The crash occurred just before 2:30 p.m. on westbound Highway 20 near Lucerne, according to the California Highway Patrol.
A witness advised that the vehicle began to speed up and served prior to the collision, based on the CHP report.
Officials reported that the subject had to be extricated from a single-vehicle crash and flown by REACH, with the original destination being UC Davis Medical Center.
CHP Officer Joe Wind said the driver was believed to have suffered possible head trauma.
However, the inclement weather required the helicopter be turned back at Lake Berryessa, with it rerouted to Sutter Lakeside Hospital, according to radio reports.
Earlier in the afternoon, just after 1 p.m., another solo-vehicle crash occurred at Highway 20 and Highway 53, with a vehicle rolling over into a nearby field, according to the CHP.
Wind said it's believed that the driver took the corner too fast.
The CHP reported that the driver suffered minor injuries, including cuts to their arms, and sought their own medical aid.
With area roadways being wet due to rain, Wind urged that drivers be cautious.
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