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Abstract There is a lack of consensus on how tropical cyclone outer winds may change, if at all, due to extratropical transition. Hence, this study examines changes in North Atlantic tropical cyclone outer size and structure using a large, multidecadal sample of cases from reanalysis data. These results suggest that tropical cyclone outer size and structure typically remain unchanged until after extratropical transition end. In those minority of cases with strong expansion during extratropical transition, increases in tropical cyclone outer winds begin first in the lower troposphere during extratropical transition and build upwards over time. This broadening of the azimuthal‐mean outer winds is also associated with an increasingly asymmetric outer wind field with the strongest winds concentrated downstream of the tropical cyclone. These storms that expand most strongly during transition are typically smaller at transition start and eventually become embedded in more strongly baroclinic environments by extratropical transition end. 

Abstract Supercells in landfalling tropical cyclones (TCs) often produce tornadoes within 50 km of the coastline. The prevalence of TC tornadoes near the coast is not explained by the synoptic environments of the TC, suggesting a mesoscale influence is likely. Past case studies point to thermodynamic contrasts between ocean and land or convergence along the coast as a possible mechanism for enhancing supercell mesocyclones and storm intensity. This study augments past work by examining the changes in the hurricane boundary layer over land in the context of vertical wind shear. Using ground-based single- and dual-Doppler radar analyses, we show that the reduction in the boundary layer wind results in an increase in vertical wind shear/storm-relative helicity inland of the coast. We also show that convergence along the coast may be impactful to supercells as they cross the coastal boundary. Finally, we briefly document the changes in mesocyclone vertical vorticity to assess how the environmental changes may impact individual supercells. 

Abstract Supercells in landfalling tropical cyclones (TCs) often produce tornadoes that can cause fatalities and extensive damage. In previous studies, many tornadoes have been shown to form <50 km from the coast, and their parent storms may also intensify as they cross the coastal boundary. This study uses WSR‐88D observations of TC tornadic mesocyclones from 2011 to 2018 to examine changes in their low‐level rotation upon moving onshore. We will show that radar‐derived azimuthal shear tends to increase in storms that cross the coastal boundary. Similar intensification trends are also found in radar‐derived (supercell) storm‐scale divergence, such that storm‐scale convergence increases as storms move onshore. It is likely changes in the near‐coast vertical wind shear and/or near‐shore convergence helps explain supercell intensification, which is important to consider particularly in operational settings. 

Abstract Tropical cyclone (TC) tornadoes are often associated with lower‐skill forecasts compared to midlatitude supercellular tornadoes. Forecasts may be improved through a greater understanding of their lightning and radar signatures. This study investigates the lightning and radar characteristics of TC tornadic cells for comparison with TC non‐tornadic cells (i.e., strongly rotating cells without tornadoes) and non‐TC tornadic cells using three lightning networks and radar data. These results show that the majority of TC tornadic and non‐tornadic cells are not associated with lightning, although the former subset occurs with lightning more often. TC tornadic cases typically have lightning maximized to its northeast, whereas the non‐tornadic subset is associated with a lower density of flashes that are more symmetrically distributed. TC tornadic mesocyclones also show stronger low‐level rotation and convergence at the time of tornado occurrence compared to non‐tornadic cases. Hourly trends in rotation and convergence show stronger increases before tornado occurrence in both variables for TC tornadic mesocyclones, yielding small, nonsignificant differences with non‐TC tornadic mesocyclones during tornado occurrence. Finally, analysis of lightning throughout the TC shows that tornadic cells often occur on the downwind edge of a broad lightning maximum, whereas non‐tornadic cases occur in the middle of a weaker lightning maximum, with these maxima propagating away from the TC in both subsets. 

Abstract There remains no consensus on whether the outer size of the tropical cyclone (TC) wind field impacts tornado occurrence. This study statistically examines the relationship between TC outer size with both the number and location of tornadoes using multidecadal tornado reports, a reanalysis‐derived TC outer size metric, and radiosonde data. These results show that larger TC spawn tornadoes that are located farther from and over a broader region relative to the cyclone center, although these changes do not entirely scale with TC outer size. Larger TCs are also associated with more frequent occurrence of tornadoes per 6 h, especially enhanced numbers of tornadoes. These changes in tornado occurrence in larger TCs may be due to a broadening of favorable helicity for tornadoes in the downshear sector, which may be partially offset by CAPE reductions in the left‐of‐shear quadrants. 

