That snowflake icon on your smartphone app might be quick and handy — it just might not be as accurate as a human when the weather gets rough.

Winter storm warnings and advisories extended from Montana to Ohio, the National Weather Service said. Forecasters warned there could be airport delays and slowed traffic with snow falling at more than an inch per hour in some areas. More than 100 homes have been damaged after a tornado touched down in a residential area outside Houston.

More than half of the United States is about to face one of the most sprawling and menacing winter storms in years, one that threatens to deliver a historic blow of snow and ice. The long-lasting storm begins Friday and,