AUSTIN, Texas—Winter weather that has overwhelmed power grids unprepared for climate change and left millions without electricity in record-breaking cold kept its grip on the nation’s midsection Wednesday. At least 20 people have died, some while struggling to find warmth inside their homes. In the Houston area, one family succumbed to carbon monoxide from car exhaust in their garage; another perished after flames spread from their fireplace. Blame the polar vortex, a weather pattern that usually keeps to the Arctic, but is increasingly visiting lower latitudes and staying beyond its welcome. More than 100 million people live in areas covered Wednesday by some type of winter weather warning, watch, or advisory, as yet another winter storm hits Texas and other parts of the southern Plains, the National Weather Service said. Utilities from Minnesota to Texas and Mississippi have implemented rolling blackouts to ease the burden on power grids straining to …