Holiday-travel hell is here. A pre-Christmas snowstorm sweeping the Midwest is threatening to upend holiday travel plans. Snow is blanketing eight states including Colorado, Nebraska, and Wisconsin, forcing airports to cancel flights. Des Moines International airport in Iowa shut down altogether. More than 400 flights routed through Chicago O’Hare were canceled due to the weather, leaving one of the busiest airports in the country operating below capacity. Travelers departing from areas with good weather may still get stranded, as many flights connect through Midwest hubs. Airlines tend to be close to fully booked this time of year, making it difficult to find seats for people whose flights have been canceled.