MEXICO CITY - Popocatépetl, the nearly 18,000 foot volcano that hovers like a sentinel on the southeastern fringe of Mexico's capital, awakened again Sunday, punctuating an especially shaky seismic season. Popo, as the mountain is widely called, spewed at least seven exhalations overnight Saturday and through the day Sunday, sending vapor, smoke and gas billowing into the clear sky. The most serious occurred just after 9 a.m. Sunday, sending a vapor cloud a mile into the air. Mexico's National Disaster Prevention Center issued a precautionary warning to residents, advising them to stay alert for a worsening situation and to keep...