The National Hurricane Center said yesterday evening that Tropical Storm Barbara was about 145 miles (233 kilometres) south-southwest of Salina Cruz, Mexico.
The storm has maximum sustained winds of 40 mph (64 kmph) and is standing still.
However, forecasters said the storm is expected to strengthen and should begin moving northward soon, bringing it to the coast of Mexico today.
A tropical storm warning is in effect in Mexico from Lagunas de Chacahua to Boca de Pijijiapan.