Winter storms have increased in frequency and intensity since the 1950s, and their tracks have shifted northward over the United States. Other trends in severe storms, including the intensity and frequency of tornadoes, hail, and damaging thunderstorm winds, are uncertain and are being studied intensively.

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 summarize 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. Current work596a7f1e-6ce5-4bdf-b144-d0715a7567bd has provided evidence of the increase in frequency and intensity of winter storms, with the storm tracks shifting poleward,57610605-1682-4827-878f-c12c6e9b674c d4efd07e-2886-41f4-bc6d-14cbbe00c382 but some areas have experienced a decrease in winter storm frequency.e251f590-177e-4ba6-8ed1-6f68b5e54c8a Although there are some indications of increased blocking (a large-scale pressure pattern with little or no movement) of the wintertime circulation of the Northern Hemisphere,8a57a9b0-a2cb-4ce7-b603-2cf81dc20736 the assessment and attribution of trends in blocking remain an active research area.d23c69fd-1a50-44fd-8ade-c636ad308b8d Some recent research has provided insight into the connection of global warming to tornadoes and severe thunderstorms.242ad1ac-9e37-44e6-9e8f-43cb318ace99 7ede85ab-94c5-46e3-8bb5-0d98844a98db

New information and remaining uncertainties: Winter storms and other types of severe storms have greater uncertainties in their recent trends and projections, compared to hurricanes (Key Message 8). The text for this key message explicitly acknowledges the state of knowledge, pointing out “what we don’t know.” There has been a sizeable upward trend in the number of storm events causing large financial and other losses.8949c886-8b4a-4845-9acf-1047f829c0ea

Assessment of confidence based on evidence: Given the evidence base and remaining uncertainties: Confidence is medium that winter storms have increased slightly in frequency and intensity, and that their tracks have shifted northward over the U.S. Confidence is low on other trends in severe storms, including the intensity and frequency of tornadoes, hail, and damaging thunderstorm winds.

References :

• Will moist convection be stronger in a warmer climate? (242ad1ac)
• Evidence linking Arctic amplification to extreme weather in mid-latitudes (8a57a9b0)
• Global Climate Change Impacts in the United States (e251f590)
• Trends and low frequency variability of extra-tropical cyclone activity in the ensemble of twentieth century reanalysis (57610605)
• Changes in severe thunderstorm environment frequency during the 21st century caused by anthropogenically enhanced global radiative forcing (7ede85ab)
• Exploring links between Arctic amplification and mid-latitude weather (d23c69fd)
• Climatology and Changes of Extratropical Cyclone Activity: Comparison of ERA-40 with NCEP–NCAR Reanalysis for 1958–2001 (d4efd07e)
• webpage Billion Dollar Weather/Climate Disasters (8949c886)
• Monitoring and Understanding Changes in Extremes: Extratropical Storms, Winds, and Waves (596a7f1e)
• Monitoring and Understanding Trends in Extreme Storms: State of Knowledge (b37557ac)
• CMIP5 Climate Model Analyses: Climate Extremes in the United States (b91893b4)
• Monitoring and Understanding Changes in Heat Waves, Cold Waves, Floods, and Droughts in the United States: State of Knowledge (e15600d0)