PHOENIX — A dangerous heat wave threatened a wide swath of the Southwest with potentially deadly temperatures in the triple digits on Saturday as some cooling centers planned to extend their hours and emergency rooms prepared to treat more people will heat-related illnesses. “Near record temperatures are expected this weekend!” the National Weather Service in Phoenix warned in a tweet, advising people to follow its heat safety tips such as drinking plenty of water and checking on family members and neighbors. “Don’t be a statistic!” the weather service in Tucson advised, noting that extreme heat can be deadly.