From USA Today (Nicole Carroll):
Specifically, our reporting finds:
At some point over the past three years, 27 states – all east of the Rocky Mountains – hit their highest 30-year precipitation average since record keeping began in 1895. A dozen states, including Iowa, Ohio and Rhode Island, saw five of their 10 wettest years in history over the past two decades. Michigan saw six of its wettest 10 years on record over the past 13 years. In June, at least 136 daily rainfall records were set during storms across five states along the Mississippi River. At the opposite extreme, eight states – including five in the West – had at least three record-dry years in the same time period. That’s double what would be expected based on historical patterns.
Michael Mann, a climatologist at Penn State University, told our reporters the greenhouse effect is important to keep Earth from freezing, but excess heat greatly reduces the temperature difference between the warmer tropics and cooler polar regions in the summer.
Mann said that reduction in the temperature difference slows down the jet stream, which makes it weaker and wavier in the summer. That means weather systems moving across the country can slow or stall more often.