In New York City, a tropical storm delivered record-breaking rains this weekend. Heavy downpours caused devastating flash floods in central Tennessee, tearing apart houses and killing more than 20 people. Yet, California and much of the West remained in the deepest drought in at least two decades, the product of a long-term precipitation shortfall and temperatures that are much hotter than usual. This divide, a wetter East and a drier West, reflects a broader pattern observed in the United States in recent decades. The map above, created using data from the National Oceanic and Atmospheric Administration, shows the Eastern half of the country has gotten more rain, on average, over the last 30 years than it did during the 20th century, while precipitation has decreased in the West. (Thirty-year averages are often used by scientists to glean big-picture climate trends from temperature and precipitation data that varies substantially year-to-year.) […]