Astronomical winter ends today and spring begins, but it won’t feel like spring because yet another cold air mass of Arctic origin has taken up residence across the Midwest (and practically all of the nation west of the Rocky Mountains). It’s a weather situation that has occurred nearly without interruption since the end of February. The current cold spell will last through the week, though temperatures will moderate slightly under the influence of the increasingly powerful March sun. Before then, Wednesday’s expected high temperature in the middle 20s places it among the the coldest 4 percent of all days in the middle of March. Looking five days ahead, computer models, after considerable disparity, are beginning to agree that accumulating snow is increasingly likely here on Sunday and Monday. It’s a situation worthy of close scrutiny in coming days.