Abstract This work investigates how the relationship between tropical cyclone (TC) tornadoes and ambient (i.e., synoptic-scale) deep-tropospheric (i.e., 850–200-hPa) vertical wind shear (VWS) varies between coastal and inland environments. Observed U.S. TC tornado track data are used to study tornado frequency and location, while dropsonde and radiosonde data are used to analyze convective-scale environments. To study the variability in the TC tornado–VWS relationship, these data are categorized by both 1) their distance from the coast and 2) reanalysis-derived VWS magnitude. The analysis shows that TCs produce coastal tornadoes regardless of VWS magnitude primarily in their downshear sector, with tornadoes most frequently occurring in strongly sheared cases. Inland tornadoes, including the most damaging cases, primarily occur in strongly sheared TCs within the outer radii of the downshear-right quadrant. Consistent with these patterns, dropsondes and coastal radiosondes show that the downshear-right quadrant of strongly sheared TCs has the most favorable combination of enhanced lower-tropospheric near-surface speed shear and veering, and reduced lower-tropospheric thermodynamic stability for tornadic supercells. Despite the weaker intensity farther inland, these kinematic conditions are even more favorable in inland environments within the downshear-right quadrant of strongly sheared TCs, due to the strengthened veering of the ambient winds and the lack of changes in the TC outer tangential wind field strength. The constructive superposition of the ambient and TC winds may be particularly important to inland tornado occurrence. Together, these results will allow forecasters to anticipate how the frequency and location of tornadoes and, more broadly, convection may change as TCs move inland. 

The impact of extratropical transition (ET) on tropical cyclone (TC) tornadoes is not fully understood with no prior tornado climatologies for ET cases. Hence, this study investigates how ET impacts tornadoes and convective-scale environments within TCs using multidecadal tornado and radiosonde data from North Atlantic TCs. This research divides ET into three phases: tropical (i.e., pre-ET), transition (i.e., during ET), and extratropical (i.e., post-ET). These results show that the largest portion of tornadoes occurs before and during ET, with the greatest frequencies during ET. As TCs progress through ET, tornado location shifts north and east in the United States but farther south or more strongly downshear right relative to the TC center. Tornadoes also tend to occur later in the day and are more likely to be associated with greater damage. Evaluation of radiosondes shows that the downshear-right quadrant of the TC is frequently the most favorable for tornado production, with sufficient entrainment CAPE (ECAPE) and strong storm-relative helicity (SRH). Specifically, the downshear-right quadrant shows slower decreases in ECAPE (associated with synoptic-scale cooling and drying) and increased SRH and associated lower-tropospheric vertical wind shear through ET, relative to the other quadrants relative to the deep-tropospheric (i.e., 850–200-hPa) vertical wind shear vector. These results inform the physical model and prediction of ET-related TC structure, both in terms of their convective-scale environments and subsequent hazard production. 

Forecasts of tropical cyclone (TC) tornadoes are less skillful than their non‐TC counterparts at all lead times. The development of a convection‐allowing regional ensemble, known as the Warn‐on‐Forecast System (WoFS), may help improve short‐fused TC tornado forecasts. As a first step, this study investigates the fidelity of convective‐scale kinematic and thermodynamic environments to a preliminary set of soundings from WoFS forecasts for comparison with radiosondes for selected 2020 landfalling TCs. Our study shows reasonable agreement between TC convective‐scale kinematic environments in WoFS versus observed soundings at all forecast lead times. Nonetheless, WoFS is biased toward weaker than observed TC‐relative radial winds, and stronger than observed near‐surface tangential winds with weaker winds aloft, during the forecast. Analysis of storm‐relative helicity (SRH) shows that WoFS underestimates extreme observed values. Convective‐scale thermodynamic environments are well simulated for both temperature and dewpoint at all lead times. However, WoFS is biased moister with steeper lapse rates compared to observations during the forecast. Both CAPE and, to a lesser extent, 0–3‐km CAPE distributions are narrower in WoFS than in radiosondes, with an underestimation of higher CAPE values. Together, these results suggest that WoFS may have utility for forecasting convective‐scale environments in landfalling TCs with lead times of several hours. 

This review article summarizes the current understanding and recent updates to tropical cyclone outer size and structure forecasting and research primarily since 2018 as part of the World Meteorological Organization's 10th International Workshop on Tropical Cyclones. A more complete understanding of tropical cyclone outer wind and precipitation is key to anticipating storm intensification and the scale and magnitude of landfalling hazards. We first discuss the relevance of tropical cyclone outer size and structure, improvements in our understanding of its life cycle and inter-basin variability, and the processes that impact outer size changes. We next focus on current forecasting practices and differences among warning centers, recent advances in operational forecasting, and new observations of the storm outer wind field. We also summarize recent research on projected tropical cyclone outer size and structure changes by the late 21st century. Finally, we discuss recommendations for the future of tropical cyclone outer size forecasting and research.