Nikki Dettmar walked her fourth-grade son to the school-bus stop at about 11 a.m. Tuesday before heading to her job at the University of Washington. But 15 minutes after getting to work, the 38-year-old medical librarian was back in the car, heading to Broadview-Thompson K-8 to pick up her son. The school in northwest Seattle, after starting two hours late, was ending two hours early. “My son was at school for 45 minutes,” Dettmar said. “All the kids had time to do today, pretty much, was eat lunch and then they had to go home.” Dettmar was among thousands of parents across the city who had to scramble when Seattle Public Schools shut down early Tuesday. Parents were notified about the early closing by robocalls and emails, but some were given as little as half an hour to get their children. Despite clear skies in parts of the city, officials decided that snowy conditions and slick roads in North Seattle made it imperative to get all kids home as soon as possible. They had earlier delayed the start of school because of concerns about early-morning conditions — worries that ultimately proved unwarranted. “Predicting winter weather in Seattle is a challenge,” Interim Superintendent Susan Enfield said in a news release. “Safety for students and staff always comes first.” The district announced later that its schools will be closed Wednesday because of a storm that was expected to bring several inches of snow. As excited students made plans for their first snow day since November 2010, some parents wondered: How exactly does Seattle Public Schools decide how to react to severe weather? The decision rests with Enfield and Assistant Superintendent of Operations Pegi McEvoy, explained district spokeswoman Lesley Rogers. They rely on communication with the city’s high-tech Emergency Operations Center in addition to Weather Service reports and school-bus driver dispatches. Officials always assume the worst-case scenario, Rogers said. If forecasts indicate that transporting kids in one part of the city could be hazardous, all schools will be closed because of the district’s interconnected bus network, she said. Once a decision to delay or close schools has been made, the district activates its emergency alert system, which includes emails and robocalls. Parents can choose which type of phone — home, cell or work — to list as their contact numbers. Elementary schools go even further. “Before we can put a child on a bus early, or before we can have a child walk home, we have to physically talk to an emergency contact,” said Zoe Jenkins, the principal at Olympic Hills Elementary, who noted that the hourlong process went smoothly Tuesday. Seattle’s system is fairly common (one exception is Lake Washington School District, which contracts with a specific meteorologist, said spokeswoman Kathryn Reith). On Tuesday, many Seattle parents said officials did the best they could. “It was an unusual situation and unusual weather,” said Beth Armitage, whose children go to Salmon Bay K-8, where she works part-time. “Seattle Public Schools has had some issues, but I’m not going to put the weather on them.” Others took issue with aspects of the district’s weather response. Some wanted officials to be more decisive — choosing either to stay open or stay closed, but not changing course midday. Others questioned the emergency alerts, which did not reach some parents at work. Still others suggested it would be wise to consider closing only some schools. Most of all, parents urged officials to remember the importance of their decisions. “At the end of the day,” said Dettmar, the Broadview-Thompson parent, “our life hinges on what the district decides to do.” Brian M. Rosenthal: 206-464-3195 or email@example.com. On Twitter @brianmrosenthal.