Mexico placed a stretch of the west coast from Mazatlan top Altata on warning status due to Sonia's northerly course, which could bring tropical storm conditions to the mainland within 24-36 hours, the National Hurricane Center, Miami, reported.
A tropical storm watch was posted from Altata to Topolobampo.
Sonia's coordinates in the in the Pacific at dawn were 18.8N and 110.0W, forecasters said in a written statement at 9 a.m. EST Sunday. The storm was expected to skirt the tip of the Baja Peninsula Sunday night before making landfall early Monday.
Sonia was packing sustained winds of 35 knots early Sunday and churning up 12-foot seas, the Hurricane Center said.