Officials are on high alert and pleading with residents to flee flood zones as a potentially life-threatening storm prepares to wallop Boston and other coastal communities today and tomorrow, soaking the coast with 15-foot tides while battering it with 80 mph winds.“This could be very dangerous. There could be casualties if people aren’t smart and don’t heed officials,” said Kim Buttrick, a meteorologist in the National Weather Service’s Taunton office.