ATLANTA, July 20 (Reuters) - The fourth tropical storm of the 2008 Atlantic hurricane season formed in the western Caribbean Sea on Sunday, the National Hurricane Center said.

Mexico, meanwhile, issued a tropical storm warning for the Yucatan Peninsula from the border with Belize to Campeche, Mexico, over tropical storm Dolly that is also crossing the Caribbean near the Gulf of Mexico, and tropical storm conditions were expected within the warning area in the next 24 hours, the Center said. (Writing by Matthew Bigg; editing by Philip Barbara) (matt.bigg@reuters.com; Atlanta bureau: 404 720-2891; Reuters Messaging: matt.bigg.reuters.com@reuters.net))

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