Weather Underground Forecast for Saturday, May 18, 2013.

Tropical cyclone activity has developed in the Eastern Pacific Basin. On Saturday, Tropical Storm Alvin will continue moving northeastward and remains well southwest of western Mexico. The system will maintain tropical storm strength with winds between 39 and 73 mph (33 and 64 knots). Elsewhere, tropical cyclone activity is not anticipated for the tropical Atlantic Ocean on Saturday. However, areas of active weather will persist for the Caribbean Sea and Latin America. In the north, a trough of low pressure over the Central U.S. will extend into Mexico, which will pull moisture in from the Gulf of Mexico. This will kick up showers and thunderstorms across central and southern Mexico throughout the day. Mexico City will be mostly cloudy with showers and thunderstorms and highs in the mid-80s(F)/ upper 20s(C). To the south, a monsoonal trough of low pressure pushes abundant moisture onshore into Central America from the Pacific Ocean. This will bring moderate to heavy shower and thunderstorm activity to Panama and Costa Rica.

In South America, moderate to heavy showers and thunderstorms will develop over Venezuela as onshore flow from the Caribbean Sea returns. This will bring periods of heavy rainfall Saturday night into Sunday. To the south, a trough of low pressure sweeps across Paraguay and southern Brazil, kicking up heavy rainfall and strong storms. Heavy precipitation will persist into northern Chile along the back side of this system.