Search for: All records

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. 

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 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. 

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 There is a lack of consensus on whether North Atlantic tropical cyclone (TC) outer size and structure (i.e., change in outer winds with increasing radius from the TC) will differ by the late twenty-first century. Hence, this work seeks to examine whether North Atlantic TC outer wind field size and structure will change by the late twenty-first century using multiple simulations under CMIP3 SRES A1B and CMIP5 RCP4.5 scenarios. Specifically, our analysis examines data from the GFDL High-Resolution Forecast-Oriented Low Ocean Resolution model (HiFLOR) and two versions of the GFDL hurricane model downscaling climate model output. Our results show that projected North Atlantic TC outer size and structure remain unchanged by the late twenty-first century within nearly all HiFLOR and GFDL hurricane model simulations. Moreover, no significant regional outer size differences exist in the North Atlantic within most HiFLOR and GFDL hurricane model simulations. No changes between the control and late-twenty-first-century simulations exist over the storm life cycle in nearly all simulations. For the simulation that shows significant decreases in TC outer size, the changes are attributed to reductions in storm lifetime and outer size growth rates. The absence of differences in outer size among most simulations is consistent with the process that controls the theoretical upper bound of storm size (i.e., Rhines scaling), which is thermodynamically invariant. However, the lack of complete consensus among simulations for many of these conclusions suggests nontrivial uncertainty in our results. 

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.