Trends in extreme precipitation and their causes were analyzed for events within the Laurentian Great Lakes for several periods: 1908–2020, 1949–2020, 1980–2019, and 1980–2020. Upward trends in extreme precipitation were found for multiple metrics, including the number of exceedances of return period thresholds for several durations and average recurrence intervals (ARI), the number of extreme basin-average 4-day precipitation totals, and the annual maximum daily station precipitation. The causes of extreme events were classified into 5 meteorological categories: fronts of extratropical cyclones (ETC-FRT), extratropical cyclones but not proximate to the fronts (ETC-NFRT), mesoscale convective systems (MCS), tropical cyclones (TC), and air mass convection (AMC). For daily events exceeding the threshold for 5-yr ARI, ETC-FRTs account for 78% of all events, followed by ETC-NFRTs (12%), MCSs (6%), TCs (2%), and AMC (1%). Upward trends in the number of events by cause were found for all categories except AMC. An examination of basin-wide 4-day extreme events (40 largest events during 1980–2019) found that all events were caused by ETC-FRTs (85%) or ETC-NFRTs (15%).

The Fourth National Climate Assessment (NCA4) is the most comprehensive report to date assessing climate change science, impacts, risks, and adaptation in the United States. The 1,500 page report covers a breadth of topics, ranging from foundational physical science to climate change response options. Here we present information on indicators and impacts of climate change in the human environment featured in NCA4 Volume II, focusing on: air quality, forest disturbance and wildfire, energy systems, and water resources. Observations, trends, and impacts of these aspects of our changing climate will be discussed, along with implications for the future. Implications: People of the United States are already being affected by our changing climate. Information on observed changes and impacts that affect human welfare and society, along with projections for the future, is highly valuable for informing decision-makers, including utility managers, emergency planners, and other stakeholders, about climate risk assessment, adaptation, and mitigation options.

An analysis of 3,104 stations in the United States shows virtually every station exhibits a positive correlation between precipitable water (PW) and extreme daily precipitation (EP) with over one‐third statistically significant. To first approximation, EP scales linearly with PW, but there is nonlinear scaling at the lower and upper ends of the PW distribution. On average, EP is amplified by twice the amount of PW, but there is substantial seasonal and spatial variability caused by dynamically forced vertical velocity with stations ranging from a one‐to‐one relationship to over three‐to‐one. These latter stations are generally found in elevated terrain or near coasts and in regions and seasons affected by strong synoptic‐scale weather systems. The results also point to PW, not vertical velocity, as the key limiting factor in the most intense EP events. This has important implications for projecting changes of the most intense EP events in a warmer world.

A new data set of monthly temperatures, adjusted for detected inhomogeneities, was used to examine whether the monthly temperature climate of the U.S. has become more extreme. During the past two to three decades, there has been a shift toward more frequent very warm months, but less frequent very cold months. Thus, overall the monthly temperature climate has not become more extreme. Midtwentieth century including the 1930s was an earlier period of frequent very warm months, a result of very warm daytime temperatures, while nighttime temperatures were not unusual. Regionally, there is a lack of century‐scale warming in the southeast U.S. annually and in parts of the central U.S. in the summer, characterized by lack of daytime warming while there has been nighttime warming. Compared to the earlier midcentury warm period, recent decades have been more (less) extreme in the summer (winter) in the west while Midwest summers have been less extreme.

Kunkel, Kenneth E.; Stevens, Laura E.; Stevens, Scott E.; Sun, Liqiang; Janssen, Emily; Wuebbles, Donald; Kruk, Michael C.; Thomas, Devin; Shulski, Martha; Umphlett, Natalie A.; Hubbard, Kenneth G.; Robbins, Kevin; Romolo, Luigi; Akyuz, Adnan; Pathak, Tapan B.; Bergantino, Tony R.; and Dobson, J. Greg, "Regional Climate Trends and Scenarios for the U.S. National Climate Assessment Part 4. Climate of the U.S. Great Plains" (2013). HPRCC Personnel Publications. 39. https://digitalcommons.unl.edu/hprccpubs/39