Assessing changes in Extreme Weather and Climate Events [INVITED]

Long-term trends in extreme weather and climate events were assessed for the 2013 National Climate Assessment report. These included heat waves, cold waves, extreme precipitation, severe convective storms, severe winter storms, drought, tropical and extratropical cyclones, and coastal winds and waves. A series of three workshops evaluated the status of knowledge and the adequacy of data to support trend analysis. Certain extreme phenomena have exhibited trends. Extreme precipitation events have increased nationally, with especially large increases in the Midwest and Northeast. The number of heat waves during the past decade has been the highest on record in the western U.S. and the second highest in most other regions (the 1930s remain the decade with the most frequent and intense heat waves in eastern and central regions). The number of cold waves during the paste decade has been the lowest on record nationally. Flooding frequencies have been upward in the northeast and central regions, downward in the southwest, and flat elsewhere. Drought has been more frequent and intense in the southwest. There is some evidence for an increase in storm frequency and intensity along U.S. coasts and an increase in extreme waves along the Pacific coast since the 1950s, but evidence along other U.S. shorelines is inconclusive.

Model simulations, backed by physical reasoning, provide high confidence in future trends for heat waves (upward), cold waves (downward), extreme precipitation (upward), and drought (upward in the southwest). There is lower confidence in future trends for other types of extremes.