Rockies starting this weekend. Snow levels could fall to as low as 5,000 feet by Sunday. Campers in this region should prepare for cold temperatures. Snow is in the forecast starting this second weekend of June in the higher elevations of the mountain West, as an unseasonably cold air mass infiltrates the northwestern United States. The cold air will build in beneath a strong southward dip in the jet stream, or upper-level trough, that will develop this weekend as a result of a weather pattern flip.
National Weather Service Sacramento CA
1039 PM PDT Sat Jun 10 2017
…Late Season Snow Over Northern Sierra and Lassen Park This Weekend…
.A cool weather system will bring late season snow to the mountains of northern California this weekend. Snow levels are expected to drop to near 5000 feet late tonight into Sunday. Although snow levels could be locally lower, the main snow
accumulations are expected above 5500 feet. There could be around 5-10 inches of snow across Lassen Park with 1-4 inches possible over the Sierra. Motorists traveling across the Sierra on Sunday should be prepared for winter driving conditions.
While this is not unprecedented, the last time I can remember having winter weather and snow in the Sierra Nevada was right after Mt. Pinatubo filled the atmosphere with particulates, and caused global cooling.