Mexico's National Water Commission reported that the storm touched land in the evening Monday just east of the town of Ayutla, in Oaxaca state.
The US National Hurricane Center said the storm's center was about 40 miles (70 kilometers) west-southwest of Salina Cruz late yesterday, and it was moving northwest at 5 mph (7 kph).
It had maximum sustained winds of 40 mph (65 kph) but was expected to weaken as it continued heading inland.
A tropical storm warning remained in effect for the coast from Salina Cruz to Laguna de Chacahua, and the center said further heavy rains and flooding were expected.
Tropical Storm Beatriz soaked the same area with torrential rains in early June.
Calvin is the third tropical storm of the eastern Pacific season, which began May 15.
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