Double Impact: When Both Tornadoes and Flash Floods Threaten the Same Place at the Same Time

Abstract

While both tornadoes and flash floods individually present public hazards, when the two threats are both concurrent and collocated (referred to here as TORFF events), unique concerns arise. This study aims to evaluate the climatological and meteorological characteristics associated with TORFF events over the continental United States. Two separate datasets, one based on overlapping tornado and flash flood warnings and the other based on observations, were used to arrive at estimations of the instances when a TORFF event was deemed imminent and verified to have occurred, respectively. These datasets were then used to discern the geographical and meteorological characteristics of recent TORFF events. During 2008-14, TORFF events were found to be publicly communicated via overlapping warnings an average of 400 times per year, with a maximum frequency occurring in the lower Mississippi River valley. Additionally, 68 verified TORFF events between 2008 and 2013 were identified and subsequently classified based on synoptic characteristics and radar observations. In general, synoptic conditions associated with TORFF events were found to exhibit similar characteristics of typical tornadic environments, but the TORFF environment tended to be moister and have stronger synoptic-scale forcing for ascent. These results indicate that TORFF events occur with appreciable frequency and in complex meteorological scenarios. Furthermore, despite these identified differences, TORFF scenarios are not easily distinguishable from tornadic events that fail to produce collocated flash flooding, and present difficult challenges both from the perspective of forecasting and public communication.

A Regime-Based Evaluation of Southern and Northern Great Plains Warm-Season Precipitation Events in WRF

“…In this latter class, the analysis domain fell within the warm sector ahead of the low pressure center. In many ways, these patterns are reminiscent of known patterns associated with severe weather and flash flood events for this region (Maddox et al 1979;Nielsen et al 2015), although it should be pointed out that the cases within this study simply represent the upper 90% of precipitating days. Along the bottom row (classes 2A-2C), 500-hPa flow FIG.…”

Strange Floods: The Upper Tail of Flood Peaks in the United States

“…Eyewitness accounts of the storm suggest that it may have been a supercell thunderstorm, with one observer describing the storm as “a broad black streak almost straight north of us which seemed to be coming straight down from the sky” (Byrd, ). Supercells are the most intense storms in the spectrum of thunderstorms and have produced some of the largest short‐term rainfall rates in the United States (Bunkers & Doswell, ; Doswell et al, ; Giordano & Fritsch, ; Nielsen et al, ; Rogash & Racy, ; Smith et al, ). Recent studies are shedding light on both the dynamics and climatology of supercells in mountainous terrain (see, e.g., Bosart et al, ; Markowski & Dotzek, ; Scheffknecht et al, ).…”

“…Downscaling simulations with WRF provide insights to common elements of storm environment for the record floods of Eastern Oregon, reducing some of the strangeness of these events. The Heppner, Meyers Canyon, and Lane Canyon storms exhibited similar structure of steering level winds, precipitable water, and CAPE fields, which play an important role in determining the potential for extreme rainfall rates (see Bunkers & Doswell, ; Nielsen et al, ; and Nielsen & Schumacher, , for additional discussion). Model simulations also suggest that storm size and motion were similar for the three events.…”

“…Cotton et al (2010) note that "storms producing the largest hailstones occur in strongly sheared environments; thus, in general, we should not expect that the storm systems producing the largest hailstones are also heavy rain producing storms" (see also Hamada et al, 2015;Zipser et al, 2006). Doswell et al (1996) and Smith et al (2001) take a different perspective, pointing to the most intense thunderstorms as underappreciated agents of extreme flooding (see also Bunkers & Doswell, 2016;Hitchens & Brooks, 2013;Llasat et al, 2016;Nielsen et al, 2015;Nielsen & Schumacher, 2018;Rogash & Racy, 2002). The 1903The , 1956The , and 1965 floods in eastern Oregon seem strange partly because we have relatively little insight to the storms that produced them.…”

Forecasting Severe Weather with Random Forests

“…Additionally, while the SPC outlooks underwent a significant change in October 2014, when ''marginal'' and ''enhanced'' categories were added to outlooks on Days 1-3 (Jacks 2014), the changes only remapped categorical definitions to the unchanged probability contours. Based on different diurnal and seasonal climatologies (e.g., Brooks et al 2003;Nielsen et al 2015;Krocak and Brooks 2018), and due to differing regimes and storm systems primarily responsible for severe weather across the CONUS (e.g., Smith et al 2012), the country is partitioned into three regions: west, central, and east ( Fig. 1).…”