Abstract
Two heavy rainfall events occurring in early 2020 brought flooding, flash flooding, strong winds and tornadoes to the southern Appalachian Mountains. The atmospheric river-influenced events qualified as extreme (top 2.5%) rain events in the archives of two research-grade rain gauge networks located in two different river basins. The earlier event of 5–7 February 2020 was an event of longer duration that caused significant flooding in close proximity to the mountains and had the higher total accumulation observed by the two gauge networks, compared to the later event of 12–13 April 2020. However, its associated downstream flooding response and number of landslides (two) were muted compared to the April event (21). The purpose of this study is to understand differences in the surface response of the two events, primarily by examining the large-scale weather pattern and available space-based observations. Both storms were preceded by anticyclonic Rossby wave breaking events that led to a highly amplified 500 hPa wave during the February storm (a broad continent-wide 500 hPa cyclone during the April storm) in which the accompanying low-level cyclone moved slowly (rapidly). Model analyses and space-based water vapor observations of the two events indicated a deep sub-tropical moisture source during the February storm (converging sub-tropical low-level moisture streams and a dry mid-tropospheric layer during the April storm). Systematic differences of environmental stability were reflected in differences of storm-averaged rain rate intensity, with large-scale atmospheric structures favoring higher intensities during the April storm. Space-based observations of post-storm surface conditions suggested antecedent soil moisture conditioned by rainfall of the February event made the widespread triggering of landslides possible during the higher intensity rains of the April event, a period exceeding the 30 day lag explored in Miller et al. (2019).
Highlights
The challenge of observing and forecasting precipitation in mountainous regions of the mid-latitudes is well documented
It was found that long periods of rainfall often linked with individual extreme rainfall events (ExtR) and sometimes with atmospheric river (AR), showed a relatively high correlation with landslide days occurring within 30 days after the rainfall event (Pearson correlation coefficient of 0.561 and p-value of 0.008 for 117 data pairs)
The landslides triggered by both events, along with post-event flooding as estimated by the Visible Infrared Imaging Radiometer Suite (VIIRS)/Advanced Baseline Imager (ABI) algorithm and post-event shallow moisture drying as estimated by the SMOPS algorithm were consistent with the soil water storage capacity having not been exceeded during the February (April) 2020 heavy rainfall event
Summary
The challenge of observing and forecasting precipitation in mountainous regions of the mid-latitudes is well documented (e.g., references [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18]). Recent increased tourism and development in these regions has highlighted the shortcomings of these capabilities To further complicate these issues, the challenges are dependent on the scale and season of the precipitation-generating storm. The variety of storms experienced locally is dependent on the geographic location of the mountains
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