The pilot reported that he was preparing to land the helicopter at an off airport landing site and encountered a wind shift or wind shear on base leg that resulted in an uncommanded yaw followed by a rapid descent rate. Before impacting the ground, the pilot was able to reduce the descent rate and level the helicopter. The accident site was desert terrain that sloped downward from the right to the left. During the accident sequence the helicopter rolled over. The pilot said that people on the ground told him that just prior to the accident there was a sudden wind storm that passed through the area that forced the people on the ground to take shelter inside an enclosure to escape the wind. The pilot stated that he had no communication with the people at the landing site and no knowledge of the wind storm. The pilot further stated that the helicopter and engine had no mechanical failures or malfunctions during the flight. The closest weather observation station, located 25 miles to the west, was reporting westerly winds at 20 knots, with gusts to 26 knots, at the time of the accident.