Heavy downpours are increasing nationally, especially over the last three to five decades. Largest increases are in the Midwest and Northeast. Increases in the frequency and intensity of extreme precipitation events are projected for all U.S. regions.

This finding is from chapter 2 of Climate Change Impacts in the United States: The Third National Climate Assessment.

Process for developing key messages: Development of the key messages involved discussions of the lead authors and accompanying analyses conducted via one in-person meeting plus multiple teleconferences and email exchanges from February thru September 2012. The authors reviewed 80 technical inputs provided by the public, as well as other published literature, and applied their professional judgment. Key message development also involved the findings from four special workshops that related to the latest scientific understanding of climate extremes. Each workshop had a different theme related to climate extremes, had approximately 30 attendees (the CMIP5 meeting had more than 100), and the workshops resulted in a paper.b91893b4-24a8-46ba-b09a-013d462caf1b The first workshop was held in July 2011, titled Monitoring Changes in Extreme Storm Statistics: State of Knowledge.b37557ac-ee97-4c28-98ca-4f1f1afe163b The second was held in November 2011, titled Forum on Trends and Causes of Observed Changes in Heatwaves, Coldwaves, Floods, and Drought.e15600d0-290f-44e2-9b58-9ffd295ee6d2 The third was held in January 2012, titled Forum on Trends in Extreme Winds, Waves, and Extratropical Storms along the Coasts.596a7f1e-6ce5-4bdf-b144-d0715a7567bd The fourth, the CMIP5 results workshop, was held in March 2012 in Hawai‘i, and resulted in an analysis of CMIP5 results relative to climate extremes in the United States.b91893b4-24a8-46ba-b09a-013d462caf1b The Chapter Author Team’s discussions were supported by targeted consultation with additional experts. Professional expertise and judgment led to determining “key vulnerabilities.” A consensus-based approach was used for final key message selection.

Description of evidence base: The key message and supporting text summarizes extensive evidence documented in the climate science peer-reviewed literature. Technical Input reports (82) on a wide range of topics were also reviewed; they were received as part of the Federal Register Notice solicitation for public input. Evidence that extreme precipitation is increasing is based primarily on analysisb37557ac-ee97-4c28-98ca-4f1f1afe163b b91893b4-24a8-46ba-b09a-013d462caf1b 0ebef171-4903-4aa6-b436-2936da69f84e of hourly and daily precipitation observations from the U.S. Cooperative Observer Network, and is supported by observed increases in atmospheric water vapor.0b3b2ff4-9ee7-45fe-8d0c-895076013715 Recent publications have projected an increase in extreme precipitation events,b37557ac-ee97-4c28-98ca-4f1f1afe163b 81dee494-06d2-4651-8ddb-f36f45dc5942 with some areas getting larger increasese251f590-177e-4ba6-8ed1-6f68b5e54c8a and some getting decreases.fe7cfee1-62d4-4a3f-8d9b-f3ec33912f87 b91893b4-24a8-46ba-b09a-013d462caf1b Nearly all studies to date published in the peer-reviewed literature agree that extreme precipitation event number and intensity have risen, when averaged over the United States. The pattern of change for the wettest day of the year is projected to roughly follow that of the average precipitation, with both increases and decreases across the U.S. Extreme hydrologic events are projected to increase over most of the U.S.

New information and remaining uncertainties: A key issue (uncertainty) is the ability of climate models to simulate precipitation. This is one of the more challenging aspects of modeling of the climate system because precipitation involves not only large-scale processes that are well-resolved by models but also small-scale process, such as convection, that must be parameterized in the current generation of global and regional climate models. Viable avenues to improving the information base are to perform some long, very high-resolution simulations of this century’s climate under different emissions scenarios.

Assessment of confidence based on evidence: Given the evidence base and uncertainties, confidence is high that heavy downpours are increasing in most regions of the U.S., with especially large increases in the Midwest and Northeast. Confidence is high that further increases in the frequency and intensity of extreme precipitation events are projected for most U.S. areas, given the evidence base and uncertainties.

References :

• Very extreme seasonal precipitation in the NARCCAP ensemble: model performance and projections (fe7cfee1)
• CMIP5 Climate Model Analyses: Climate Extremes in the United States (b91893b4)
• A sea ice free summer Arctic within 30 years? (81dee494)
• Increasing drought under global warming in observations and models (0b3b2ff4)
• Global Climate Change Impacts in the United States (e251f590)
• Regional Climate Trends and Scenarios for the U.S. National Climate Assessment: Part 9. Climate of the Contiguous United States. NOAA Technical Report NESDIS 142-9 (0ebef171)
• Monitoring and Understanding Trends in Extreme Storms: State of Knowledge (b37557ac)
• Monitoring and Understanding Changes in Heat Waves, Cold Waves, Floods, and Droughts in the United States: State of Knowledge (e15600d0)
• Monitoring and Understanding Changes in Extremes: Extratropical Storms, Winds, and Waves (596a7f1e)