Trends in Environments Conducive for Large Hail

The large hail parameter (LHP) is used to assess 39-year trends (1979–2017) in environments conducive for large hail. The LHP is a composite index, used to differentiate environments conducive for significantly severe hail (≥ 5 cm) versus smaller hail, that is comprised of three thermodynamic parameters and three kinematic parameters that are associated with hail growth. We define a LHP-day as any day (12 UTC – 12 UTC) that has a maximum LHP that exceeds 5.8, 10.6, and 18.4, representing the 25^th, 50^th, and 75^th percentile of past environments that coincided with significantly severe hail. Using the lowest threshold (5.8), there has been an significant increasing trend in LHP-days in the central Plains (3–5 more LHP-days per decade) and in the northeast U.S. (1–3 more LHP-days per decade). There has been a significant decreasing trend in LHP-days in the immediate lee of the Colorado Rocky Mountains (3–5 days per decade). Using the higher thresholds (5.8 and 18.4), there has been an significant increasing trend in LHP-days in the central and southern Plains (1–4 more LHP-days per decade). These trends are attributed to changes in the thermodynamic parameters of the LHP. The results suggest that there is has been increasing hail risk due to an increasing frequency of days that are conducive for significantly severe hail over the time period examined. Implications on future large hail risk are also discussed.