Compounding Heatwave‐Extreme Rainfall Events Driven by Fronts, High Moisture, and Atmospheric Instability.

Heatwaves have been shown to increase the likelihood and intensity of extreme rainfall occurring immediately afterward, potentially leading to increased flood risk. However, the exact mechanisms connecting heatwaves to extreme rainfall remain poorly understood. In this study, we use weather type data sets for Australia and Europe to identify weather patterns, including fronts, cyclones, and thunderstorm conditions, associated with heatwave terminations and following extreme rainfall events. We further analyze, using reanalysis data, how atmospheric instability and moisture availability change before and after the heatwave termination depending on whether the heatwave is followed by extreme rainfall, as well as the location of the heatwave. We find that most heatwaves terminate during thunderstorm and/or frontal conditions. Additionally, atmospheric instability and moisture availability increase several days before the heatwave termination; but only if heatwaves are followed by extreme rainfall. We also find that atmospheric instability and moisture after a heatwave are significantly higher than expected from climatology for the same time of the year, and that highest values of instability and moisture are associated with highest post‐heatwave rainfall intensities. We conclude that the joint presence of high atmospheric instability, moisture, as well as frontal systems are likely to explain why rainfall is generally more extreme and likely after heatwaves, as well as why this compound hazard is mainly found in the non‐arid mid and high latitudes. An improved understanding of the drivers of these compound events will help assess potential changing impacts in the future. Plain Language Summary: Extreme rainfall which can lead to flash floods is more likely to occur if it is preceded by a heatwave. The exact reasons behind this connection, however, are not fully clear. In this study we investigate the mechanistic drivers connecting heatwaves to extreme rainfall in Europe and Australia by analyzing which types of weather (e.g., fronts, cyclones, thunderstorms) are present during the transition from heatwaves to extreme rainfall. We also analyze how atmospheric characteristics associated with extreme rainfall during thunderstorms change depending on if a heatwave is followed by extreme rainfall or not. We find that heatwaves are usually followed by extreme rainfall when there are thunderstorm conditions and/or when there is the presence of a front. Further, we find that high amounts of moisture are present if heatwaves are followed by extreme rainfall and that atmospheric conditions favorable for thunderstorms, including high amounts of moisture are generally increased after heatwaves. These findings help understand how heatwaves are connected to extreme rainfall and can help assess how the risk from these events might change in the future. Key Points: The transition from heatwaves to extreme rainfall is usually associated with fronts and/or thunderstorm conditionsAtmospheric instability and moisture are larger than during non‐heatwave conditions and increase over several days before the terminationAvailability of moisture is important for producing extreme rainfall after heatwaves and explains the spatial variability for this event [ABSTRACT FROM AUTHOR]